compact see cryptocrystalline
compaction a) In geology, compaction (= geological compaction) refers to the densification
process caused by the gradual increase of overburden (weight) resulting from continuous
deposition of sediments or by pressures from the movement of rock within the crust. b) The
geologic process of changing fine-grained sediment to consolidated rock, as in clay to shale. c)
In soils engineering, compaction is the artificial process to improve the engineering
properties of a clay or soil by mechanical energy (e.g., vibration, static pressure). The clay or soil
is usually partially saturated and thus contains mineral particles, air, and water. Compaction
reduces the volume of air in the pores so that the dry density of the soil/clay increases. In
practice, to maximize compaction, the water content is adjusted to a suitable value, called the
optimal water content, so that the dry density approaches a maximum value. See water, optimal
content for compaction; consolidation, clay or soil
complex A complex is a dissolved or surface species that forms by association of a cation and
either an anion or a neutral molecule, the latter anion or molecule is often referred to as a ligand.
The complex may have an overall charge that is positive, negative, or neutral.
compressibility a) In soils engineering, compressibility is a mechanical property that defines the
resistance of a water saturated clayey soil to compressional deformation under effective stress.
The change in effective stress leads to the flow of porewater, resulting in the change in soil
thickness or volume. Usually the compressibility of clay is measured by the one-dimensional
(i.e., K0 condition) consolidation testing, and is defined by the compression index. b) In
geophysics, compressibility is defined as the reciprocal of the bulk modulus, which relates the
change in volume of a material to the hydrostatic state of stress. See also effective stress, void
ratio, compression index
compression index the ratio of change in void ratio to the change in the logarithmic effective
stress when a clay is subject to consolidation. Cf., compressibility
compression, primary In clay science, the initial stage of consolidation compression of a clay,
which is governed by the dissipation of excess pore water pressure (i.e., pore water pressure in
excess of the hydrostatic water pressure). During this stage, the excess pore water pressure
changes (usually approaching the hydrostatic water pressure), and the clay either produces pore
water (in the case of positive excess water pressure) or adsorbs water into the pores (in the case
of negative excess water pressure). During this stage, the effective stress changes, and the clay
volume changes as well. Cf., compression, secondary
compression, secondary the stage of consolidation compression occurring under constant
effective stress after primary compression, where a change in effective stress occurs. Syn.,
drained creep; Cf., compression, primary; creep.
concretion see glaebule
conditional stability constant although not a true equilibrium constant, a conditional stability
constant describes the equilibria of complex formation at ambient solution conditions. For
example, a conditional stability constant may be used when pH is fixed at a specific value; the
conditional stability constant would then vary with pH. Thus, the complex will be either strong
or weak depending on solution conditions, in this case with pH and possible protonation of the
ligand. In other cases, temperature or ionic strength may alter metal to ligand strength, which
changes the conditional stability constant. Conditional stability constants are commonly used to
describe surface complexation reactions. Syn., apparent stability constant; See also ligand
congruent dissolution dissolution where the same ratio of atoms present in the parent phase is
released to the solution. This type of dissolution may be referred to as “stoichiometric”
dissolution. Cf., congruent melting, incongruent dissolution, incongruent melting
congruent melting Upon heating to form a liquid, the liquid (melt) has the same composition as
the solid. Cf., congruent dissolution, incongruent dissolution, incongruent melting
consolidation, clay or soil the process by which a clay unit deforms as a result of drainage of
pore water with non-equilibrium pressure produced by loading. During consolidation loading, the
positive non-equilibrium pore water pressure causes water to flow out of the soil, resulting in a
decrease in soil volume, also called consolidation settlement. The process of consolidation is
usually accompanied by an increase in effective stress. This is different from drained shearing. In
general, the loading rate for consolidation must be much smaller than drained shearing. Cf.,
drained shearing, effective stress
consolidation loading see consolidation, clay or soil
consolidation settlement see consolidation, clay or soil
constrained polymer region For polymer nanocomposites, the volume around nanoparticles
where the degree of freedom of movement of the polymer has been lowered. This region
involves the interaction between the polymer and nanoparticle and is distinguished by having
properties that are different from the bulk polymer properties. The volume of this region is
dependent upon the strength of the interaction of the polymer and nanoparticle and the flexibility
of the polymer. The concept, for example, explains why a gas diffusion coefficient of a polymer
nanocomposite may differ from the bulk polymer. Gas diffusion may also be affected by the
arrangement of clay particles that leads to a “tortuous path” and a reduction of diffusion through
continuously stirred tank reactor (CSTR) a reaction vessel with inlet and outlet flow and
whose contents are stirred. CSTRs are commonly operated at steady state, but they may alos
function transiently. Cf., batch reactor, chemostat, plug flow reactor
contraction, soil or sand In soil science, soil or sand contraction involves the volumetric
reduction of a saturated clayey soil or sand body by drained shearing. For a soft, normally
consolidated or lightly over consolidated saturated clayey soil subject to drained shearing, pore
water generally flows out of the soil (or sand) owing to shearing-induced positive excess pore
water pressure, and hence its total volume decreases, indicative of contraction behavior. Cf.,
converse piezoelectric, see piezoelectric
cosmetics The United States Federal Food, Drug, and Cosmetic Act (FD&C Act) of 13 March
2013 defines cosmetics by their intended use, as "articles intended to be rubbed, poured,
sprinkled, or sprayed on, introduced into, or otherwise applied to the human body...for cleansing,
beautifying, promoting attractiveness, or altering the appearance" [FD&C Act, sec. 201(i)]. The
act specifically excludes soap as a cosmetic. Clays and clay minerals used in cosmetics include
bentonite, illite, kaolin, iron oxides, etc., often characterized by color or locality, and each
purported to function or behave differently.
Coulomb interactions For classical calculations or simulations of atomic structures, Coulomb
interactions account for the attraction or repulsion between pairs of atoms in accord with the
positive or negative charge on each atom. Coulomb’s Law is used to calculate the potential
energy between each pair of atoms in the model system. For periodic systems, the long-range
component of these interactions is often treated in reciprocal space by Ewald summation or a
similar technique. Syn., Coulombic interactions, electrostatic interactions; Cf., Ewald sum, force
counter ion an ion with a charge opposite in sign to the charge of the solid interface to which it
is attracted. Cf., co-ion
covalent bond a chemical bond that involves sharing of one or more electrons, generally an
electron pair. Covalent bonds require the appropriate geometric arrangement of coordinating
atoms so that orbitals can overlap. Diamond, which involves carbon atoms, exhibits the ideal
characteristics of covalent bonds. However, many elemental pairs involve covalent character and
other bonding character (i.e., covalent plus ionic character as in the Si-O bond).
critical radius In crystal growth and heterogeneous geochemical kinetics theory, the “critical
radius” refers to the smallest size that atoms or ions must come together to produce a stable
nucleus for crystal growth or to allow bubbles to form.
cross striated b-fabric see b-fabric
granostriated b-fabric see b-fabric
critical state a) In physics, the critical state (or critical point) is where a substance has two
physical states) with the same temperature, pressure and volume. For example, it is where a
liquid and gas phase of a substance cannot be differentiated because the volume is the same at a
fixed temperature and pressure. The critical state for water occurs at 374 oC at 218 bars, and the
liquid state cannot be differentiated from steam. b) In soil science, the critical state is where the
shear stress remains at a constant value (i.e., steady) while the shear strain continues to increase,
when a fully saturated clayey soil or sand body is subject to a relatively large shear strain. If the
shearing process is drained, the total volume also remains constant. Syn., constant volume state
crossed polarizers “Polars” or “polarizers” are devices that permit passage of light with
vibrations in only one direction. In an optical “polarizing microscope”, two polarizers (the lower
device is often referred to as the “polarizer” and the upper device is the “analyzer”) are oriented
such that light vibrations are at 90 degrees from each other (and the two polarizers are said to be
“crossed”). If nothing is present in the optical path between the two devices to change the light
vibrations, all light is prevented from passing and the image is black. Material placed in the
optical path changes the behavior of the light rays, with the behavior dependent on the
crystallography of the material (e.g., crystal structure, crystallinity), the chemical composition,
the thickness of the material, and refractive indices. If the analyzer is removed from the optical
path, then the devices are not considered “crossed”. Syn., crossed nicols, crossed polars
crossed nicols (archaic) The original polarizing prisms of petrographic microscopes were the
early 19th century design of William Nicol, and each was referred to as a Nicol prism (Bloss,
1961). When both Nicol prisms were introduced into the optic path in mutually perpendicular
orientations, the examination was said to be using “crossed Nicols”. The phrase “crossed nicols”
is still used for this geometry of polarized light, regardless of whether the polarizing optics use
Nicol prisms or another polarization device. See petrographic microscope, crossed polarizers
crossed polars see petrographic microscope, crossed polarizers, crossed nicols
cryptocrystalline a rock-texture term for which the individual mineral grains in an aggregate are
too small to be distinguished in an ordinary light microscope. If grains can be distinguished in a
light microscope, the texture is “microcrystalline”. The rock is said to have a “compact” texture
if grains cannot be distinguished by the naked eye. Carbonate petrographers use a discrete crystal
dimension for individual carbonate grains for a rock to be classified as having “cryptocrystalline”
texture, although the dimension does not appear to be in universal agreement.
crystal structure the atomic arrangement for a crystalline material
crystal class see point group
crystal an element or chemical compound that is crystalline and shows planar faces that express
this crystallinity. If the solid lacks faces, it is referred to as “anhedral” and if it is completely
bounded by faces, then it is referred to as “euhedral”, and it is “subhedral” if bounded by poorly
defined faces. The term “single crystal” is often used in a colloquial sense for crystalline grains
which are anhedral.
crystal system Crystal systems are defined based on the symmetry of a crystal. There are six
crystal systems, given in decreasing symmetry: cubic (or isometric), hexagonal, tetragonal,
orthorhombic, monoclinic, and triclinic. Minimum symmetry requirements are: four 3-fold or -3
axes (cubic), one 3- or 6-fold axes (hexagonal), one 4-fold axis (tetragonal), three mutually
perpendicular directions with 2-fold and/or mirror plane symmetries (orthorhombic), one 2-fold
axis and/or mirror plane (monoclinic), and center of symmetry or identity operation only
(triclinic). Consequently, because of the symmetry present, the relative lengths of the
crystallographic axes and the values of interaxial angles may be constrained: cubic, a1 = a2 = a3, α
= β = γ = 90 o; hexagonal, a1 = a2 = a3 ≠ c, α, β = 90 o, γ = 120 o, β = 90 o; tetragonal, a1 = a2 ≠ c,
α = β = γ = 90 o; orthorhombic, a ≠ b ≠ c, α = β = γ = 90 o; monoclinic, a ≠ b ≠ c, α = γ = 90 o, β
> 90 o; triclinic, a ≠ b ≠ c, α ≠ β ≠ γ. The term “isometric” is sometimes used as a
morphological term where the measured crystal is equant. Cf., crystallographic axes
crystal growth, driving force In thermodynamics, the driving force (Fd) associated with crystal
growth is: Fd = Dm/kBT = s, where Dm is the change in chemical potential, s is the
supersaturation state, kB is the Boltzmann constant, and T is absolute temperature.
crystalline a solid consisting of atoms, ions, or molecules packed together in a periodic
arrangement. The material must have sufficient atomic ordering such that a (X-ray, electron,
neutron, etc.) diffraction pattern containing well-defined maxima can be indexed using Miller
indices (Nickel, 1995). Cf., non-crystalline
crystallinity index an attempt to describe the state of crystallinity of a solid as a value of some
characteristic, usually relating to diffraction. The term is a misnomer because it suggests that the
complex idea of crystallinity may be represented by a single value. The use of the term
“crystallinity index” should be avoided, although it may be placed within quotation marks when
referring in a limited way to previously referenced work (Guggenheim et al., 2002). Some
indices are useful to describe e.g., crystallite size or grade of diagenesis. Indices were derived by
Hinckley (1963) to distinguish between different samples of kaolinite, by Kübler (1964) to
describe certain origins of samples of illite, and by Árkai (1991) to describe different origins of
chlorite. It is recommended to refer to the author describing the procedures necessary to define
the value, regardless of what the index may actually be describing, such as the Hinckley index.
See Guggenheim et al. (2002) and references therein. Cf., Árkai index, Kübler index, Hinckley
crystallographic axes a set of reference axes used in crystallography. These axes are usually
three in number, although in some cases, they may be four. The axes are generally mutually
perpendicular, coincide with symmetry axes or the normals to symmetry planes, and in cases
where the crystal lacks symmetry, parallel to lines of intersection of two faces with greatest areas.
These axes are designated as a, b, c, and angles between axes are designated α, β, and γ where α
is located between axes b and c, β is located between axes a and c, etc. according to the right-hand rule. See also: crystal system.
CSTR see continuously stirred tank reactor
cubic see crystal system
Curie temperature see ferromagnetism, ferroelectric
cutan A pedofeature involving material (commonly oriented clay coatings, but also gels,
amorphous, etc. coatings) that covers the surfaces of voids, grains, and aggregates, and are
common in paleosols. Cutans may be derived and differentiated as diffusion cutans, illuviation
cutans, and stress cutans. Other cutans may describe compositional variations. Cutans may be
identified in a paleosol or soil under the optical microscope. See pedofeature. argillan an (oriented) clay coating over grains, voids, or aggregates. Compositional
characteristics may be used as prefex modifiers, as “ferri-argilan” (iron oxide
stained) or “organo-argillan” (stained by organic matter). See cutan, pedofeature,
ferran, mangan. Syn., clay skin.
calcan a cutan composed of calcite.
ferran a cutan of iron oxide and hydroxide
mangan a cutan of manganese oxide and hydroxide
organan a cutan composed of organic matter
silan a cutan formed of opal or chalcedony
sesquan a cutan composed of Al-Fe oxides
soluan a cutan of soluble salts (e.g. gypsum)
Debye length see diffuse double layer
deflocculant a substance that disperses particles to form a colloidal or near-colloidal suspension
when added to a slurry that contains clumps of particles that have aggregated to form larger non-fused particles. Common deflocculants for simple (non-exchanged) clay systems include sodium
carbonate or sodium phosphates, such as sodium pyrophosphate. These deflocculants produce a
buffered, high pH solution and are especially useful when size-separating clay particles. Cf.
deflocculate to disperse clumps of small particles adhering together in a suspension to form a
colloid or near-colloid suspension. The resulting suspension is more fluid than the original.
dehydration the removal of H2O from a phase. The H2O may be present as an integral part of
the atomic structure or as “free” (adsorbed) water. Dehydration commonly is achieved by
heating or by evaporation.
dehydroxylate any phase obtained by elimination of the hydroxyl from phyllosilicates prior to
recrystallization. (Quot Guggenheim et al., 2006)
dehydroxylation the removal of (OH)- groups from the atomic structure of a phase.
Decomposition/recrystallization usually results, but there are some materials that form
dehydroxylate structures prior to recrystallization. Cf., dehydroxylate
delamination Delamination is a term used to describe a layer-separation process between the
planar faces of adjacent layers of a particle. Delamination describes a process whereby
intercalation occurs with the introduction of guest material and the stacking of layers remains.
This term differs from exfoliation. Exfoliation implies that the orientation between the layers of
the host structure is lost, and that interlayer cohesive forces are overcome. Where delamination
or exfoliation cannot be distinguished, use “intercalation” or “delamination/exfoliation” to
describe the process. See intercalation, exfoliation, 1:1 layers, 2:1 layers. (From AIPEA
Nomenclature Committee, 2011, unpublished)
density functional theory (DFT) an approximate method of incorporating quantum mechanics
in calculating the electronic structure and related properties of a finite or periodic molecular
model. This method differs from ab initio methods (i.e., a first principles method typically using
a molecular orbital model) in that electron-electron interactions are approximated using
functional mathematics in vector space. Cf., molecular dynamics, quantum calculation
devitrification crystallization from a glass
deposits, eluvial In geology, sedimentary deposits (or eluvium) derived by weathering, either
with or without significant movement by the effects of gravity. In soil science, a soil horizon
developed by the removal of soil material in suspension or solution (leaching) from a layer of a
deviator stress The deviator stress is the difference between the major principal stress (σ1) and
minor principal stress (σ3). The deviator stress usually defines the shear resistance of a clayey soil
under triaxial shear loading.
DFT see density functional theory
diagenesis the chemical, physical, and biological reactions incurred by sediment during burial,
after initial accumulation. Diagenesis reactions may involve addition and removal of material,
transformation by dissolution and recrystallization or replacement (authigenesis), or both, and
phase changes (See Ostwald ripening). Weathering, incurred by sediments at the Earth's surface
under ambient conditions, is not part of the diagenesis process and represents the lower
temperature limit of diagenesis. Hydrothermal, geothermal, and contact metamorphism are not
considered part of the diagenesis process. The lowest grade of metamorphism limits the
diagenesis process at high temperature and high pressure. In clay-rich rocks, the boundary
between diagenesis and very low-grade metamorphism (anchizone is the transitional zone) has a
Kübler index of 0.42 - 0.25 degrees two theta. Reduction of smectite interlayers in illite-smectite
interstratifications to <10% is typical of the diagenetic zone-anchizone transformation (Merriman
and Peacor, 1999). Weaver and Brockstra (1984) proposed a boundary between diagenesis and
metamorphism as that point at which disordered illite (1Md) has been converted to ordered (1M,
3T or 2M1). “Retrograde” diagenesis was described by Nieto et al. (2005) as “fluid-mediated
retrograde processes occurring under diagenetic conditions”. See anchizone, epizone,
interstratification, Ostwald ripening, smectite-illite Cf., Kübler index
diamagnetism describes a property of material where there is repulsion by a magnet. Cf.,
magnetic susceptibility, magnetism
diatomaceous earth a soft, naturally occurring, sedimentary rock, of marine or freshwater origin,
composed of biogenic silica derived from tests (shells) of single-celled algea (diatoms).
Diatomaceous earth has a high melting point (1715 oC) and is used in filters, mild abrasives,
thermal insulators, and absorbants. Syn., diatomite
diatomite see diatomaceous earth
dielectric see insulator
diffuse double layer All surface charges are balanced by oppositely charged counter ions.
Some of these counter ions are bound, usually transiently (see cation exchange), in the so-called
Stern or Helmholtz layer, some of them may diffuse away from the surface and form a diffuse
layer. Both the charged surface and the diffuse ions form the electric double layer, which in total
is uncharged. The thickness of the diffuse layer is solely a function of the ionic strength of the
solution. The distance 1/kappa, where the thickness has decreased to roughly 1/e is called the
Debye (screening) length. The thickness decreases with increasing ionic strength because of the
screening effect of ions. Cf., Stern layer
diffuse layer see diffuse double layer, Stern layer
diffusion, volume the temperature dependent process of mass transfer of matter in solids from
areas of high concentration to low concentration. Volume diffusion commonly occurs along
interstitial channels or grain boundaries, or by vacancy exchange with an atom, ion, or molecule.
diffusion the process of mass transfer of matter from areas of high activity to low activity, or of
energy where heat is transferred from higher temperatures to regions of lower temperature, over
time. Diffusion is generally driven by thermal vibration of the mass.
dilatancy a property of a non-Newtonian fluid where the fluid shows a reduction in viscosity
over time if not agitated (e.g., stirred) and an increase in viscosity when agitated. Cf., Newtonian
fluid, quick clay, thixotropy
dilation, soil or sand In soil science, soil or sand dilation involves the volumetric expansion of
a saturated clayey soil or sand body when subject to drained shearing. For a stiff, highly over
consolidated saturated clayey soil subject to drained shearing, pore water may generate a negative
pressure and water external to the body tends to flow into the soil (or sand), and hence its total
volume increases, indicative of dilation behavior. Cf., contraction, soil or sand;
dipole moment, electrical a measure of the unequal distribution of negative (electrons) and
positive (protons) charge in an atom, molecule, or solid, with units of charge times distance (1
Debye, 1 D = 3.335641*10-30 C.m = 10-10 esu . Å, where esu = electrostatic valency units). A
molecule has a nonzero dipole moment if the individual bond dipole vectors do not cancel (e.g., a
water molecule). Cf., polarization
dioctahedral chlorite a species of the chlorite mineral group with dioctahedral sheets only (e.g.,
donbassite) Cf., trioctahedral chlorite, di,trioctahedral chlorite, dioctahedral sheet
dioctahedral sheet In the ideal case, the smallest structural unit in a phyllosilicate contains
three octahedra. If two such sites are occupied with cations and one site is vacant, then the
octahedral sheet is considered “dioctahedral”. If all three sites are occupied, the sheet is
considered “trioctahedral”. (Quot Guggenheim et al., 2006; see also references therein). A
dioctahedral sheet generally contains predominantly trivalent cations. Cf., trioctahedral sheet
disilicic not a valid term, previously used as a classification of the micas where the number of
silicon atoms per formula unit is two per four tetrahedral sites, see Rieder et al. (1998). Cf.,
mica, true mica, brittle mica, interlayer-deficient mica, group names
dislocation see line defect
dispersed phase see suspension
di,trioctahedral chlorite a species of the chlorite mineral group with a dioctahedral 2:1 layer
and a trioctahedral interlayer (e.g., cookeite, sudoite). Cf., trioctahedral chlorite, dioctahedral
chlorite, dioctahedral sheet, trioctahedral sheet
domain microfabric see microfabric, clay
double metal hydroxides a group name for naturally occurring and synthetic compounds with
the general formula of M2+(1-x)M3+x(OH)2A-x where M2+ is a divalent metal with Mg being the
most common in nature (others include Ni, Cu, Ca Fe), M3+ is a trivalent metal normally Al or Fe
(also Cr), A- is a monovalent anion, and x is commonly near 0.3. The most common anion in
nature is CO32- [SO42-, Cl-, (OH)- also occur]. For divalent anions, the formula is altered to (x/2).
The positively charged portion of the structure is brucite-like, with the anion portion analogous to
the interlayer in the phyllosilicates. The interlayer materials are readily exchangeable and may
include H2O and occasional cations. Syn., layer double hydroxides, LDH, double layer
hydroxides, hydrotalcite-like group, HT, anionic clay, and various similar versions
double metal hydroxide organoclays see double metal hydroxides
double layer hydroxides see double metal hydroxides
drained shearing refers to the process of external loading of a soil (or sand) when a soil body is
subject to external loading (or shearing) and the boundary condition of the soil allows drainage of
porewater. If porewater flows out of the soil, the soil exhibits contractive behavior; otherwise the
soil exhibits dilative behavior. See contraction, soil or sand; dilation, soil or sand
drilling mud fluids (e.g., water, oils, organics) plus dissolved additives and appropriately sized,
suspended solids (e.g., Na-rich bentonite, palygorskite, organoclay, barite, sand) needed to
produce bulk physicochemical and rheological properties appropriate for deep bore holes and for
the removal of “cuttings” generated during the drilling process. For example, one such
physiochemical requirement for drilling mud is a high specific gravity to prevent blowout if high-pressure strata is encountered during drilling at depth.
driving force, crystal growth see crystal growth, driving force
drug The United States Federal Food, Drug, and Cosmetic Act (FD&C Act) of 13 March 2013
defines drugs, in part, by their intended use, as "articles intended for use in the diagnosis, cure,
mitigation, treatment, or prevention of disease" and "articles (other than food) intended to affect
the structure or any function of the body of man or other animals" [FD&C Act, sec. 201(g)(1)].
Clays and clay minerals are used in pharmaceuticals to enhance bioavailability, either as an
excipient or an active principle, or as a way to control the release of the drug (Rodrigues et al,
d spacing see d value
d value As defined in the Bragg equation for diffraction, nl = 2d Sinq, where n is an integer, l is
the wavelength, θ is the glancing angle of incidence, and d is the (perpendicular) spacing
between the diffracting planes. Because d is defined as a spacing, the term d spacing is
redundant. See Bragg equation
earthenware nonvitreous, opaque ceramic whiteware, which may or may not be glazed. Water
adsorption is variously defined as greater than 3% or greater than 5%.
earthy having a dull luster, similar to soil, usually involving an aggregate of fine-grained
edge dislocation see line defect
effective stress, soil In classic soil mechanics, effective stress of a saturated clay body is the
difference between the total stress and the pore water pressure. However, at the microscopic
scale, effective stress must consider the actual stress involving forces transferred through particle
contacts. In the stress range of interest to soil engineers, both water and soil particles are assumed
incompressible, and hence the soil properties and mechanical behavior are controlled by forces
involving inter-particle contacts only.
efflorescence In geology, the weathering process where salt laden ground water is brought to
the surface of a geologic material by evaporation, allowing the dissolved salts (e.g., halite,
gypsum, calcite, natron) to crystallize forming a white/grey, often fluffy powder. Efflorescence
is common in arid climates where rocks or soils of marine origin are exposed at or near the
elastic a descriptive term for tenacity where an applied force deforms a crystal, but the crystal
resumes its original shape after the applied force is released. Cf., flexible, brittle
elastic constants general sets of properties that describe a response of a material to elastic stress.
Elastic properties are described by the modulus of elasticity (Young’s modulus), bulk modulus
(modulus of incompressibility), and the modulus of rigidity (μ). In the elastic regime, the
induced strain is linearly proportional to the stress. Cf., bulk modulus, Young’s modulus.
electrical double layer see diffuse double layer
electric dipole For atoms or molecules, an electric dipole occurs where there is a separation of
positive and negative charge over a short distance. For example, an electric dipole occurs for
H2O where (the bent) molecule forms a small negative charge near the oxygen atom whereas an
equal, but positive charge forms on the opposite side of the molecule associated with the
electrical potential (y) the potential energy per unit of charge; the difference in electrical
potential between two points is the voltage (V).
electrode An electrode is a charged wire or plate. An anode is a positively charged electrode
and a cathode is a negatively charged electrode. Charged particles are attracted to the electrode
of opposite charge.
electronegativity Electronegativity (electron affinity) is the ability for an atom to attract
electrons. The original definition was specific to atoms, but it is often applied to functional
groups, although reference to the dipole moment, polarity, and/or Lewis acid/base character may
be more appropriate to describe the attraction of electrons within a particular molecule or
electroneutrality a constraint imposed on classical calculations or simulations of atomic
structures in which the model system contains no net electric charge.
electron tunneling Electron tunneling is a quantum mechanical property that occurs because
electrons behave as waves of energy and are thus capable of passing through an energy barrier
that would not be possible in classical mechanics. In scanning tunneling microscopy (STM), the
tip and sample wave functions overlap so that when a bias voltage is applied, there is some finite
probability of finding the electron on the other side of the energy barrier. Depending upon the
direction of the bias voltage in STM, an electron may jump the gap or ‘tunnel’ across the barrier
from tip to sample or from sample to tip.
elephantiasis, nonfilarial see podoconiosis
elutriation see air classification
eluvium see deposits, eluvial
embryo In crystal nucleation theory, an incipient and metastable particle with a size smaller than
a critical radius. See critical radius
emulsion see suspension
enantiomorph or enantiomorphic pair see mirror plane
enaulic c/f-related distribution see c/f-related distribution
end member a mineral with a formula that is stoichiometric with respect to the asymmetric unit
of the cell, and therefore reflects a lack of solid solution. See solid solution for an example, also
see asymmetric unit
endothermic see enthalpy
enthalpy Enthalpy, H, is a thermodynamic state function equal to internal energy plus the
product of pressure times volume or H = E + PV, i.e., the heat content of the system. For
example, the change in enthalpy, ΔH, is the heat involved, either released (exothermic) or
absorbed (endothermic), in a system for a constant pressure process (e.g., a reaction at a fixed
pressure). Exothermic reactions have a negative enthalpy change, whereas endothermic reactions
have a positive enthalpy change. See state function, entropy
entropy a thermodynamic state function, which is classically defined as a change, ΔS, in a
system involving reversible heat (qrev) in a process and absolute temperature (T), such that ΔSsys =
qrev/T. A reversible heat process implies an equilibrium process, and thus the change in entropy
allows the determination of the favored direction of a process or reaction. If the total entropy
change is positive from the initial state to the final state, the process spontaneously occurs.
Where an increase in entropy, ΔS, is positive, this indicates a greater degree of motion or
randomness of atoms, molecules, or ions in a system. See enthalpy, Gibbs energy, state function
enzyme a (bio)molecule that serves as a catalyst for a chemical reaction.
epitaxy an oriented crystalline overgrowth forming on a monocrystalline substrate. This
overgrowth occurs when the lattice dimensions within the common plane of the overgrowth and
the substrate are similar. For moderate differences, dimensional variations of the two phases may
still result in a continuous contact. However, for larger differences in the lattices of the two
phases, small particles may form of the overgrowth phase if only limited accommodation occurs.
epizone the beginning of low-grade metamorphism. The epizone is defined by Kübler indices
less than 0.25 in mudrock lithologies. Cf., Kübler index
equipoint or equivalent point symmetry equivalent points, which commonly refers to positions
of symmetrically equivalent atoms in a crystal structure
equivalent spherical diameter Clay particles are often size-separated using Stokes’ law,
although Stokes’ law applies strictly to spherical particles only. Clay particles with platy shapes
settle at slower rates than spheres of equal density in a fluid, however. If Stokes’ law is used in
the size-separation analysis, it is appropriate to describe the platy clay particle as settling at
velocity equal to that of a sphere of “equivalent” diameter (equivalent spherical diameter or esd
or e.s.d.). See Stokes’ Law
equivalent point see equipoint
erosion movement and removal of natural materials by the action of erosive agents, such as
water, ice, and wind. See weathering
esd or e.s.d. see equivalent spherical diameter
essential element (medical/biological) a term broadly defined as a major element (e.g., H, O, C,
N, Na, K, Ca, Mg, P, S, Cl) that plays physiologically important functions in human health.
Essential elements can be derived through diet or through percutaneous migration from peloids.
Cf., essential trace element; see peloid
essential trace element (medical/biological) a term for trace elements (e.g., Li, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, W, Mo, Si, Se, F, I, As, Br, Sn) required for human health and specific to the
function of certain proteins and enzymes. An essential trace element may be derived through
percutaneous migration or through diet. Cf., trace element
euhedral see crystal
eutectic the lowest (minimum temperature) point of a liquid field in a phase diagram. Cf., phase
Ewald sum The Ewald sum is an approximate method used to calculate the long-range portion
of a potential energy term (e.g., electrostatic) in classical simulations where periodic boundary
conditions are used. Energy terms at distances less than a defined cutoff value are calculated
directly in real space. However, for distances greater than this cutoff (extending to infinite
distance), the energy terms are calculated in reciprocal space.
excipient The inactive ingredient in a pharmaceutical product. The excipient, often clay, may be
the vehicle for the active principle or may be added to improve the physical appearance, taste,
smell, or smoothness of a drug or to promote disintegration (time release) of capsules or pills.
Cf., active principle
exfoliation a) In clay science, exfoliation involves a degree of separation of the layers of a host
structure where units, either individual layers or stacking of several layers, are isotropically
dispersed (freely oriented and independent) in a solvent or polymer matrix (Bergaya et al., 2011).
This may be achieved by intercalation, by mechanical means, or by other methods. Where
delamination or exfoliation cannot be distinguished, use “intercalation” or
“delamination/exfoliation” to describe the process. See intercalation, delamination, 1:1 layer, 2:1
layer. (From AIPEA Nomenclature Committee, 2011, unpublished) b) a process of physical
weathering where large granitic plutons break into onion-like sheets along joints that lie parallel
to the exposed surface.
exothermic see enthalpy
expandable clay see swelling clay
extensive property a thermodynamic property that depends upon the amount (e.g., number of
moles) of a substance, for example, internal energy, E. Cf., intensive property
extra-framework see zeolite
extrusion the process of pushing a plastic material through a die to change its shape. Commonly
used in the clay industry to manufacture bricks.
F center defect see point defect
Fahrenheit A temperature scale where the freezing point of pure water at one standard
atmosphere is set to 32 o and the boiling point is set to 212 o. Cf., Celsius, Kelvin
failure loss of cohesiveness
failure envelope To determine how an applied stress can cause failure of a material, such as a
soil mass, a failure envelope is drawn by determining a line of best fit tangent for several Mohr
circles at failure. Thus, the failure envelope is the locus of all shear and normal stress points at
failure for a given material. Soil strength depends upon the effective stress, and the size of the
stress circles at failure varies with the stress level. By varying the stress level (e.g., the minor
principle stress), the failure stress circle changes location and size, and the line that envelopes all
these failure stress circles is the failure envelope. Usually it can be defined base on either
effective stress or total stress, and hence it can be an effective stress envelope or a total stress
envelope. The most widely used linear (Mohr-Coulomb) failure envelope is defined as tau = c
+ sigma[tan (phi)], where phi is the friction angle or the slope of the failure envelope, and c is the
cohesion (soil strength) or the intercept of the failure envelope at the vertical axis (or the shear
stress axis). Syn., Mohr failure envelope, See also Mohr’s circle
Faraday constant the magnitude of electric charge per mole of electrons; has the value of F =
fat clay a qualitative industrial term used to distinguish clays possessing high plasticity (i.e.,
“fat”) or only slight plasticity (i.e., “lean”). The term originates from a soil classification system
(Unified Soil Classification System used in both engineering and geology).
felsic a geologic term describing magmas, igneous rocks and silicate minerals that have a
relatively high concentration of lighter elements such as oxygen, sodium, aluminum, silicon and
potassium. Felsic magmas typically are highly viscous when compared to mafic magmas. Felsic
rocks are typically light colored when compared to mafic rocks Cf., mafic
ferran see cutan
ferrimagnetism a property of material where there is an antiparallel alignment of magnetic
moments but these alignments are unbalanced either because one direction has stronger moments
or a larger number of moments. Inverse spinel structures (“ferrites”) are often ferrimagnetic,
where magnetic moments of ferric iron cations in the tetrahedral sites and one-half the cations in
the octahedral sites (also ferric iron) are opposed and cancel, whereas the remaining occupied
octahedral sites can have either balanced (e.g., Mg, Cd, Zn, zero unpaired electrons) or
unbalanced (e.g., Mn, Fe2+, Co, Ni, Cu) magnetic moments, approximately proportional to the
number of unpaired electrons.
ferrites, see ferrimagnetic
ferroelectric describes a property of materials whereby an electric dipole moment remains even
after a strong electric field is removed. The electric dipole moment (polarity) results from
electronic polarization of individual atoms or ions, and/or a reorientation of polarizable
molecules in the crystal. Dipole moments in ionic crystals can also form from initial cation
positional disorder within a site from unit cell to unit cell; domains (and a dipole) develop when
an applied electric field causes a favored positioning of cations within the site cavities. Examples
of ferroelectric materials include KDP (potassium dihydrogen phosphate), colemanite, and
barium titanate. Dipoles are randomized at the “Curie temperature”.
ferromagnetism describes a property of material where permanent magnetism develops when a
magnetic field is applied. Ferromagnetism results by a parallel alignment of magnetic moments
which remains after the material is removed from the applied magnetic field. The phenomenon
occurs with iron, and hence the name, but other elements (in general, elements between atomic
numbers of 23 and 29) and alloys (e.g., MnBi) also show the effect. Upon heating, the effect is
diminished, and when heated to the “Curie temperature”, the magnetic effects are fully removed.
fibrosis see pneumoconiosis
fibrous a crystal habit describing thin, flexible filaments
fill In geotechnical engineering, fill is geological material in containment formed by human
activities, such as dumping, compaction, and hydraulic deposition (via pumping).
fire clay a kaolinitic-rich clay with excellent refractory properties, and the fired product is
commonly used as fire bricks. The refractory clays are classed as low, medium, high, and super
duty with the latter heat resistant from 1430 to 1804 °C. Fire clays are low in alkali, alkali earth,
and transition metals. Typically found in association with coal layers. See refractory material.
Cf., underclay, seat rock
firing heat treatment of clay materials that cause partial melting and fusion to create ceramics
fissile the property of breaking or cleaving into flake-shaped, nearly planar paper-thin fragments.
Fissility reflects fabric and texture such as the parallel alignment of clay minerals and
phyllosilicates and fine-grain size laminations. See lamination.
fissility see fissile
flame retardancy Flame retardancy is the property of an additive that lowers the flammability of
a material. Flame retardancy is measured by several standard testing methods, with the most
common test being the UL94 test. However, more information is obtained by utilizing a cone
calorimeter. Clay/polymer nanocomposites lower the flammability of the polymer significantly
in both types of flammability tests. Clay layer-by-layer composites have been shown to greatly
lower the flammability of textiles. The flammability of materials for construction and clothing is
of particular interest for the interiors of airplanes, commercial and residential construction, home
furnishings, children sleepware, and clothing for industrial workers. Cf., layer-by-layer
flexible a descriptive term for tenacity where a mineral may bend without breaking, but does not
return to its original shape after the force is released. Cf., elastic
flint clay a fine-particle size, non-plastic, dense, brittle kaolinitic clay layer or deposit, that
breaks with a conchoidal fracture. Typically found in association with coal layers. Cf. underclay,
flocculation In the original, flocculation refers to the destabilization of a suspension by the
clumping of small particles to larger ones, but without fusion, owing to small amounts of
polymers (e.g., see Lyklema, 1991). Common usage is no longer restricted to polymers, and may
involve changes of the electrolyte. However, the mechanisms of destabilization differ where
polymers vs electrolytes are considered: polymers are adsorbed and form bridges between
particles, whereas electrolyte changes involve ionic strength variations that reduce repulsive
forces between particles. Both processes produce clumping of small particles in a suspension to
produce larger particles. The total surface area does not essentially change. Syn., aggregation
floccule A cluster of loosely bound particles in a suspension that combine to produce a larger
particle. In clay mineralogy, a floccule, or “floc”, is comprised of very fine-grained clay minerals
in association with fine silt and organic debris, held together by electrostatic forces or organic
sheaths (after Potter et al., 2005). See flocculation.
flow quick-clay landslide see quick-clay landslide
flux a) In metallurgy, a material which chemically cleans a metal surface to prepare it for
welding, brazing or soldering. b) In ceramics, a material which lowers the meting point of
ceramic materials to facilitate glass formation. c) In physics, the rate of transfer of heat, mass,
magnetism, etc. that passes a unit area per unit time. Cf., fusion
fluxing melting of a substance
fluxional bond a dynamic bond where atoms exchange between symmetry-related sites. In cases
where the configurations are non-equivalent, the result is an isomer or tautomer, whereas a
fluxional molecule involves chemically equivalent configurations.
fly ash fine particulate, airborne, typically amorphous, siliceous residue from burning coal in
industrial burners. The chemistry of the coal and the type/chemistry of the fly ash collection
system determines the composition of the fly ash. Pozzolanic (cementitious) fly ash is
commonly used as an additive in cement. Non-pozzolanic fly ash is often used as a filler in
wood and plastic products, in asphaltic concrete, in roofing tiles, and in other composite
manufactured materials. Fly ash commonly contains a variety of heavy metals that were present
in trace concentration in the unburned coal. Cf., bottom ash
foam see suspension
fog see suspension
force field A force field is derived from a set of parameters determined from an approximate
energy expression and then used to calculate interatomic or intermolecular energies in a classical
calculation or simulation of an atomic structure. Most force field methods include pairwise
interatomic interactions (e.g., van der Waals, electrostatic), and some include intramolecular
interactions (e.g., bond stretch, angle bend) present in polyatomic species. Interaction parameters
are adjusted so that results obtained from the force field, such as structural, mechanical, and
spectroscopic properties, match those from experiment or quantum mechanical calculations as
closely as possible.
form a) in mineralogy, a crystal shape that is an expression of the ordered pattern of the atomic
structure. The crystal form is commonly a regular geometric shape. b) in crystallography, a form
consists of a group of symmetry-related crystal faces.
formality the number of gram formula weights (= molecular weights) of the solute in one liter
of solution. Useful where experiments use measured volumes and where temperature effects are
not being studied. Cf., molarity, normality, molality, mole fraction
free-swell test A free-swell test is a process- or quality-control test to measure the compatibility
of a clay with various solvents by placing a specified amount of clay in the fluid of interest,
allowing it to remain undisturbed, and by determining the amount of swelling after a specified
time. A graduated cylinder is often used to judge the amount of swelling after one or two hours,
and 1 gram of clay is typically used, although each manufacturer/supplier follows their own
protocols. The test is relative to a clay with no swelling behavior, as there are no standard-swelling behaviors. The test is especially useful to determine the optimum organoclay for paint
or grease formulations.
freeze drying a dehydration process where material is dried via sublimation of contained water.
The material is frozen in a vessel, subjected to gas-pressure reduction often followed by slight
heating, and dried by removal of sublimated water vapor. In clay mineralogy, samples of clay are
dispersed in water as a suspension prior to freeze drying. The residual clay often shows the
aggregate structure and fabric of the dispersed phase.
Frenkel defect see point defect
friable the characteristic of a solid material that allows its particle size to be easily reduced
fugacity see activity
fuller's earth a clay or other earthy material of fine particle size that is highly absorbent and/or is
a natural bleaching earth. The term is derived from the historical use of certain clays to “full” or
remove oils and other contaminants from raw woolen cloth.
fulvic acid Fulvic acids occur as a result of the decay of plants and microorganisms in some
soils, sediments, peats, coals, shales, and surface waters. Unlike humic acids, fulvic acids are
soluble in water over a large range of pH values (and this is the operational distinction between
humic vs fulvic acids). Along with humic acid, fulvic acid may be extracted from solids using a
strong base, such as NaOH or KOH. Fulvic acid may actually originate from hydrolysis of large
molecules by the strong base during extraction. However, at pH 1, fulvic acid remains in solution
but humic acid will precipitate. Cf., humic acid, humification, humic substances, humin
fundamental particle a sequence of 2:1 dioctahedral or trioctahedral layers which are coherently
related by rotations of multiples of 60o. Such particles, as observed in sample separates, are
assumed to be identical to the individual growth units in unseparated samples. Some data have
shown that coherent scattering domains undergo separation during sample disaggregation, and
that particles that result are not fundamental, but many investigators assume that there is a one-to-one relation between fundamental particles in separates and coherent scattering domains in
unseparated soil or rock samples. Cf., MacEwan crystallite, interparticle diffraction, coherent
fusion the unification of two or more particles such that the substances between the two particles
is of the same material as the particles. Fusion often refers to the melting together of two
substances. In clay science, aggregates may form without fusion with bridges forming between
particles by H2O or by polymers, such that they behave as an apparently larger particle. Cf.,
gallery see pillared clay
gangue In mining, the non-economic host rock in which valuable minerals (usually
metalliferous ore minerals) are found. When the gangue is discarded, this material is referred to
gastrolith a small mineral concretion in the stomach. Some gastroliths may form if excess
(ingested) clay cannot be eliminated by the digestive system. Some species, such as birds,
reptiles, or dinosaurs, swallow small stones (gastroliths) that are believed to aid in digestion.
These gastroliths are generally not concretions and are rounded and polished owing to the
abrasive action within the stomach.
GCMC see Grand Canonical Monte Carlo
gefuric c/f-related distribution see c/f-related distribution
gel strength A measure of the ability of a colloidal system (i.e., a gel or sol) to withstand a load
(shear force) without loss of the high slurry viscosity and/or the mechanical elasticity, often
measured with a rheometer and expressed in units of mass per area. The gel strength is a
function of the inter-particle forces present in the solid-liquid system. Cf., colloid, suspension,
gel see suspension
gelling clay an industrial term for clay minerals (especially palygorskite, sepiolite, Na- or Na-exchanged montmorillonite, hectorite, and organoclay) having a high slurry viscosity at low
percent solids when mixed with fluid. Such clay minerals form a colloid where the dispersed
phase and the dispersion medium produce a semisolid material, similar to jelly. Industrial
applications include thickening/suspension agents, adhesives, sealants, putties and glazing
compounds. Jelling viscosity is typically measured on Fann or Brookfield viscometers. For
aqueous applications and some fibrous clay minerals such as palygorskite, the gelling behavior
and viscosity is enhanced by adding magnesium oxide or by extruding the slurry to align the
needle-shaped crystallites. For montmorillonite, gelling behavior can be enhanced by soda ash
treatments to increase the exchangeable Na-cation content and the swelling capacity of the clay
mineral. For solvent-based applications, various smectites can be surface modified by
quaternary-amine compounds to form organoclays that display superb gelling characteristics.
geophagy the deliberate ingestion of (specifically) soil/clay; a form of pica. Cf., pica
geotherapy commercialized therapies using geologic materials, such as peloids, or processes,
e.g., hot springs or mud baths. Cf., peloid, pelotherapy
Gibbs free energy See Gibbs energy
Gibbs energy a thermodynamic state property useful to determine the spontaneity of a reaction
within a system (without regard to the surroundings as is the case with entropy changes) and the
direction of the reaction. The change in Gibbs energy, ΔG, is equal to ΔH - TΔS, where ΔH is
the change in enthalpy (cal/mole), T is the absolute temperature (K), and ΔS is the change in
entropy (cal deg-1 mole-1). A substance reacts if the change in Gibbs energy is negative going
from the initial state to the final state. Gibbs energy is often referred to as Gibbs free energy.
See enthalpy, entropy, state function
glaebule a pedofeature forming segregated lumps of material with diverse composition (similar
to cutans) as part of a soil groundmass. The non-planar shapes and more distinct outlines
differentiate glaebules from cutans. Three common glaebules include mottles, nodules, and
nodule a glaebule that is irregular to nearly spherical, with a massive internal structure.
concretion a nodule-shaped glaebule showing an internal structure of concentric layers.
mottle poorly differentiated glaebules occurring as diffuse patches in the groundmass.
glass a solid with a degree of order intermediate to the highly ordered arrangement of atoms,
molecules, or ions in a “crystalline” solid and the highly disordered arrangement as found in a
“gas”. Most glasses are in a metastable state and can be described as a supercooled liquid, which
lack a melting point. Rapid cooling from a molten state (e.g., magma, lava) may result in a glass
and this commonly depends on the volatile content of the melt. Not all glasses are formed from a
melt. Although most opal forms from silica-saturated fluid under near-surface conditions, a rare
“non-crystalline opal” (e.g., opal-AN) forms by transport of silica via steam to cold surfaces.
glaze a glassy coating produced on a ceramic by firing or the slurry used to make the coating
glide plane symmetry involving reflection across a plane coupled with a translation of a/2, b/2 or
c/2 parallel to an axis (a-glide, b-glide, or c-glide, respectively) or between two axes (n-glide
with translations of a/2 + b/2, b/2 + c/2, or a/2 + c/2; d-glide or diamond glide with translations
of a/4 + b/4, b/4 + c/4, or a/4 + c/4) or between three axes for tetragonal or isometric crystals
with a translation component of a/4 + b/4 + c/4.
Grand Canonical Monte Carlo simulation (GCMC) a Monte Carlo molecular simulation that
is based on the grand canonical thermodynamic ensemble, an ensemble (e.g., a system of
particles) in statistical mechanics that describes the possible states of the particles in equilibrium.
The model system properties of chemical potential, volume, and temperature are held constant,
but the number of particles in the system (e.g., interlayer water molecules) is allowed to vary as
equilibrium is achieved. In this context, equilibrium implies both potential energy and system
density (number of particles). Monte Carlo moves are accepted based on the energy change of the
move according to a Boltzmann probability (so that some moves resulting in higher energy are
accepted). Cf., force field, molecular dynamics, Monte Carlo molecular simulation
green sand strength see green strength
green strength a) In clayware manufacture, the ability of an unfired, molded clay body to resist
mechanical deformation. b) In metal casting, a measure of the ability of a bentonite-bound sand
mold to resist deformation (also called “green sand strength”).
greenware an unfired, molded clay body
grit a qualitative term in the clay-mining industry that refers to small, hard accessory minerals
occurring in the bulk clay deposit, such as quartz, feldspar, rutile, ilmenite, and apatite, which
imparts an undesirable “abrasive” character to the bulk clay.
groundwater water existing underground in voids or pore spaces in rock or sediment
group names Phyllosilicates are classified on the basis of characteristics involving planar
structures, non-planar structures and regular interstratifications (e.g., Guggenheim et al., 2006).
For planar structures and regular interstratifications, the layer type (e.g., 1:1, 2:1) is further
divided by interlayer material present that is required to offset the net negative charge on the
layer, and each division is given a group name. In addition, each group has a generally
characteristic spacing [based on the d(001)] perpendicular to the stacking direction, i.e., csinβ.
The group names (x ~ layer charge per formula unit) for the planar structures (interstratifications
are not given here) are: serpentine-kaolin (x ~ 0, csinβ ~ 7.1-7.3 Å), talc-pyrophyllite (x ~ 0,
csinβ ~ 9.1-9.4 Å), smectite (x ~ -0.2 to -0.6, csinβ ~ 14.4-15.6 Å), vermiculite (x ~ -0.6 to -0.9,
csinβ ~ 14.4-15.6 Å), true mica (x ~ -1.0, csinβ ~ 9.6-10.1 Å), brittle mica (x ~ -2.0, csinβ ~ 9.6-10.1 Å), interlayer-deficient mica (x ~ -0.6 to -0.85, csinβ ~ 9.6-10.1 Å), and chlorite (x ~
variable, csinβ ~ 14.0-14.4 Å). Groups are further divided into subgroups (e.g., serpentine,
kaolin, talc, pyrophyllite, trioctahedral smectite, dioctahedral smectite, etc.) by mineral species
based on the octahedral character (i.e., dioctahedral, trioctahedral) and subgroups are divided
based on chemical composition to mineral species. Bailey (1980) designated the trioctahedral
smectite subgroup as saponite and the dioctahedral smectite subgroup as montmorillonite.
H2O - see water, adsorbed
H2O+ see water, structural
habit Habit refers to the general shape of mineral particles, as in the “platy” habit of clay
particles, inclusive of shapes that reflect the internal arrangement of atoms as well as crystal
growth features. Examples of habits include acicular, lamellar, and equant.
Hamaker constant The Hamaker constant, A, describes the attractive force between two solid
surfaces due to temporary dipole forces (van der Waals interactions). For ideal planar surfaces
separated by length L, the van der Waals potential energy is proportional to A/L2.
hardness In mineralogy, hardness is the resistence to scratching. Mohs’ scale of hardness is
generally used to obtain relative hardness information by comparing (i.e., by a scratch test) an
unknown to one of ten minerals defined in the Mohs’ hardness scale. Hardness tests are
performed on a smooth surface of the unknown. Hardness is not an isotropic property and thus,
may depend on the direction in which the surface is scratched, although the differences in most
minerals are small. Cf., Mohs hardness, Vickers hardness
half life the time, t1/2, required for the number of radioactive atoms in a system to decay and thus
decrease to half of the original number.
healing clay clay used therapeutically, for example, to alleviate pain, absorb toxins, reduce
swelling, or protect skin. Common healing clays are montmorillonite (in bentonite),
palygorskite/sepiolite, and kaolin. Cf., active principle, aesthetic medicine, antibacterial clay,
geotherapy, medicinal clay, pelotherapy
heat capacity a measure of the heat energy required to raise the temperature of a substance by 1
oC. For example, the specific heat energy or capacity is the amount of heat needed to raise 1
gram of a substance by 1 oC. The heat capacity is, therefore, equal to the mass of the substance
times the specific heat. Thus, the molar heat capacity is the heat capacity per mole.
heat of fusion the energy (often heat) required to transform a given quantity of a solid to a melt
without changing temperature. Cf., heat of sublimation, heat of vaporization
heat of sublimation the energy (often heat) required to directly transform a given quantity of a
solid to a gas without changing temperature (or going through the liquid state). The heat of
sublimation is the sum of the heat of fusion and heat of vaporization at the given temperature.
Cf., heat of vaporization, heat of fusion
heat of vaporization the energy (often heat) required to transform a given quantity of a liquid
into a gas or vapor without changing temperature. Cf., heat of sublimation, heat of fusion
Helmholtz plane see Stern layer
hemostatic wound dressing Hemostatic wound dressings are used to control bleeding when
tissue damage occurs. One active ingredient applied either directly or with gauze bandages to a
wound is clay (commonly, kaolins, zeolites, and smectites), which is used to absorb fluid and to
promote blood clotting.
heterogeneous nucleation precipitation from solution of one solid phase on another.
heterogeneous reaction a reaction that involves more than one physical state (i.e., solid, liquid,
gas). Cf., homogeneous reaction
hexadecyltrimethylammonium organoclay expandable 2:1 clays (e.g. smectite, vermiculite)
treated with solutions of the quaternary alkylammonium salt, hexadecyltrimethylammonium
(HDTMA) bromide (also called cetyltrimethylammonium bromide), form organophilic-type
organoclay. HDTMA organoclay can effectively remove organic contaminants, such as toluene,
from water; see Boyd et al. (1988). Cf., adsorptive organoclays, organoclay, organophilic-type
hexagonal see crystal system
high-activity clays In geotechnical engineering, high-activity clays have activities that range
from about >1 to 7+, with smectitic clays (i.e., smectite or mixed-layer clays with a smectite-like
component) being the most common having activities near 3 (where Ca-saturated, or in high
salinity environments) to 7+ (where Na-dominated and at low salinity). See activity, clay; low-activity clays; quick clays
Hinckley index Hinckley (1963) attempted to define the “crystallinity” of kaolinite by
describing changes in the powder X-ray diffraction pattern for various samples of kaolinite. The
procedure, however, does not quantify the diversity of defects present and, if the procedure is
used, it should not be characterized as a “crystallinity” index. See also, Plancon and Zacharie
(1990); Guggenheim et al. (2002). Cf., crystalline, crystallinity index, Kübler index, Arkai index
homogeneous nucleation direct precipitation of a solid phase within a solution and not on the
surface of another phase.
homogeneous reaction a reaction that involves only one physical state (i.e., solid, liquid, gas).
Cf., heterogeneous reaction
honeycomb microfabric see microfabric, clay
hormite a mining term, now obsolete and not used in the geologic literature, that refers to the
palygorskite-sepiolite group (Brindley and Pedro, 1970; Bailey et al., 1971a).
HT see double metal hydroxides
humic acid Humic acids are chemically diverse acids that result from the decay of plants and
microorganisms in some soils, sediments, peats, coals, shales, and surface waters. These acids
are insoluble in water at low pH, can be precipitated at a pH = 1, and extracted from solids using
a strong base, such as NaOH or KOH. cf., fulvic acid, humic substances, humification, humin
humic substances the general term that includes (large-molecule) humic acid, fulvic acid, and
humin, all of which form the natural organic matter (NOM) of some soils, sediments, peats,
coals, and shales. Non-humic materials, such as amino acids, peptides, and sugars, may also be
present in some soils as NOM. Humic substances impart the brown or black color to some soils
and surface waters as well as biological productivity. Humic substances do not further
biodegrade readily. Humic substances are quite heterogeneous and complex, and studies often
provide average properties and chemical compositions. However, Lehmann and Kleber (2015)
have suggested that large-molecule humic substances do not actually exist in soils, and NOM
form from a progressive breakdown of organic compounds. cf., fulvic acid, humic acid,
humidity amount of water vapor contained in the atmosphere. “Relative humidity” is the ratio,
usually expressed as a percent, of the amount of water vapor in the atmosphere to the maximum
water vapor in the atmosphere possible at a specific temperature. “Absolute humidity” is the
mass of water vapor per unit of dry air. When unspecified, reference is usually being made to
humification a process involving the decay of plant and microorganism matter to produce the
natural organic matter (NOM) as found in some soils, sediments, peats, coals, and shales. Plant
lignin, related transformation phases, polysaccharides, melanin, cutin, proteins, lipids, nucleic
acids, and fine char particles, are materials involved in humification. cf., fulvic acid, humic acid,
humic substances, humin
humin Humin occurs as a result of the decay of plants and microorganisms in some soils,
sediments, peats, coals, and shales, but not aquatic waters (humin is not soluble in water).
Humin may be formed also in the dehydration of some sugars. Humin, along with fulvic acid
and humic acid, is a humic substance. However, humin cannot be extracted from humic
substances with either a strong base or a strong acid, whereas fulvic or humic acids can be
extracted. cf., fulvic acid, humic acid, humic substances, humification
hydraulic conductivity In geotechnical engineering, hydraulic conductivity refers to the ease of
fluid flow (for example, permeability) through sediment, rock or soil, and is affected by the
viscosity of the fluid (water). Cf., permeability
hydraulic gradient In an aquifer, the ratio of total hydraulic head loss (or difference) to the
length of flow path in a given direction, or in a simple description, the slope of the surface of the
hydraulic head The hydraulic head is the total energy of moving water (including porewater or
groundwater), usually represented by the height of water column with a unit of length. The total
hydraulic head is the summation of the velocity head (kinetic energy), pressure head (potential
energy), and elevation head (potential energy), all converted to the height of the equivalent water
column: h = hv + hp + hE = v2/2g + u/γw + z, where h is the total hydraulic head, and subscripts V,
P, and E denote velocity, pressure, and elevation; v is the velocity; g gravity, u water/fluid
pressure, γw unit weight of water at 25 oC, and z elevation.
hydrogen bond The hydrogen (H) atom in solids is often asymmetrically located between an
electronegative donor atom (D) and an electronegative acceptor (A) atom, such that the D-H
distance is smaller than the H-A distance. The H atom can penetrate the electron cloud of the
donor atom. The D-H--A configuration may be linear or bent, and there may be multiple A
atoms. The overall bond is considered essentially ionic in character, with an overall bond
strength (for D-H--A configurations) considerably weaker than a (sigma) covalent bond but
considerably stronger than van der Waals interactions. The H atom may serve to balance overall
charge, for example as an O-H in oxygen-based structures, in which case the H does not link a
donor to an acceptor atom.
hydrolysis a) occurs when a molecule reacts with water to break a chemical bond, and thereby
often incorporating H2O into the substance. b) Hydrolysis at a surface involves the breakdown
of the H2O molecule into hydroxides and protons to form protonated/deprotonated surface sites.
hydrometer a device used to measure the specific gravity of a fluid, usually containing
suspended particles. In soil analysis, a hydrometer is used to measure the particle size distribution
of fine-grained (< 75 mm) soils such as clays, on the basis of the Stokes’ Law: the settling
velocity of suspended particles depends upon the particle size. With a time sequence of
measurements of the suspension density, the quantity of particles settling out of the suspension
can be estimated, and hence the size – quantity relationship (or particle size distribution) of
different particles can be obtained.
hydrophilic surfaces or molecules with strong attraction for H2O. Hydrophilic solutes are
polarizable and tend to dissolve more readily in water than in oil. Cf., hydrophobic
hydrophobic hydrophobic materials, either surfaces or molecules, have little or no affinity for
H2O. Cf., hydrophilic
hydrotalcite-like group see double metal hydroxides
hypo-coating In micromorphology of soils, hypo-coating is a pedofeature that consists of a layer
of material that is related and immediately adjacent to the surface of, for example, voids, grains,
or aggregates. Syn., neo-cutan; Cf., coating, cutan, quasi-coating
identipoint an identical point (a point whose environment is exactly the same as the
environment around each other point) placed on an array. A set of identipoints within an atomic
structure shows the scheme of repetition, or translational periodicity, of an array. Cf., array,
structure, Bravais lattice
iep see point of zero charge
improper rotation axis see rotoinversion axis
impurity defect see point defect
incongruent dissolution dissolution where the release of constituents from the dissolving phase
into the solution does not reflect the stoichiometry of the original phase. Cf., congruent melting,
congruent dissolution, incongruent melting
incongruent melting a reaction where the solid phase reacts to form a mixture of liquid plus
crystals, with neither having the composition of the original solid. Cf., congruent melting,
indurate hardening of a rock or sediments by the effects of temperature, pressure, cementation,
inheritance a) the phenomenon by which minerals are formed in another environment in space
or time from that where they are now found. b) The term is also used when some element of a
pre-existing mineral structure is inherited by another mineral via the phenomenon of
transformation. See also neoformation, transformation
inner sphere complexes Inner sphere complexes are ions, which adsorb in the inner Helmholtz
plane. There is spectroscopic evidence that these ions come so close to the surface that, e.g.,
water molecules from a hydration shell have to be removed from the contact plane. Since
adsorption of ions on an increasingly charged surface requires energy from bonding, one speaks
of specific adsorption.
insulator, electrical materials that are poor conductors of electricity. Syn., dielectric
intensive property a thermodynamic property that is independent of the amount of a substance,
such as the property of heat capacity. Cf., extensive property
intracrystalline swelling Intracrystalline swelling involves an increase in the layer-to-layer
spacing of a phyllosilicate mineral to accommodate H2O or other molecules in the interlayer. For
example, in swelling clays, an increase in the relative humidity around the clay mineral particle,
causes interlayer expansion, resulting in partially or fully hydrated interlayer ions. Cf., osmotic
ion Atoms or molecules become ions by the gain or loss of electrons, which have a negative
charge. A positively charged ion, or cation, involves a loss of one or more electron(s). A
negatively charged ion, or anion, involves a gain of one or more electron(s).
intercalation Intercalation is a general term to describe the movement of atoms, ions or
molecules into a layered host structure, often a swelling clay mineral. This process can be
reversible or non-reversible. The host-structure layers are essentially unchanged with the inserted
material going between the layers. The layers must remain semi-contiguous via stacking.
Intercalation commonly involves cation exchange or solvation reactions. Intercalation may
involve, for example, H2O molecules or surfactants of single planes (monolayers) to paraffin-type
layers between the layers of the host phase. The resulting structure is an “intercalated structure”.
See delamination, exfoliation. (From AIPEA Nomenclature Committee, 2011, unpublished)
interference colors In optical crystallography, an interference color results with crossed
polarizers where light enters an appropriately crystalline medium and refracts (separates into two
ray fronts); thus, each wave front travels at slightly different velocities with a change in both
speed and direction. Upon leaving the medium, the wave fronts interfere (recombine) and
produces a component of light where there is a difference, or retardation, between the two wave
fronts. This difference results in a change in wavelength in the final wave front, which produces
a change in color, called an interference color.
interlayer A general term that implies either the region between the two adjacent layers or the
relation between the two adjacent layers (quot Guggenheim et al., 2009). “Interlayer distance” is
more precise to describe the distance between the adjacent layers (tetrahedral sheet to tetrahedral
sheet, as shown in Fig. 2), and is measured by taking the average of the z coordinate of the basal
oxygen plane. The “interlayer displacement” describes the displacement portion or lateral shift
from tetrahedral sheet to tetrahedral sheet across the interlayer space. Cf., layer, layer
Figure 2. Illustration of terms used to describe interlayer, layers,
and intralayer topologies. From Guggenheim et al. (2009).
interlayer shift see interlayer
interlayer distance see interlayer
interlayer material For phyllosilicates, interlayer material separates the 1:1 or 2:1 layers and
generally may consist of cations, hydrated cations, organic material, hydroxide octahedra, and/or
hydroxide octahedral sheets (see fig. 2). The interlayer material offsets the net negative charge of
the layer. In certain cases (e.g., talc, pyrophyllite, where the net layer charge is zero), there is no
interlayer material, and an interlayer separating the layers is empty. After Guggenheim et al.
(2006). Cf., layer
interlayer displacement see interlayer
interlayer-deficient mica abbreviated from interlayer-cation-deficient mica (Rieder et al.,
1998). A group name for platy phyllosilicates of 2:1 layer and a layer charge of between -0.6 to
-0.85 per formula unit. Interlayer-deficient micas do not have swelling capacity. The subgroups
of the brittle micas are based on dioctahedral or trioctahedral character (wonesite is the only
known member of the trioctahedral subgroup), and species within the subgroups are based on
chemical composition. The value of -0.6 is a general limit, as wonesite appears to be an
exception with a layer charge of -0.5, although it has no swelling capacity. The value of -0.85
has been fixed from examples of dioctahedral species only. Previous to Rieder et al. (1998), the
term “hydromica” was used to express a perceived excess in H2O above the O10(OH)2 formula
unit, but these phases are either interstratifications (e.g., hydrobiotite = interstratification of
biotite and vermiculite) or micas that exhibit a deficiency in interlayer cation occupancy. Thus,
the term “hydromica” was abandoned in favor of interlayer-deficient mica. The term for species
“hydrobiotite” was confusing, and the use of the prefix “hydro” is now avoided. Cf., mica, true
mica, brittle mica, group names
interparticle diffraction a concept introduced by Nadeau et al. (1984) which theorizes that
individual clay mineral particles of oriented samples used in X-ray diffraction studies are stacked
in aggregated composite particles which coherently scatter radiation so as to give rise to 00l
diffraction patterns similar to those of uncleaved McEwan crystallites. The composite particles
are theorized to be produced during sample preparation following disaggregation of original rock
or soil samples.
interstratification a clay material where two or more kinds of layers are interleaved in a
coherently diffracting structural domain with a degree of ordering which may vary from random
to completely ordered. Layers may be 1:1 layers or 2:1 layers and interlayers may be of swelling
or non-swelling nature. X-ray diffraction and transmission electron diffraction (TEM) are two
common techniques used to determine layer proportions and regularity of interleaving. Energy
dispersive techniques are commonly used to obtain chemical information. Regular
interstratifications may be designated as mineral species, provided that they conform to specific
criteria as specified by the International Mineralogical Association. Non-regular
interstratifications are commonly characterized according to the information available, e.g., mica-smectite irregular interstratification, and do not warrant species status.
intralayer displacement see layer displacement
intrinsic stability constant a stability constant, K, for a complex is determined from the activity
of the complex in solution divided by the activity of the reactants in a system at equilibrium.
The larger the value of K, the greater is the stability of the complex. For multiple complexes that
may result in a reaction, several constants may be determined, thus Koverall is the product of
multiple constants: K1 x K2 x K3... >Intrinsic= is used because the stability constant is an essential
physical chemical parameter that relates concentrations of the components of a reaction at
intumesence an irreversible expansion of a solid upon heating. Vermiculite commonly shows
expansion upon rapid heating (intumesence) owing to the loss of interlayer H2O at relatively low
temperatures and the separation of the layers. Slow heating of vermiculite may produce worm-like threads, and it is this characteristic that gives the vermiculite group its name: vermiculari, to
breed worms, in Latin.
ion exchange see cation exchange
ion-dipole interaction a fluxional bond formed between an ion and a polar molecule. The classic
example is where water molecules hydrate a sodium cation. However, the interaction is not
restricted to H2O, as many polar organic molecules will form ion-dipole interactions. See
ionic potential the ratio (Ip) of charge (z) on an ion to the radius (r) of the ion, IP = z/r.
isoelectric point (iep) see point of zero charge
isometric see crystal system
isomorphic see isomorphous
isomorphous Isomorphous literally means “having the same form”. The idea was first
described by F. Beudant around 1800 (see Zoltai and Stout, 1984, p. 5-6) to describe crystals
having the same form but having compositions between FeSO4 and ZnSO4. Thus, the term
subsequently became commonly used to describe a series of crystals having continuously
varying composition even where crystal faces are lacking. Such homogeneous chemical mixtures
display a continuous range of properties, e.g., from the Fe to Zn end members. Such a series of
solids is referred to as an “isomorphous substitution” series (archaic) or “solid solution” series.
See “solid solution” for additional discussion. Isomorphic (syn)
isotropy a description of material with physical properties that are the same regardless of
direction of measurement. Cf., anisotropy
K0 condition see at-rest condition
K-bentonite see bentonite
kandite discredited term, use kaolin-serpentine group (Bailey, 1980)
kaolin, soft see kaolin, hard
kaolin, hard a white to gray clayey-textured rock predominantly composed of kaolin group
minerals (primarily kaolinite). Hard kaolin is fine grained, difficult to break, and commonly with
sharp, protruding (jagged) edges. Hard kaolin requires more complex mine extraction and
mineral processing techniques than “soft kaolin” (kaolin-rich rock that is smooth to the touch,
weak, and friable).
kaolin a) Petrologic term: rock composed primarily of kaolinite, nacrite, dickite, or halloysite
(i.e., minerals of the kaolin group). In most case, the identification of the specific species is
unknown. The rock is commonly white, earthy, and soft. b) Mineralogic term: a sub-group name
(within the group “serpentine-kaolin”) for those phyllosilicates that are dioctahedral, with 1:1
layers, and with a net layer charge of approximately 0.0. Species of this sub-group include
kaolinite, nacrite, dickite and halloysite. Previously, the group name was “serpentine-kaolinite”,
and the subgroup name was “kaolinite”, but this scheme created confusion because it was unclear
if “kaolinite” was referring to the more general sub-group or the species “kaolinite”. See also
Part 2 of the Glossary. Cf., dioctahedral, 1:1 layers
kaolinite subgroup superceded by kaolin subgroup
kaolinite-serpentine group superceded by kaolin-serpentine group
Kaopectate® a commercial product (made by Chattem, Inc.) involving a formulation of kaolin
and pectin that was used as an anti-diarrheal. Since 2003, the formulation was changed to
bismuth subsalicylate, but some formulations containing attapulgite as the active principle are
sold in Canada.
Keggin ion An aluminum-rich Keggin-type structure is commonly used as a pillaring agent in
clays. This complex has a composition of Al13O4(OH)24(H2O)12)7+, and is often referred to as
Al13. Other compositions with the Keggin structure are possible, but they have not been as
extensively studied in pillared clays. Keggin structures are of commercial interest because they
are catalysts. See pillared clay.
Kelvin A temperature unit where one K equals 1/273.16 of the thermodynamic temperature of
the triple point of pure water, which is the temperature at which ice, water and water vapor can
coexist in thermodynamic equilibrium (definition from the 13th Conference of the Générale des
Poids et Mesures). 0 K, termed “absolute zero”, is the temperature at which atoms do not
thermally vibrate. Cf., Celsius, Fahrenheit
kinetics see chemical kinetics
Kübler index Kübler (1964, 1967) attempted to define a “crystallinity” index for illite (“IC”) by
examining the powder X-ray diffraction of intergrown illite and muscovite, originally to identify
the anchizone (diagenesis) and the anchizone-epizone (metamorphic) boundaries. Measured
values are expressed as small changes in the d value based on the width for the 10-Å peak at half
height above the background for Cu radiation. If the procedure is used, it should not be
characterized as a “crystallinity” index, as it is unclear if “crystallinity” is actually being
measured because such patterns also reflect the presence of smectite and other K-rich micas,
different mean crystallite sizes, lattice strain, layer stacking order, instrument parameters and
other features. Cf., crystalline, crystallinity index, Hinckley index, Arkai index
laminae see lamination
laminar microfabric see microfabric, clay
lamination sequences of thin bedding (or “laminae”) occurring because of variations in the
sediment supply in sedimentary rocks. Potter et al. (2005) suggests that laminae are <1 cm.
Laponite® Laponite is a synthetic clay mineral with a hectorite-like composition,
Na0.7Si8Mg5.5Li0.3O20(OH)4, and structure. Laponite is manufactured by BYK Additives &
Instruments to modify the rheology of aqueous fluids. Laponite-S482 is a common excipient in
lattice a collection of equivalent points (i.e., identipoints) which are distributed periodically in
space, and this term, in three dimensional space, commonly refers to Bravais lattices. From
Guggenheim et al. (2006) and references therein. The term “layer lattice” is incorrect because it
implies a structure rather than a lattice. Cf., array, Bravais lattice, identipoint, structure
lattice misfit A lattice misfit is where there are (one or more) dimensional mismatches between
a substrate crystal and an overgrowth crystal that has formed by heterogeneous nucleation. A
lattice misfit parameter, d, may be calculated from d = Da/a, where a is the lattice parameter of
the (stress-free) substrate crystal, and Da is the difference in lattice parameters between the
underlying substrate and the overgrowth precipitate. Cf., epitaxy, lattice
lattice parameters see cell parameters
layer For phyllosilicates, a layer (see Fig. 1) contains one or more tetrahedral sheets and an
octahedral sheet. There are two types of layers, depending on the ratios of the component sheets:
a “1:1 layer” has one tetrahedral sheet and one octahedral sheet, whereas a “2:1 layer” has an
octahedral sheet between two opposing tetrahedral sheets. Quot Guggenheim et al. (2006); also
see references therein. Cf., plane, sheet, tetrahedral sheet, octahedral sheet
layer-by-layer composite composites produced on nearly any substrate, including textiles, where
the composite is fabricated by successive dipping/rinsing/drying of the substrate in two different
solutions, one solution containing a clay, usually montmorillonite, and the second solution
containing a complimentary polymer (e.g., any polycationic polymer). These composites are
typically transparent, and generally 40 to 50 bilayers thick. Layer-by-layer composites lower
flammability substantially and improve gas barrier properties. See flame retardancy
layer charge In phyllosilicates, the “layer charge” or “net layer charge” is the total negative
charge deviation from an ideal, unsubstituted dioctahedral or trioctahedral composition. For
example, for an R3+-rich dioctahedral 2:1 layer, the layer composition is ideally: R2Si4O10(OH)2.
In muscovite mica where R = Al and there is an Al substituted Si site, the layer composition is:
Al2(Si3Al)O10(OH)2 and because an Al3+ substitutes for an Si4+, there is an unsatisfied residual
charge on the layer that results, a layer charge of -1. In muscovite, this residual charge is
compensated by an interlayer cation, K+, so that the structure is charge neutral. Because of the
anion framework of O10(OH)2, layer charges are always negative, and may be reported in the
literature as either a positive or a negative value. A negative layer charge results from either a
solid solution where a cation of lesser positive charge substitutes for a cation of greater charge or
by a vacancy (no charge) substitution for a cation. Anion substitutions [e.g., O for (OH)] are also
possible but uncommon. The location and size of the substitution has a profound effect on the
physical properties of clays. The layer charge is used in the classification scheme for
layer displacement the sum of the intralayer displacement plus the interlayer displacement,
which defines the total relative displacement between adjacent layers, as shown in Figure 1. For
2:1 layers, the layer displacement is measured from the geometric center of the ditrigonal ring.
The “intralayer displacement” is the shift that originates from the octahedral slant within one
layer and is measured from the geometric center of the ditrigonal ring from the lower to the upper
tetrahedral sheet of that layer (Figure 1). Layer displacement should be used instead of
“interlayer shift”. Cf., interlayer, layer
layer double hydroxides see double metal hydroxides
LDH see double metal hydroxides
lean clay see fat clay
Lennard-Jones potential a description of the interactive forces occurring between a pair of
neutral atoms or molecules. The potential is comprised of force-field terms: at long-separation
distances, van der Waals attraction predominates, whereas at short-separation distances, strong
repulsion predominates as a result of the Pauli exclusion principle. The Lennard-Jones potential
is accurate for noble gas interactions and a relatively good model for most neutral atoms and
molecules. The Lennard-Jones potential is computationally simple and thus commonly used in
LHC see liquid holding capacity
ligand see complex
line defect A line defect involves a series of structural imperfections that produces a linear
feature and, commonly, offsets or “dislocations”, within an atomic structure. An “edge
dislocation” forms where a plane of atoms or ions that would normally extend throughout the
crystal terminates along a line within the crystal. Distortions are at a maximum along the line of
termination. A “screw dislocation” is a localized screw axis involving atoms or ions (or blocks
of atoms or ions) to form a spiral, much like a spiral staircase, with a step at the outer surface of
the crystal. Because the screw dislocation is a local feature, the screw axis is not related to the
space group (overall symmetry) of the structure, and the crystal regains its overall atomic
periodicity at distances further from the central line of the dislocation. The step at the surface of
the crystal is believed to enhance crystal growth because atoms or ions can better adhere and
bond to sites associated with greater surface area at the step. Cf., point defect, screw axis
liquid limit one of the Atterberg Limits tests. The water content of a fine-grained soil/water
mixture that defines the boundary between the liquid and plastic states for that soil, as defined by
the test method described in ASTM Standard D4318 - 05. See Mitchell (1993). See also
activity, Atterberg Limits, consistency number, plastic limit, plasticity index, shrinkage limit.
liquid holding capacity the maximum capacity that a fine-grained, porous, granular material
can absorb a liquid into the pore structure of the individual grains and still remain free-flowing.
The point where the liquid impregnated granules adhere to the sides of a container (i.e. become
“wetted”) represents a rough measure of internal liquid holding capacity of the material (i.e., the
point beyond where granules begin adhering together and lose free-flowing properties).
liquidity index (water content – plastic limit) ÷ index of plasticity, or: (w – wp) ÷ (wl – wp). See
also Atterberg Limits, plastic limit.
liquidus a term describing the lower limit where only a liquid phase exists in a phase diagram.
Cf. Phase diagram, solidus
lithomarge a sectile and compact kaolin clay when wetted, often white to red/pink in color and
mottled. When dry, lithomarges are friable.
LOI see loss on ignition
loss on ignition (LOI) weight loss after heating, and (usually) subsequent cooling, to determine
the presence of volatiles in a solid. Cf., water, structural; water, adsorbed
low volatile matter (LVM) an industrial term referring to superheated but not fully calcined
clays. Cf., superheating
low-activity clay In geotechnical engineering, low-activity clays have activities of < 1, and
include illite, chlorite, and kaolinite. Fe and Al oxide minerals and clay-sized primary minerals
are also considered low activity. See activity, clay; high-activity clays; quick clays
lumen a) Industrial/clay: A lumen is the bore of a tube. Thus, halloysite is a tubular mineral
with an approximately 30 nanometer diameter lumen that runs the length of the tube. b) Medical:
A cavity of passage in a tubular organ; e.g., the lumen of the intestine. Also, commonly applied
to those clay minerals where medications are loaded into the tube bore, and the clay particle is
coated with an excipient to control the timing of delivery.
luster appearance of a mineral surface in reflected light
lutite Lutite is an older field term for an argillaceous, fine-grained rock that is equivalent to
claystone and mudstone. In clay mineralogy, the term “lutite” is commonly applied to authigenic
clays. An equivalent term is “argillite”.
LVM see low volatile matter
macronutrient a general term for major dietary nutrients required in relatively large quantities
per day. For example for human consumption, macronutrients include Ca K, Na, Mg that may be
potentially provided by ingestion of clays, whereas clays in soils provide these nutrients for
plants. Cf., micronutrient
mafic a geological term describing magmas, igneous rocks and silicate minerals that have a
relatively high concentration of magnesium and iron and a low silica concentration. Mafic
magmas typically have low viscosity, when compared to felsic magmas. Mafic rocks are
typically darkly colored when compared to felsic rocks. Cf., felsic
magnetic susceptibility the ratio of the magnetization, M, relative to the macroscopic magnetic
field intensity, B. “Diamagnetic” substances have negative values, “paramagnetic” substances
have positive susceptibilities. Cf., diamagnetism, paramagnetism
magnetism describes a property of material where there is attraction by a magnet. Cf.,
mangan see cutan
matrix see clay groundmass
matrix microfabric see microfabric, clay
matured muds see peloid
maximum consolidation stress/pressure see preconsolidation pressure
MC see Monte Carlo molecular simulation
MD see molecular dynamics
mean square displacement a representation of the average (squared) distance traveled by an
atom or particle during a defined period of time at a specific temperature. In a molecular
dynamics simulation, the mean square displacement is used to calculate the diffusion coefficient
for a given atomic or molecular species (e.g., interlayer cation or H2O). In an atomic structure
determination, the mean square displacement describes the time averaged movement of an atom
about its mean equilibrium position at a given temperature. This “movement” involves not only
(dynamic) thermal vibrations, but also an apparent movement caused by (static) disorder from the
random distribution of atoms over different equilibrium positions at (or near) a site from all the
unit cells sampled in the experiment (where perfect periodicity is lacking in a crystal). Cf.,
density functional theory, Grand Canonical Monte Carlo simulation, molecular dynamics, Monte
Carlo molecular simulation, quantum calculation
medicinal clays a general term for a clay used as an active principle or excipient ingredient in
medication. Cf., active principle, aesthetic medicine, antibacterial clay, geotherapy, healing clay,
mica a general term for platy phyllosilicates of 2:1 layer and a layer charge of ~ -1.0 per formula
unit (true mica group) or ~ -2.0 per formula unit (brittle mica group) or between -0.6 to -0.85 per
formula unit (interlayer-cation-deficient mica group, abbreviated to interlayer-deficient micas).
Micas do not show swelling capability. Octahedral character, either trioctahedral or dioctahedral,
further divides the mica groups into subgroups, whereas chemical composition separates the
species within the subgroups. Cf., true mica, brittle mica, interlayer-deficient mica, group names
micaceous A mineral habit where individual grains are platy in shape, as often caused by a
single plane of cleavage. Cf., habit
micelle an aggregate of surfactant molecules (each molecule consisting of a non-polar
hydrophobic tail and polar hydrophilic head) dispersed as a liquid colloid. In aqueous solutions,
a micelle forms with the tails oriented inward and the heads facing outward to solution.
Michaelis-Menten kinetics equation The Michaelis-Menten kinetics equation describes the
kinetics of many enzyme-catalyzed reactions and involves an enzyme binding to a substrate to
form a complex. This complex produces a product and additional enzymes, which produces
more complexes with a consequential increase in the reaction rate. The equation has been
successful in describing the rates of many biochemical reactions (e.g., protein-protein reactions)
other than in complexes involving enzyme binding substrates.
microcomposite, organoclay an organoclay with at least one dimension in the micrometer range
dispersed in a polymer. Cf., organoclay, organoclay nanocomposite
microcrystalline see cryptocrystalline
microfabric, clay Clay microfabric is the description of the spatial distribution of clay particles
in either clay-rich rock or sediment, commonly observed directly by electron microscopy of
aggregates, domains, or layer stacking effects within the material. Microfabric variations may
affect physical properties, conditions of formation, and evolution of the material. See Bennett et
al. (1977) and Potter et al. (2005) and see microstructure. Types of clay microfabric are (after
Grabowska-Olszewska et al., 1984):
domain microfabric A domain microfabric is composed of unoriented, coarse domains of
kaolinite crystals with parallel axial orientation. Domain boundaries involve face-to-edge and edge-to-edge particles. Pore shapes are complex, with equidimensional
interdomain pores (2-8 µm in diameter) and smaller fissure-like intradomain pores
(<0.5 µm). This microfabric is characteristic of eluvial kaolinitic clays; domains
often form from weathered feldspar crystals.
honeycomb microfabric Honeycomb microfabric consisting of unoriented, high porosity (60-90%), nearly equidimensional cells or domains commonly 2 - 12 µm in size in
sedimentary clay-rich soils. Cell walls are comprised of microaggregates of face-to-face and face-to-edge clay particles of montmorillonite-illite mineralogy. Silt/sand
grains are rare and are distributed throughout the soil. Most cell contacts are of
flocculation type (clumping of small particles), which promotes the high porosity.
The fabric is syngenetic (i.e., formed during sedimentation) and forms in recent
marine and lacustrine sediments.
laminar microfabric A sedimentary clay soil with a laminar-flow appearance from well
developed bedding/sorting of its structural components, mostly microaggregates with
face-to-face, and occasional face-to-edge, boundaries. Pores between aggregates are
fissure- and wedge-shaped parallel to the lamination. This microfabric is common of
clay-rich deposits (>50%) of varying mineral composition and forms in syngenetic
and/or post-depositional environments.
matrix microfabric A microfabric characterized by a continuous unoriented clay matrix with
a non-uniform distribution of silt/sand grains. The clay (illitic and mixed-layer
particles) forms microaggregates with face-to-face, face-to-edge, and edge-to-edge
boundaries. Pore sizes range from 8 to 2 µm for poorly compacted and compacted
sediment/soil respectively. Formation can be syngenetic and/or post-depositional.
pseudoglobular microfabric Pseudoglobular microfabric is formed by the weathering of
iron-rich igneous or metamorphic rocks with neoformation of iron phyllosilicates
(e.g., nontronite). This microfabric contains spheroidal microaggregates ranging in
diameter of 5 - 20 µm, sometimes composed of sheet-like particles with face-to-edge
contacts or with face-to-face and face-to-edge interactions. Porosity is made of
equidimensional interglobular (10 - 15 µm) and equidimensional or elongate
skeletal microfabric Skeletal microfabric is comprised of a generally uniform, porous
structure of unoriented silt/sand grains (to 60%) and clay (10 - 30%), the latter
forming a discontinuous matrix and commonly accumulating along the larger-grain
boundaries to tenuously bond the grains together. This microfabric is more compact
than the honeycomb microfabric with porosities ranging from 40 - 60%. The skeletal
microfabric occurs more commonly in recent clay deposits of varying mineral
composition (but often illitic). Formation can be syngenetic and/or post-depositional.
sponge microfabric Sponge microfabric consists of coarse aggregates (> 80 µm in diameter)
of sheet-like microaggregates with mostly face-to-edge and face-to-face contacts,
forming a continuous fine-cellular network resembling sponge. The clay material is
not orientated and pores are irregular in shape and < 3 µm in size. For example, this
microfabric has been reported in smectite clays of hydrothermal origin. See
turbulent microfabric A microfabric with a turbulent-flow appearance derived from clay
microaggregates that are well oriented along deformed laminations of clay-coated
silt/sand grains within a matrix of deformed bedded clay. Clay microaggregates are
bounded by face-to-face clay particles, and locally, by face-to-edge contacts at very
small angles. The clay content is > 20%. The pores are primarily fissure-like and
elongated along the lamination. This microfabric is commonly formed during the
diagenesis (compaction) of clay sediments with precursors of honeycomb and matrix
micrometer (μm) an International System (SI) unit of measure equal to 10-6 meters.
micron (μm) an old name for micrometer
micronutrient a general term for dietary essential nutrients required in relatively small amounts
(less than 50 milligrams per day). For example, micronutrients include vitamins (organic
compounds) and trace elements (e.g., Fe, Cu, Zn, I, Se, Mn, Mo, Cr, F, Co) for human
consumption that may be potentially provided by ingestion of clays, whereas clays in soils
provide these nutrients for plants. Cf., macronutrient
mineral an element or chemical compound that is normally crystalline and that has been formed
as a result of geological processes (quot Nickel, 1995). A mineral forms by a naturally occurring
process (e.g., “geological” process); phases formed by the interaction of individuals (even if
outdoors under conditions not fully controlled by the individual) are not minerals. Nickel (1995)
discussed exceptions to the requirements, such as the equivalence of extraterrestrial and
“geological” processes, metamict (non-crystalline) minerals, mercury (a liquid mineral), and
others (quot Guggenheim et al., 2006). Some naturally occurring processes, but without a
geological component, such as compounds that form biologically (e.g., oxalate crystals in certain
plants, marine animal shells), are not minerals. Synthesized materials are not minerals, but may
be referred to as “synthetic minerals” (e.g., “synthetic diamond”, “synthetic halite”) because the
use of “synthetic” negates the naturally occurring/geological aspects specifically. Likewise,
“biomineral” is acceptable for similar reasons as synthetic mineral. See crystalline, Cf.,
biologically controlled mineralization, biologically induced mineralization, biomineralization
mineraloid a natural solid with insufficient long-range atomic ordering to be classified as a
mineral. For example, limonite (FeO . OH . nH2O) is often considered an amorphous “mineral”
mirror plane or symmetry plane, used to describe a repetition of features whereby identical
points occur an equal but opposite distance along any line perpendicular from this imaginary
plane. Consequently, an object is “bilateral” in that it shows a matching of features (also referred
to as “reflection”) but a change in “handedness”, e.g., most people, standing with arms by their
sides and feet side-by-side, have a mirror (bilateral or reflection) relationship between the left
side and the right side. Cf., symmetry, center of symmetry, rotation symmetry
Mississippi bentonite an obsolete term, see Southern bentonite
mist see suspension
mixed layer see interstratification
Mohr’s circle Mohr’s circle is a two-dimensional graphical representation of the three-dimensional state of stress of a soil element subject to load in the normal stress - shear stress
coordinate system. Components (i.e., orientations of the normal and shear stresses) of applied
stress may be graphically obtained across various inclined planes at a given point in a stressed
body. Syn., Mohr circle of stress, stress circle
Mohs hardness Mohs’ scale of hardness is composed of a series of common minerals with
increasing hardness: (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite, (6) orthoclase, (7)
quartz, (8) topaz, (9) corundum, and (10) diamond. Hardness is determined by a scratch test, as
minerals of greater or equal hardness can scratch those of equal or lesser hardness. Cf., hardness,
moisture content see water content, degree of soil
molality the number of moles of solute per kilogram of solvent. Because weights of a
solute/solvent are generally fixed at all temperatures, this scale is useful for experiments where
physical properties (e.g., boiling point, freezing point) are examined over a temperature range.
Cf., molarity, formality, normality, mole fraction
molarity the number of moles of the solute in one liter of solution. Useful where experiments
use measured volumes and where temperature effects are not being studied. Cf., formality,
normality, molality, mole fraction
mole fraction number of moles of one component divided by the total number of moles of all
components Cf., molarity, formality, normality, molality
molecular dynamics (MD) a molecular simulation for calculating time-averaged properties of a
molecular system. The model system is simulated by solving Newton’s equations of motion,
including kinetic and potential energy terms. The energy terms can be obtained by using
classical (force field) or quantum (Density Functional Theory) methods. System constraints (e.g.,
constant volume, temperature, pressure) usually correspond to a thermodynamic ensemble. Cf.,
density functional theory, force field, Grand Canonical Monte Carlo simulation, Monte Carlo
molecular simulation, quantum calculation
molecular orbital method a method of calculating the electronic structure and related
properties of a finite molecular system consisting of two or more atoms by solving the
Schrodinger equation to determine the electronic structure. Various methods exist to
approximate the simultaneous solution of the Schrodinger equation for all electrons in the
molecular sieve see zeolite, pillared clay
molecular simulation a set of computational methods used to calculate physical or chemical
properties of a model system consisting of atoms or molecules. Computed properties can include
chemical reactions, structure, spectroscopy, and transport. Many techniques exist for such
simulations based on model system size and desired accuracy, from quantum methods to
classical (force field) methods. Cf., force field, quantum calculations
molecularity the number of reacting molecules, atoms, or ions in a single-step chemical
reaction. For example, a unimolecular reaction may involve radioactive decay of a single atom,
or one molecule producing other molecules. A bimolecular reaction involves the collision and
reaction between two molecules, atoms or ions to form other products. Cf., Transition State
monic c/f-related distribution see c/f-related distribution
monoclinic see crystal system
monolayer see alkylammonium organoclay monolayer
mononuclear complex see binuclear complex
monostriated b-fabric see b-fabric
Monte Carlo molecular simulation (MC) a classical molecular simulation approach, based on
the Metropolis Monte Carlo algorithm, for calculating ensemble averages of structural properties.
The model system evolves based on random moves (e.g., atom or molecular translation, system
volume) that are accepted or rejected using a Boltzmann algorithm. System constraints (e.g.,
constant volume, temperature, pressure) usually correspond to a thermodynamic ensemble. Cf.,
force field, Grand Canonical Monte Carlo simulation, molecular dynamics, quantum calculation
montmorillonite-saponite group old group name superceded by smectite. See also Part 2 of
the Glossary. Cf., smectite
morphology (soil micromorphology) In soil science, the geometry and surface topology of a
soil particle. The term may also be used as a study of soil horizons and soil properties.
mosaic-speckled b-fabric see b-fabric
mottle see glaebule
muck indicates a soil composite that is largely organic in nature, lacks recognizable plant
structures and is very darkly colored. It differs from peat in being darker, lacking plant structures
and having increased mineral content. It differs from ball clay in having significantly higher
organic content and in being much darker.
mud soft, plastic, sticky or slippery matter, generally containing water and clay minerals derived
from fine-grained soil/regolith/sediment materials. Mud is commonly composed of clay-size
particles, but often contains mixtures of sand- and silt-size particles as well as microorganisms.
Potter et al. (2005) considers mud a sedimentological field term for unconsolidated fine-grained
sediments of any composition and consisting of 50% or more of particles smaller than 0.0063
mm (i.e., clay and silt size fractions). Cf., see geotherapy, matured mud, pelotherapy, poultice,
mudrock An indurated, sedimeantary clay-bearing rock with 33-65% clay-size constituents and
stratification of >10 mm thickness (bedding). An individual mudrock bed may be internally
massive (after Potter et al., 2005). See clayshale, claystone, mud, mudstone, mudshale, silt,
mudshale An indurated, sedimentary clay-bearing rock with 33-65% clay-size constituents and
lamination (stratification of <10 mm thickness), after Potter et al. (2005). See clay, clayshale,
claystone, mud, mudstone, mudrock, silt, siltstone.
mudstone A general term for a rock which is similar to shale but used to describe a clay-rich
rock without laminations. The fraction of clay and/or silt components are not well defined. The
silt component may be of major proportions. See clay, clayshale, claystone, mud, mudrock,
mudshale, silt, siltstone.
mud therapy therapeutic use of healing mud (hydrated clay) in an empirical way and without
medical supervision for palliative or cosmetic purposes. Traditionally, mud therapy was
practiced in the natural environment where the mud occurs, but this is no longer the case. Also,
see aesthetic medicine, antibacterial clay, geotherapy, healing clay, pelotherapy, peloid
mull In soil science, mull is a type of humus of the A horizon, without distinct layering, and
composed of organic matter intimately mixed with the fine-grained mineral fraction. Syn., mull
nanocomposite Nanocomposites are fine-particulate materials that are heterogeneous at the
nanoscale level (i.e., less than 10-7 meters). Such materials have structures and properties that are
composite-particle-size dependent. The composite-particle size is sufficiently large that it is not
considered a chemical (atom) cluster or complex.
nanocomposite, organoclay an organoclay with at least one dimension in the nanometer range
dispersed in a polymer (e.g. nylon-6 clay-nanocomposites; see Gilman et al.,1997). The less
hydrophilic surface of organoclays facilitates dispersion in a polymer. Dispersing ~5%
organoclay in a polymer can make the polymer more flame retardant and improve the physical
properties. See Gilman et al., 1997; Picken et al., 2008; Ruiz-Hitzky and Van Meerbeek, 2006.
Cf., organoclay, organoclay microcomposite
nanotube a natural or man-made, nanometer-scale tubular structure. See nanocomposite,
nanometer (nm) an international system (SI) unit of measurement equal to 10-9 meters.
natural peloid see peloid
Néel temperature see antiferromagnetism
neo-cutan see hypo-coating
neoformation The phenomenon by which secondary minerals are formed by crystal growth
entirely from solutes without any incorporation of pre-existing structural components from other
preexisting (parent) minerals. See inheritance, transformation
Nernst Equation a model to determine the equilibrium reduction potential of a half-cell in an
electrochemical cell, such that Ered = Eored - (RT/zF) ln(ared/aOx), where Ered = half-cell reduction
potential, E0red = standard half-cell reduction potential, R = ideal gas constant, T = absolute
temperature, z = number of electrons transferred in the half-cell, F = Faraday constant, aRed =
activity of the reduced species, and aOx = activity of the oxidized species. An alternative form of
the equation can be used for the total cell potential where Ecell = Eocell - (RT/zF) lnQ, where Ecell is
the cell potential or electromotive force, E0cell is the standard cell potential, Q is the reaction
quotient, and the other parameters are given above. The Nernst equation is most useful for dilute
solutions because concentrations are approximately equal to activities. The equation only applies
where there is no current flow at the electrode and also because concentration is approximately
equal to activity. Otherwise, corrections must be applied.
Newtonian fluid a fluid where the viscosity, equal to the ratio of shear stress to shear rate, is
constant. A Newtonian fluid is approached in suspensions of water and clay where the clay is
fully defloculated. Cf., dilatancy, quick clay, thixotropy
nodule see glaebule
non-crystalline a solid where the constituent components are randomly packed. Many
variations can occur between the two extremes of crystalline vs. non-crystalline. For example, a
non-crystalline material, such as many glasses, consists of atoms arranged as groups of tetrahedra
or octahedra. However, although such groups have almost the same mutual arrangements, they
are displaced without periodicity with respect to each other (i.e., limited order or short-range
order). (Quot Guggenheim et al., 2006) Cf., crystalline
normality the number of gram equivalent weights of the solute in one liter of solution. Useful
where experiments use measured volumes and temperature effects are not being studied. Cf.,
molarity, formality, molality, mole fraction
normally consolidated clays a clay mass that has been compressed by the stress expected from
just overburden, without any previous overloading. Cf., preconsolidation stress,
normal mode the correlated motion of atoms in a molecule or crystal identified by a specific
frequency and with the same phase relations in the vibrational spectrum. The symmetry of the
normal mode is related to the symmetry of the molecule or crystal. Cf., vibrational spectroscopy
obliquity the ratio of the major principal effective stress to the minor principal effective stress
among three orthogonal planes at any stressed point, where there are zero shear stresses.
octahedral layer inappropriate usage for an octahedral sheet, see octahedral sheet
octahedral sheet For phyllosilicates, an octahedral sheet contains edge-sharing octahedra that
are connected in a continuous two-dimensional arrangement. These octahedra generally contain
small (e.g., Al, Fe3+) or medium size (e.g., Mg, Fe2+, Li) cations with coordination anions (e.g.,
O, OH, Cl, S). In some phyllosilicates (e.g., modulated phyllosilicates), the octahedral sheet may
be only partially continuous, for example, forming one-dimensional strips of octahedra or islands
of octahedra. There are two types of octahedral sheets, dioctahedral and trioctahedral. After
Guggenheim et al. (2006) and references therein. Cf., dioctahedral sheet, trioctahedral sheet,
oedometer a one-dimensional, consolidation testing apparatus commonly used to measure the
compressibility and consolidation behavior of soft clayey soils. In such tests, the soil is confined
to one-dimensional compression and one-dimensional flow. Only vertical deformation or strain
occurs, whereas the lateral or horizontal deformation is restricted or zero.
organan see cutan
organoclay, grafted compound-type see organoclay
organic clay In civil engineering, an organic clay is a clayey soil containing a specific range of
organic matter content (ASTM Standard D2487). Organic clay is usually distinguished by
determining the change in the liquid limit of a clay that was oven-dried at 105 - 110 oC. If the
ratio of the liquid limit of the oven-dried clay to that of the natural (i.e., without oven drying)
clay is < 0.75, then the soil is classified as organic clay. Cf., liquid limit, organoclay, organic soil
organoclay, organophilic Using organic contaminant adsorption isotherms and sorptive
behavior, organophilic-type organoclays (which produce simple, linear isotherms) are defined as
having large exchanged organic-cation alkyl groups. These groups seem to act as a solvent phase
(e.g. partitioning phase, i.e., solubilized), but are located in the interlayer, to absorb organic
contaminants (Jaynes and Boyd, 1991b). a) Synthetic systems. These clays are prepared from
smectite or vermiculite by using large quaternary (> C-10) organic cations, such as
hexadecyltrimethylammonium (C-16) or dioctadecyltrimethylammonium (C-18). These higher
charge clay minerals (e.g., vermiculite, illite, high-charge smectite) adsorb greater numbers of
large organic cations and yield organoclays that are more effective in absorbing organic
contaminants compared to the unmodified clay. See also Boyd and Jaynes (1994). b) Natural
systems. Soil organic matter and organic compounds adsorbed to (internal or external) clay-mineral surfaces can act as a solvent phase for organic contaminants dissolved in water. Organic
compounds (e.g., benzene, toluene, xylenes in gasoline) are more soluble in soil organic matter
or the organic phase derived from organic compounds adsorbed to mineral particles than in
water. Nonionic organic compounds may be partitioned (i.e. solubilized) into soil organic
matter; see Chiou et al. (1979). Cf., organoclay, adsorptive; organoclay
organoclay, adsorptive Based on organic contaminant adsorption isotherms and sorptive
behavior, the adsorptive-type organoclays (generally producing nonlinear and Langmuir-type
isotherms) have exchanged organic cations that act as interlayer props to hold the interlayer open.
This arrangement facilitates additional organic contaminant adsorption onto the siloxane
surfaces, which are relatively hydrophobic except near exchangeable cation sites (Chen, 1976;
Jaynes and Boyd, 1991a). These clays are prepared from smectite by replacing inorganic
exchangeable cations with small organic cations, such as tetramethylammonium or
trimethylphenylammonium. Lower charge clay minerals (i.e., lower charge smectite) with
adsorbed small organic cations yield organoclays that more effectively adsorb organic
contaminants compared to the unmodified clay. See Boyd and Jaynes (1994). Cf., organoclay,
organoclay a phyllosilicate, typically smectite, vermiculite, or kaolin, but also other minerals
(e.g., double metal hydroxides) with sorbed organic molecules, such that the properties of the
mineral are altered. Commonly, the sorption occurs between the 2:1 or 1:1 layers. The
mechanism for sorption may vary depending on the organic molecule and concentration. In
alkylammonium organoclays, alkylammonium cations replace inorganic cations, and these
organic cations are bonded to the layers via electrostatic (= Coulombic) forces. In organophilic
alkylammonium organoclays where large organic cations completely fill the interlayer, van der
Waals interactions between the alkyl groups augment the Coulombic forces, which increase both
adsorption and organoclay stability. Adsorptive type organoclays have small organic cations that
partially fill the interlayer, are stabilized by Coulombic forces, and act as pillared materials where
there are accessible areas between the pillars for additional adsorption. These pillars increase
surface areas relative to untreated clay or organophilic clays. In grafted compound-type
organoclays with organic compounds such as silanes in the interlayer, bonding to the clay layer is
covalent. Organoclays with adsorbed neutral polymers are attached to the clay layer by weak
multiple dipole-induced dipole type bonds. In organoclays prepared from charged polymers,
such as proteins, the polymers are bonded by both electrostatic and dipole-induced dipole bonds.
Organoclays can be used as adsorbents, thickening and thixotropic agents, in nanocomposites,
and in new materials with catalytic, optical, and electronic properties (Lagaly et al., Ogawa, and
Dékány, 2006). Syn., organo-clay, clay-organic complex; Cf., organoclay, adsorptive; organoclay,
alkylammonium; organoclay, organophilic alkylammonium phyllosilicate; pillared clay;
organo-peloid see peloid
organophilic a characteristic property of a clay whereby the clay can sorb an organic solvent.
These clays are usually surface modified, commonly by sorbing various quaternary ammonium
compounds, which allow the clay to swell in organic liquids. See organoclay, Cf., organophobic
organophilic organoclay partition phase see organophilic, organoclay
organophobic a characteristic property of a clay whereby the clay repels an organic liquid. Most
naturally occurring clays are organophobic and are not wetted by nonpolar organic liquids. See
organoclay, Cf., organophilic
orthorhombic see crystal system
osmosis A physical process by which a solvent (typically, water) diffuses through a semi-permeable membrane (a porous material which is permeable to the solvent, but not the solute),
owing to differences in solvent activity of two solutions which are separated by the membrane.
osmosis, reverse The process of forcing a solvent (typically, water) through a semi-permeable
membrane from a solution with lower water activity to a solution with higher water activity by
applying a pressure opposite to and in excess of the osmotic pressure. Reverse osmosis is a
process commonly used to purify (i.e., to physically separate and remove dissolved ions from)
osmotic pressure The differential pressure exerted by a solvent across a semi-permeable
membrane owing to the difference in solvent activity between two solutions separated by the
osmotic swelling interlayer expansion that occurs in phyllosilicate minerals responding
to the effects of temperature, pressure and composition (e.g., chemical potential) of an electrolyte
surrounding the mineral with respect to the interlayer H2O (i.e., the “osmotic effect” or “osmotic
pressure”). At equilibrium, the chemical potential of H2O in the electrolyte and in the interlayer
is related to the salt concentration of the electrolyte (the osmotic pressure is zero). At a given
temperature, pressure, or composition away from equilibrium, there will be a difference in
chemical potential between the electrolyte and clay interlayer and the resulting osmotic pressure
will be the driving force to affect the layer-to-layer distance. Under certain conditions, the
resulting osmotic pressure drives the clay layers further apart than the interlayer distances
common to intracrystalline swelling. Cf., intracrystalline swelling
Ostwald ripening a recrystallization process where the smallest crystals dissolve and the
chemical constituents are added to the larger crystals of the same phase. This process of crystal
growth and coarsening occurs in a solution. The driving mechanism is a shift to lower surface
free energy (Ostwald, 1900 in Baronnet, 1982).
Otay-type montmorillonite Defined by Schultz (1969) for montmorillonite samples [current
nomenclature for montmorillonite is that it is an Al-rich, dioctahedral smectite with an ideal
structural formula of (Al3.15Mg0.85)Si8O20(OH)4X0.85.nH2O with layer charge from primarily
octahedral substitutions of Mg] with a large net negative layer charge [-0.80 to -1.20 per unit
cell, O20(OH)4] obtained almost entirely (<0.15 charge from tetrahedral sheet) from cation
substitutions in the octahedral sheet, although exceptions were noted. This classification scheme
was based on chemical and thermal analysis. The term is obsolete and should not be used.
Terms used in this obsolete classification are: Wyoming-type, Otay-type, Chambers-type,
Tatatila-type, beidellite-type (ideal and non-ideal), and non-ideal montmorillonite. See also Part
2 of the Glossary.
outer sphere complexes Ions adsorbing in the outer Helmholtz plane form outer sphere
complexes. The larger distance to the surface relative to the shorter distances of inner sphere
complexes may result from retaining H2O molecules (via hydration) between the surface and the
outer Helmholtz plane see Stern layer
overburden In mining, overburden refers to the unusable material(s) that is above the ore of
interest. Overburden may be removed or left in place (by tunneling below it).
overconsolidation ratio In soil science and soils engineering, the overconsolidation ratio (OCR)
is the ratio of the preconsolidation effective stress to the current consolidation effective stress to
which the soil is being subjected. For normally consolidated sediment the OCR = 1.0, thus for
overconsolidated sediment the OCR >1.0.
palliative Palliative refers to the relief of pain (or more generally, other side effects of an
illness), but without treatment of the underlying condition. Clay baths (balnea) are commonly
used as a palliative treatment for chronic pain (e.g., arthritis).
papule A pedofeature formed by fragments of laminated clay coatings. See pedofeature.
paraffin complex see alkylammonium organoclay paraffin complex
parallel striated b-fabric see b-fabric
paramagnetism a weak magnetic attraction that develops in the presence of a magnetic field
when magnetic moments align in the material. Paramagnetism is found in substances with
atoms, molecules and lattice defects possessing an odd number of electrons. Hence, transition
metals with partly filled inner shells show paramagnetic behavior. Metals (e.g., Fe) may be
paramagnetic. Cf., magnetic susceptibility
paramud hydrated clays mixed with paraffin, heated to 40-45˚C before topical application in a
<5 cm thick layer for ~30 min. Paramud applications are usually covered by a non-permeable
cover to conserve heat. The procedure is often claimed to result in moisturizing the skin and
parting a mechanical property where a mineral breaks along a planar or near-planar structural
weakness. Parting usually results from plane-like defects, often twinning, or is produced by
anisotropic stress. In contrast, cleavage is a property of the periodic crystal structure. Cf.,
pearly having a luster similar to that of mother-of-pearl with an interplay of rainbow-like colors
peat a) Peat is partially decomposed plant (humus) matter that formed in a nominally oxygen-free, water-saturated environment, often found in northern regions where microbial activity is
limited. Peat has more than 30% dry mass of dead organic matter ranging from colloidal masses
to recognizable plant residues. b) In soil science, peat soil has a high content of plant-derived
organic matter (humus), usually measured by the weight loss (or loss on ignition) by dry
combustion in atmosphere. In peat soils, plant residues are recognizable. If the residual weight
of an oven-dried (at 105-110 oC) soil after dry combustion at 440 oC is less than 75% of the
weight of the oven-dried soil, then the soil is considered to be peat.
peat soil see peat
ped individual, natural soil aggregates composed of primary particles that form a soil structure.
Adjoining peds are separated by planes of weakness formed by voids or cutans.
ped, primary A ped that can not be divided into smaller peds is a primary ped. Several primary
peds may be packed together to form compound peds (referred to as secondary and tertiary peds).
pedofeature A pedofeature refers to a unit fabric in a soil that is easily differentiated from the
adjacent fabrics by a difference in concentration of a component, such as variations of chemical
(e.g., organic, ferric iron, manganese) components, grain-size fraction, or internal fabric. The b-fabrics are commonly not considered as pedofeatures. See b-fabric, cutan, papule
pedoplasma see soil plasma
pedoplasmation production and redistribution (either physical or chemical) of soil plasma
during soil formation and development. See soil plasma
peloid a maturated mud or muddy dispersion composed of a mixture of fine-grained material
(commonly clay), mineral water (or sea water) and often organic compounds that result from
biological metabolic activity, with healing and/or cosmetic properties. Peloids involve a
maturation or ageing process, which may occur naturally, although commercially available
peloids are often processed for weeks to several years in special ponds, typically containing
mineral waters or brines of varying composition. Based on the composition of the fine-grained
components, the terms inorganic peloid (phyllo-peloid, sulpho-peloid), organic peloid (organo-peloid), and mixed peloid have been proposed. Also, see mudtherapy, pelotherapy, peloid
maturation. Cf., ageing. Syn., matured mud
Various types of peloids have been described:
natural peloid a healing mud formed in the natural sedimentary environment
and used at the source. syn., healing mud
phyllo-pelloid an inorganic peloid mostly composed of clay-sized
sulfo-peloid or sulpho-peloid an inorganic peloid mostly composed of sulfur or
organo-peloid a peloid composed primarily of organic-rich materials (e.g.
peloid maturation a process that occurs with the intimate blending of fine-grained materials
(commonly clays and/or organic material) with medicinal mineral water or sea water for variable
periods of ageing. Biological activity during maturation may play a role in enhancing healing
properties and in shortening ageing time. Cation exchange between minerals and water may also
occur during maturation, affecting the fluid chemistry, and this may enhance the physical
properties (e.g. plasticity) of the suspensions.
pelotherapy therapies involving the use of peloids (muds or poultices). Cf., active principle,
aesthetic medicine, antibacterial clay, geotherapy, healing clay, medicinal clay, peloid
peptize a) The formation of a stable dispersion of colloidal particles in water, usually by
chemical additives; b) In bentonite clay-product manufacturing, the use of chemical additives to
improve the performance characteristics of the clay for a particular end use. Cf., peptized clay.
peptized clay a clay to which chemicals have been added to improve the performance
characteristics of the clay for a specific end use. Cf., peptize
percutaneous migration As applied to clay science, percutaneous migration refers to the
diffusion of elements from clay poultices (peloids) through the skin.
peripheral replacement Replacement of a reactant mineral by a product mineral beginning at
the outermost margin of the reactant mineral’s grain or fracture-bounded remnants. See
perlite hydrated volcanic glass containing 2-5 wt. % H2O. Because of their high viscosity,
rhyolitic melts form glasses upon cooling. Devitrification produces a “perlitic structure”, which
is characterized by concentric cracks. Perlite has a pearly luster, and is glassy gray to black.
When heated, H2O is released and glass shards dehydrate and expand to volumes to 20 times
their original size. Associated zeolites are common. Perlite, after heating, is used as aggregate,
fillers, in plaster products and in light-weight concrete, as coatings for steel beams as fire
protection to reduce the possibility of melting or partial melting, etc.
permanent charge see point of zero charge
permeability In soil science, permeability is a property of a soil or porous medium, which
expresses the ease of a fluid flow (e.g., water, crude oil, gas, or air) through the medium, given in
units of (length)2. In contrast, hydraulic conductivity refers specifically to water as the permeant.
Cf., hydraulic conductivity
petrographic microscope an optical (transmitted visible light) microscope with polarizing filters
or prisms along the optical path. The petrographic microscope is commonly used to examine
thin sections and grain mounts. See thin section, crossed Nichols, plane light.
pH the negative logarithmic (log10) measure of hydrogen ion (H+) activity which defines the
acidity or alkalinity of a solution. In pure water, a value of 7 is neutral and represents an exact
balance between the activities of hydrogen (H+) and hydroxide (OH-) ions. Values lower than 7
represent an increase in hydrogen ion activity and are acidic. Values higher than 7 represent a
decrease in hydrogen ion activity and are alkaline.
pharmacognosy The branch of knowledge concerned with medicinal drugs derived from natural
products. This knowledge base includes the study of physical, chemical and biological
properties of natural substances, for example, clays and clay minerals, used medicinally.
phase diagram a graphical representation of the relationships involving possible phases that
occur in a system, typically based on parameters such as temperature (T), pressure (P),
composition (X), and typically at equilibrium conditions. However, other parameters may be
used where convenient and non-equilibrium diagrams may be useful to show important phase
phase a part of a system with distinct chemical and physical properties that is mechanically
separable from other parts of the system
phenyltrimethylammonium organoclay see trimethylphenylammonium organoclay
phreatic surface the groundwater table in soil or sediment. The phreatic surface is the interface
between the vadose zone (unsaturated water content) and the phreatic zone (saturated with water)
of a soil/sediment, usually characterized with a zero porewater pressure.
phyllo-peloid see peloid
phyllosilicate a family of minerals contain continuous two-dimensional tetrahedral sheets of
composition T2O5 (T = Si, Al, Be...) with tetrahedra linked by sharing three corners of each, and
with a fourth corner pointing in any direction. The tetrahedral sheets are linked in the unit
structure to octahedral sheets, or to groups of coordinated cations, or individual cations.
Although continuous tetrahedral sheets often form six-fold rings, other ring configurations are
considered part of the phyllosilicate family. See Guggenheim et al. (2006) and references
therein. Cf., clay mineral
physical weathering the breakdown of rocks to fragments through primarily non-chemical
processes. Non-chemical processes may include, but are not limited to, ice and/or root wedging,
thermal expansion, stress release (e.g., exfoliation), and physical consequences of crystal
wedging, and volume change caused by hydration and/or dehydration. Physical weathering may
be biologically influenced. See weathering, chemical weathering
pica a disorder involving the eating of material that does not have nutritional value. Pica is the
more general term for the eating of odd things, such as clay, ice, hair, starch, etc. Cf., geophagy
piezoelectric a property of crystals where an electric moment forms in proportion to tension or
compression, with a reversal in polarity depending on the direction of the stress. The
phenomenon is dependent on certain non-polar crystal classes or symmetries of the material. A
“converse piezoelectric” effect is known where the crystal changes shape if an electric current is
applied along the polar axis. Quartz exhibits piezoelectric and converse piezoelectric effects.
pillar see pillared clay
pillared clay a clay mineral intercalated with small organic or inorganic complexes (or “pillars”),
which do not completely fill the interlayer space. The size and shape of the resultant cavities (or
“galleries”) are determined by the size, shape, and orientation of the pillars. Pillared clays are
potentially useful to remove organic molecules based on shape-selective adsorption (“molecular
sieves”). For clays, intercalations involve positively charged complexes (either organic or
inorganic “cations”) to offset the negatively charged layers of the clay. Common types of
complexes include those of the methylammonium group (organic) and the Keggin ion (Al13,
inorganic). Other types of layered materials can also be pillared. See Keggin ion. Cf., zeolite
PL or PPL see plane light
plane of vibration see plane light
plane polarized light see plane light
plane in phyllosilicate mineralogy, a set of one or more types of atoms (e.g., a plane of Si and Al
atoms, a plane of basal oxygen atoms) that form a two-dimensional flat or nearly flat surface. See
Guggenheim et al. (2006) and references therein. Cf., sheet, layer
plane light Polarized light, or plane light, is light that vibrates within a single plane (“plane of
vibration”) which is defined as the plane parallel to the ray’s path and its vibration direction. The
polarized light results from the introduction of a single polarizing device into the optic path of a
petrographic (polarized light) microscope, with the plane of vibration obtained from the
privileged direction of the polarizer. See petrographic microscope. Abbr. PL, or PPL (plane
plasma, soil see soil plasma
plasma see soil plasma
plastic limit one of the Atterberg Limit tests. The water content of an homogenous, fine-grained
soil/water mixture where the mixture begins to exhibit plastic behavior upon deformation, as
defined by the test method described in ASTM Standard D4318 - 05. Syn. “lower plastic limit”.
See Mitchell (1993). See also activity, Atterberg Limits, consistency number, liquid limit,
plasticity index, shrinkage limit.
plasticity index the numerical difference between the liquid limit and the plastic limit; i.e., the
difference between the water content of a fine-grained soil/water mixture at the boundary
between its liquid and plastic states and the boundary between its plastic and brittle states, based
on tests outlined by Atterberg and standardized by ASTM Standard D4318 - 05. See Mitchell
(1993). Syn. Plasticity number. See also activity, Atterberg limits, consistency number, liquid
limit, plastic limit, shrinkage limit.
plasticity When referring to clay, this is a property where moistened material, when deformed
under the application of pressure, will retain the induced deformed shape when the applied
pressure is removed.
plug flow reactor a column or cylindrical reactor used to describe the reaction kinetics within a
continuous, flowing system. Cf., batch reactor, chemostat, continuously stirred tank reactor
pneumatolitic a petrologic term that refers to alteration or crystallization involving a gas phase,
typically forming from cooling magma.
pneumoconiosis Pneumoconiosis is a lung disease caused by the inhalation of (mineral) dusts,
characterized by lung inflammation, coughing and development of fibrous connective tissue
(fibrosis). Examples of pneumoconiosis include silicosis, caused by respirable silica, and
asbestosis, caused by respirable asbestos. Also see asbestosis, silicosis
podoconiosis derived from the Greek podo (foot) and konia (dust), and refers to a condition in
which some nano-particles enter the bloodstream through skin causing a condition characterized
by gross enlargement of areas of the body (mostly limbs) by blockage of the lymphatic system or
by a non-parasite immune system response. This condition is similar to elephantiasis, which is
caused by parasite blockage of the lymphatic system. Syn., nonfilarial elephantiasis
point defect Point defects are structural imperfections that occur at a specific point within an
atomic structure, and may produce a variation in the ideal chemical composition of the crystal.
“Schottky defects” occur where a vacancy replaces a cation or an anion from their ideal sites in
an atomic structure. In such cases, charge neutrality must be maintained, and thus for example,
where a cation is replaced by a vacancy either higher valence cations must be substituted for
lower valence cations or a corresponding anion must be replaced by a vacancy. “Frenkel defects”
involves a misplacement of a cation (“cationic Frenkel defect”) from its site to an interstitial
position where a site does not normally reside. Like the Schottky defect, the Frenkel defect must
involve charge neutrality (for example, producing a “cationic Frenkel defect” and an “anionic
Frenkel defect”). “Impurity defects” affect the chemical composition of the crystal and involve
an atom or ion of a different type either in place of an atom or ion that belongs to the crystal or in
an extraneous (interstitial) position. An “F center defect” in alkali halides involves a trapped
electron in an otherwise vacant site that was formerly occupied by an anion. This defect is
thought to cause a color change in the halide. Cf., line defect
point group The ten basic operations (center of symmetry, mirror plane, proper and improper
rotation axes) and their 22 allowable combinations (total = 32) are called “point groups” or
“crystal classes”. A combination is allowable only if “closure” is produced. See “rotation
symmetry” for the definition of “closure”.
point of zero net proton charge see point of zero charge
point of zero charge (pzc or zeropoint of charge) the pH value of a solution where the negative
variable charge equals the positive variable charge for a mineral. The variable charge results
from unsatisfied bonds at grain boundaries and any compensating negative (OH-) or positive (H+)
ions, and thus is a function of the solution surrounding the mineral grain. The variable charge of
a phyllosilicate involves the edges of the particle, whereas the layer charge is the “permanent
charge” and not of interest in determining the point of zero charge (“total charge” is the sum of
the variable and permanent charges). Surface properties change with the presence and types of
ions satisfying the residual charges at the crystal surfaces. For example, the type of ions
attaching to the surface can affect flocculation/dispersion properties and therefore, sedimentation
rates. Sposito (1998) defines the point of zero charge more succinctly as “the pH value of a
solution, where the net surface charge of a particle is zero”. The point of zero charge should not
be confused with the “point of zero net proton charge”, which refers to particles where only
protons are charge determining. The point of zero charge is not necessarily identical to the
isoelectric point (iep), in part because of how they are derived experimentally. The point of zero
charge is usually determined from titrations at various ionic strengths, which yield intersecting
curves at a single point (the “common intersection point”) which, in the absence of sorption of
other charge-determining ions, is identical to the point of zero charge. The iep is determined by
electrokinetic methods as the pH where the particle mobility is zero. Both points are only
identical if specific adsorption of other ions is absent.
polarizability the ability of an atom, ion, or molecule to become polarized (where there is a
distortion of its charge distribution). For example, an anion has greater polarizability than a
cation because of the tendency of the electron cloud about an anion to be easily distorted by a
local electric field. Generally, an anion is unable to hold its outer electrons tightly and therefore
it is more readily polarizable than a cation.
For a monoatomic atom or ion and some molecules, the polarization vector P has the
same direction as the applied electric field vector E, and P = aE, where a is the polarizability. If
a molecule has an anisotropic polarizability, that is the polarization P may not follow the
direction of the applied electric field E, the polarizability a is a symmetric tensor, and the
polarization P is given as:
and because the polarizability tensor is symmetric: alpha(xy) = alpha(yx), alpha(xz) = alpha(zx),
and alpha(yz)=alpha(zy). Cf., polarization, dipole moment
polarization Polarization is the distortion of the charge distribution about an ion or molecule.
In effect, the electron cloud about, for example, an anion can become distorted by a neighboring
cation, thereby affecting their determined ionic radii. The anion, now polarized by the cation,
shares its electrons with the cation and covalent character increases. Polarization may be
temporary if the electric field is temporary, and thus a temporary dipole moment may form. Cf.,
polarized light see plane light
polymer-clay nanocomposite see clay nanocomposite
polymorphism the ability of a given element or compound to crystallize in more than one form,
with each form having a distinct crystal structure. Cf., polytypism
polynuclear complex see binuclear complex
polytype refers to a structure that develops from layer-stacking sequences via polytypism. See
polytypism an element or compound in two or more layer-like crystal structures that differs in
layer-stacking sequences. Polytypism differs from polymorphism in permitting small differences
in chemical composition between structures, not to exceed 0.25 atoms per formula unit of any
constituent element. Layer structures that differ from one another by more than this amount are
to be called polytypoids rather than polytypes. (Quot Bailey, 1977) Cf., polymorphism
polytypoid see polytypism
popcorn texture Popcorn texture is a term for a macro-texture observed on exposed and
weathered clay surfaces commonly associated with deposits of sodium bentonite (usually on
open-pit mine-faces, bed outcrops, or stockpiles). These high-swelling bentonites develop
irregularly rounded lumps resembling mounds of popped corn caused by successive periods of
swelling and shrinkage from the cyclical uptake and loss of water within the clay. See also
alligator skin texture.
porcelain a high-strength and low-absorption glazed or unglazed ceramic whiteware of high
porosity the ratio of the void volume (empty space) in a material to its total volume (i.e.,
including particles and the empty space between the particles), usually expressed as a percentage.
Cf., void ratio
porostriated b-fabric see b-fabric
porphyric c/f-related distribution see c/f-related distribution
poultice a moist, often medicated, mass applied to a wound or a painful area to reduce pain or to
help in healing. The medicated mass may produce soothing vapors to aid in healing. Poultices
are commonly hydrated clays, often bentonite. Poultices are also used to dissolve stains from
porous decorative stone by the long-term application of a solvent via the poultice. Syn.,
preconsolidation pressure the maximum consolidation pressure or stress that a clay mass has
ever been subject to, including the current stress acting on the clay. Syn., preconsolidation stress,
maximum consolidation stress/pressure
preconsolidation stress see preconsolidation pressure
preconsolidation stress pressure see preconsolidation pressure
primary mineral A primary mineral is a mineral of igneous, metamorphic or sedimentary origin
now residing in weathering, sedimentary, diagenetic or hydrothermal environments where many
alteration processes operate. A primary mineral is present in the parent rock; thus, in soils and
other weathering or alteration environments, a primary mineral is one that remains from the
parent rock whereas a secondary mineral is one that forms as the rock weathers. Cf., secondary
prismatic a crystal shape where one dimension is considerably greater than the other two
proper rotation axis see rotation symmetry
pseudoglobular microfabric see microfabric, clay
pseudomorph A pseudomorph is an alteration product of a crystal that preserves the external
euhedral crystal form of the precursor mineral. The pseudomorph may have a different chemical
composition and/or crystal structure than the precursor crystal. The pseudomorph can result from
replacement of the original structural bonds by formation of new bonds that minimize the energy
toward equilibrium in response to changes in the chemical (T, P, X) environment. cf.,
alteromorph, centripetal replacement
pseudotrilayer see alkylammonium organoclay pseudotrilayer
pyroelectric a property of crystals where an electric dipole moment develops in response to a
temperature change. The material cannot exhibit temperature gradients, and the property
diminishes over time at temperature. Only polar crystal classes exhibit this property. An
analogous magnetic property, “pyromagnetism”, can also exist.
pyromagnetism, see pyroelectric
pzc see point of zero charge
quantum calculation a calculation of atomic or molecular properties of a model system based
on first principles (ab initio) quantum mechanics. Because quantum calculations involve either
the solution or approximation of the Schrodinger equation to determine the electronic structure,
this is the highest level of theory that can be used to calculate molecular-scale properties of a
model system. Cf., density functional theory
quasi-coating In micromorphology of soils, quasi-coating is a pedofeature that consists of a
layer of material related to surfaces (i.e., voids, grains, aggregates) but not immediately adjacent
to the surface. Syn., quasi-cutan. Cf., hypo-coating
quasi-cutan see quasi-coating
quaternary alkylammonium salt alkylammonium salts, such as hexadecyltrimethylammonium
bromide, have four alkyl or methyl groups attached to nitrogen with a permanent positive charge.
The similar n-hexadecylamine hydrochloride used in the alkylammonium layer charge method is
only positively charged in neutral to acidic solutions. See alkylammonium layer charge method
quaternary phosphonium salt a quaternary organic salt based on the phosphonium (PH4+) ion.
Tetramethyl phosphonium chloride is the phosphonium analog of tetramethyl ammonium
chloride. Cf., quaternary alkylammonium salt
quick clay Landslide-prone, silty-clay to clayey-silt size sediments that become liquid upon
failure (shear strength <0.5 kPa). Pre-failure strength exceeds post-failure strength by 30
(minimum) to hundreds of times. Quick clay is not thixotropic; it cannot reform after failure.
‘Quick clay’ applies only to the undisturbed material.
Quick clays of Scandinavia, Eastern and Western Canada, and Alaska developed in fine-grained, glacial rock flour that accumulated in marine and brackish water during Pleistocene
glacial retreat (Torrance, 2012). The salt induced a flocculated microstructure that gained
strength as additional sediment accumulated. The water content approximates the high-salinity
liquid limit of the sediment. Quick clays in the down-glacial-flow direction from the magnetite-rich, iron-ore deposits of Quebec and Labrador gained unusually high undisturbed strengths
owing to iron-oxide cementation. Isostatic uplift elevated the deposits above sea level, and an
oxidized weathered crust formed. In broad, level areas, downward percolation of rain displaced
the salty pore waters. In areas adjacent to uplands, artesian pressures have displaced the salt
upwards to the surface drainage system. The oxidized zone is thinner where salt removal was
upward than where it was downward. During salt removal, the structure and water content
remained nearly unchanged, whereas the liquid limit of the sediment decreased to its low salinity
value, and the liquidity index increased from about 1.0 (high salinity) to 1.2 – 4+ (low salinity).
At liquidity indices >2, the thoroughly disturbed material flows like motor oil.
Chemical, mineral and microstructural factors are critical to quick-clay development. The
mineralogy must be dominated by “low activity minerals” (illite, chlorite, quartz, feldspars,
amphiboles, iron oxides, and carbonates). Other requirements include: flocculated
microstructure (salt-induced), leaching of salt (which decreases the liquid limit), and reducing
conditions that inhibit the formation of swelling clays. Experiments to produce quick clay using
‘pure’ clay, such as illite, have been unsuccessful, but using dispersed material from what had
once been ‘quick clay’ has succeeded. Whereas non-swelling phyllosilicates must be present,
clay-sized primary minerals also appear to be necessary.
A mineralogical variant is a quick clay that developed contemporaneously in marine
sediment in Japan that is dominated by low-activity (<1.1), high-ferrous-iron, non-swelling clay,
and volcanic ash (Torrance and Ohtsubo, 1995). However, where oxidizing conditions develop
that allow conversion of the ferrous iron to ferric iron to decrease the layer charge sufficiently to
produce a swelling smectite, the resultant liquid-limit increase renders the oxidized zone no
longer ‘quick’. Cf., liquid limit, liquidity index, plasticity index, quick-clay landslide, shear
strength, thixotropy, Udden-Wentworth scale.
quick-clay landslide A landslide in which quick clay plays an important role. They usually start
with a small triggering landslide along a riverbank or terrace, but may also be triggered by human
actions or earthquakes. “Flow quick-clay landslides” occur where a substantial depth (a few
meters, or more) of quick clay underlies a relatively thin surface-weathered zone. Flow failures
commonly occur stepwise and retrogressively over a substantial time period (minutes to more
than an hour). The liquid debris carries the thin crust out of the landslide scar and along the river
valley. Very little debris remains within the scar. Flows are the norm in Scandinavia and
constitute about half of the quick-clay landslides in eastern North America. In “spread quick-clay landslides”, the non-quick overburden is thicker and the large chunks of crust are difficult to
transport. Once failure is initiated, the failure plane advances rapidly into the quick-clay zone
and, as the quick clay liquefies and starts to move, the overburden breaks into a series of slices
that are oriented perpendicularly to the direction of movement. In most cases, a large proportion
of the landslide debris remains within the landslide scar. Ridges of nearly intact slices alternate
with zones of liquid and plastic debris to create a ribbed, horst and graben-like topography.
Spread-type landslides are rapid (tens of seconds to minutes in length). Cf., quick clay
Ramsdell-style notation a method commonly used to describe phyllosilicates, where a set of
related polytypes is designated by a single name, usually a species name or a group name,
followed by a structural symbol suffix that defines the layer stacking differences (after Guinier et
al., 1984). The symbolism is based on the number of layers (first part of the suffix), which is
followed by an italicized capital letter that defines the crystal system: C (= cubic), H (=
hexagonal), T (= trigonal with hexagonal Bravais lattice), R (= trigonal with rhombohedral
Bravais lattice), Q (= quadratic or tetragonal), O (= orthorhombic, previously Or), M (=
monoclinic), and A (= anorthic or triclinic, previously Tc). A subscript “d” indicates disorder and
a subscript “1" or “2" indicates that another polytype exists with the same number of layers and
symmetry. Cf. Ramsdell-style notation for chlorite
Ramsdell-style notation for chlorite a method commonly used for chlorite where a set of
related polytypes is designated by a single name, usually a species name (e.g., clinochlore,
chamosite) or the group name (in this case, chlorite), followed by a structural symbol suffix that
defines the layer stacking differences. Unlike the Ramsdell-style notation for phyllosilicates, the
chlorite notation was developed for one-layer polytypes; although multi-layer chlorite polytypes
are known, they are rare. The first part of the symbolism (I or II) designates the orientation of the
interlayer sheet, the italicized second part (a or b) describes how the interlayer sheet cations
project on to the hexagonal ring of the adjacent 2:1 layer, and the third part (1 through 6)
indicates how the next 2:1 layer resides relative to the interlayer sheet. Although there are 24
possible combinations of regular one-layer polytypes, only 12 of these are unique. A dash
separates the second and third parts of the symbol, when the third part can be determined. Some
polytypes do not have 2:1 layers that are symmetrically disposed about the interlayer, in which
case the second part of the symbol may be given as ab or ba. Examples: clinochlore-IIb-4,
chamosite-Ibb, pennantite-Ia. Cf. Ramsdell-style notation
reaction order The order of a reaction is empirically observed and determined by the sum of the
powers of the concentrations of the rate equation. If the rate of a reaction, R, is proportional to
the concentration of one species, R = k[A], where k = constant and [A] = concentration of
species A, then this is a first-order reaction. A second-order reaction may have a rate equation of
R = k[A][B] for two reactants, A and B, and the powers of each sum to 1 + 1 = 2. Sometimes,
the rate determining step can be deduced from knowing the reaction order of a series of reactions.
reciprocal lattice The reciprocal lattice, first constructed by P. Ewald, involves a set of points,
each of which represents a set of planes in space, 1/d from the origin. The value of d is the
spacing between a set of planes in a unit cell. This lattice is useful to better visualize a diffraction
pattern and its geometric relationship to the unit cell of the crystal under study. The relationship
is obtained from the modified Bragg equation (1/dhkl = 2sin θ/λ), which is the condition where a
possible X-ray reflection can occur. Thus, the point located at 1/d represents the cross-section of
the pole of this set of planes, hkl, and corresponds to a possible X-ray reflection from the crystal.
The unit cell as determined by the reciprocal lattice (referred to as the “reciprocal unit cell”), by
construction, is defined in relation to the unit cell of the atomic structure (referred to as a the
“direct cell” or “real cell”) of the crystal under study: a* is perpendicular to the plane containing
b and c, b* is perpendicular to the plane containing a and c, and c* is perpendicular to the plane
containing a and b, where the * (referred to as “star”, as in “a star”, “b star”, etc.) indicates a
reciprocal lattice measurement. Cf., crystallographic axis, Bragg’s law
recrystallization a) Senso stricto. solid-state transformation(s) of crystalline material to another
crystalline material. In this process, larger, more defect-free grains result than the predecessor
grains. Although the bulk composition does not change, the resultant assemblage may be of the
same mineralogy or different (e.g., polymorphs) mineralogy. In rocks, this is a mechanism by
which plastic deformation can ultimately produce, via recrystallization, an assemblage of strain-free grains. It is unclear if recrystallization is truly “solid state” because the process may involve
the formation in inter-granular fluid films. A “secondary recrystallization” also can result where
there is an increase in particle size of grains by subsuming neighbors. It is unlikely that clay
minerals transform in this manner; low-temperature transformations involving clays usually
require the presence of water. b) Senso lato. Conversion of pre-existing chemical and
mineralogical composition (either crystalline, poorly crystalline, or amorphous) either to new
crystalline material of the same mineralogy or to a new phase assemblage, commonly involving
limited amounts of aqueous fluids. For clays, the crystal-surface energy to crystal volume is
reduced to drive recrystallization, even at low temperatures. See Ostwald ripening
reduced cell see unit cell
reflection see mirror plane
refractory material A refractory material retains its chemical and physical properties at high or
very high temperatures (ASTM, v. 15.01 indicates high temperature at >1,000 oF). The high-temperature materials are generally non-metallic and are commonly composed of, but not limited
to, oxides of aluminum, calcium, magnesium, and silicon. Refractory materials are common in
linings for furnaces, kilns, and incinerators, and are used in some crucibles. Fire clays are often
used to manufacture refractory materials. See fire clay
refractory clay see fire clay, refractory material
regolith crustal material above unweathered bedrock, including unweathered rock where it is
entirely surrounded by unconsolidated or weathered natural material. The term is not related to
the geologic age of the bedrock, the individual constituent materials of the regolith, or the
formation or assembly of the ensemble of constituent materials.
regular volatile matter (RVM) an industrial term referring to dehydration of untreated, porous
clay until it contains between ~5 - 20 wt % free moisture, with dehydration achieved by heating
below or near the boiling point of water (< 105 °C) to preserve the integrity of the clay and to
create empty pore spaces so that the material may resorb fluids.
Reichweite (or R, R0, R1, etc.) literally means “the reach back”. The expression of both the
probability of finding a B layer after an A layer in a two-component system of layers containing
A and B layers, and the influence A has on the identity of the next layer, after Jadgozinski, 1949.
When flipping a coin, R is equal to 0 because there is no influence at all of one flip on another.
The probability of getting a head depends only on the proportion of heads and tails, in this case
0.5, and thus there is a null relation between the influence of A on B. For perfect order of 50% I
(illite) and 50% S (smectite) layers, ISISIS... the R = 1. A sequence of ISII... is R = 3, with one S
layer surrounded by three I layers. Common usage is R0 for R = 0, R1 for R = 1, etc.
relative humidity see humidity
relict soil a soil formed on a preexisting landscape under a previous pedogenetic regime, and not
subsequently buried by geologically younger materials. See also paleosol.
remolded quick clay “Remolded quick clay” refers to the material after the flocculated
microstructure of the quick clay has been destroyed. By definition, remolded quick clay behaves
as a liquid. The flow properties of remolded quick clay are required for analysis of the flow
behavior of landslide debris as it leaves the landslide scar and flows. Most remolded quick clays
behave in the ‘modern viscometric’ thixotropic manner of shear resistance increasing and
decreasing, respectively, as the shear rate increases and decreases. The change is never more than
a trivial portion of the quick clay strength prior to collapse. Remolded silt-rich, quick clay may
exhibit extreme dilatancy at high shear rates, leading to shear blockage. Cf., quick clay, quick
clay landslide, dilatancy
residual soil soil formed in place by in-situ weathering of parent rock material. Cf., sedimentary
residual strength The residual strength is the shear resistance or the capacity of a clay, soil, or
rock to withstand significant pressure that develops after strain-softening at large strains.
Residual strength is controlled by the characteristics of the internal failure surface where either
significant particle movements have occurred (as in soils) or the fracture interface is the peak
failure surface (as in rocks). The residual strength is typically smaller than the peak failure
strength of a clay, soil, or rock.
reticulate striated b-fabric see b-fabric
reticulate a texture visible at hand-specimen or thin-section scale in which one generation of
secondary minerals or soil plasma is arranged in a net or network geometry. See soil plasma
rheology the study of the deformation and flow of materials. For clay scientists, this may
involve the study of the plasticity (i.e., creep, rupture) of clay, clay-water interactions, clay
suspensions and interparticle forces, the electrical double layer, etc.
rotation axis see rotation symmetry
rotation symmetry symmetry involving a repetition of features about an axis. The axis type
may be either a one-fold, two-fold, three-fold, four-fold, or six-fold axis, where the angle of
repetition may be determined by 360/n, where n is equal to the axis type (1, 2, 3, 4, or 6). A one-fold rotation axis is the identity operation. Another name for this type of axis is “proper rotation
axis”. Cf., symmetry, rotoinversion axis
rotational stacking disorder Phyllosilicates commonly show rotations between adjacent layers
involving n60° (where n is an integer from 1 to 6), and with registry or partial registry between
layers. Stacking disorder or partial disorder involves the lack of regularity in the n value.
Equivalence to this type of stacking order/disorder may be achieved also by random or partially
random translations along the pseudohexagonal a or b axes (commonly along a, and very rarely
along b) of the layer silicate. See turbostratic stacking
rotoinversion axis A general type of symmetry axes that involves a rotation followed by an
inversion operation, with a repeating set of rotation/inversion operations until “closure”
(returning to the starting position in the rotation/inversion or repetition process) is reached. The
one-fold rotoinversion axis () is equivalent to a center of symmetry, and the two-fold
rotoinversion axis () corresponds to a mirror plane; only the latter are used, respectively. In
addition, the implies that the object possesses a center of symmetry and a 3-fold axis and a is
equivalent to a 3-fold and a mirror plane perpendicular to it. Thus, only the is a unique
symmetry operation. Cf., symmetry
rubification a weathering process wherein minerals in a soil release iron which subsequently
precipitates to form free iron oxides, such as hematite (red) and goethite (yellow brown), to
produce reddening of a soil horizon.
RVM see regular volatile matter
safety glass see tempered glass
saprolite A residual, sedentary, in situ regolith developed by chemical weathering of rocks, most
often primary crystalline (igneous and metamorphic) bedrock. Saprolite preserves parent-rock
textures in the form of abundant, predominantly argillaceous, and commonly pseudomorphous
weathering products of individual primary minerals. Preservation of parent-rock mineral textures
and fabrics in saprolite is often associated with high microporosity and may be a consequence of
isovolumetric weathering. Saprolites are typically some meters thick but can be hundreds of
saturation see saturation (water)
saturation index a measure of how far from equilibrium a solution is with respect to a given
mineral. The saturation index, SI, is determined from SI = Q/Keq, where Q is the reaction
quotient and Keq is the equilibrium constant, often given as log10 (Q/Keq). When SI = 0, then the
system (water + mineral) is at equilibrium, if SI < 0, the mineral will dissolve, and if SI > 0, then
the mineral will precipitate.
saturation (water) the condition where the voids within a material are filled with water. The
degree of water saturation is the ratio of pore water volume to the total pore volume, usually
expressed as a percentage.
Schottky defect see point defect
screw dislocation see line defect
screw axis symmetry involving a rotation about an axis followed by a translation parallel to the
axis. The rotations correspond to a 2-, 3-, 4- or 6-fold axis and requires a translation following
each rotation. The symbol for a screw axis is nm, where n is the rotation component and m/n is
the translation component. For example, in a 41 screw axis, the translation is 1/4 of a unit cell
and the rotation corresponds to a 4-fold axis. Thus, symmetry consistent with a 41 screw axis is
generated by a 90 o rotation followed by a 1/4 cell translation, with the process continuing until
closure. Cf., rotation symmetry, rotoinversion axis, symmetry, unit cell
seat rock a rock or sedimentary layer underlying a coal bed with physical characteristics
suggesting that it served as a soil which supported the vegetation that produced the coal.
(Modified from Huddle and Patterson, 1961)
sedimentary clay In civil engineering, a clayey soil formed by transport and sedimentation of
particles by water, air, or ice. Loess is an example of a sedimentary soil because it forms via
wind transport and sedimentation. Sedimentary marine clay forms by deposition of fine-grained
particles through seawater.
septechlorite an invalid term, use kaolin-serpentine group (Bailey, 1980)
seat earth a British term for underclay Cf., underclay
secondary mineral a mineral formed by an alteration process; the chemical constituents of a
secondary mineral may be derived from the decomposition of a primary mineral and/or derived
from outside the system during the alteration process. Cf., primary mineral
secondary crystal growth crystal growth from nanoparticles to larger particles that occurs by an
assembly of aggregates, sometimes known as “oriented attachment”.
self-activating clay an organoclay + organic solvent with an optimum gel strength that has not
been augmented by additives, such as polar molecules (e.g., ethyl alcohol, acetone, propylene
carbonate). See gel strength, organoclay
self-dispersing clay Self-dispersing organoclays are clays that have additives to develop
viscosity at much lower shear and energy input than other, non self-dispersing organoclays. An
example of additives to form an appropriate complex are EDTA(ethylenediaminetetraacetic acid)
and dimethyldihydrgenated tallow quaternary ammonium. This molecule intercalates into the
organoclay and props the interlayer open to allow exfoliation.
sensitivity (St) A geotechnical term that evaluates the degree of strength loss when natural
silty-clay and clayey-silt size sediments are thoroughly disturbed by natural or human actions.
Sensitivity (St) of sediments is the ratio of the undisturbed in-situ shear strength to the shear
strength after thorough remolding. Generally, the higher the sensitivity, the greater the
geotechnical challenge, for example, in leading to flow landslides or excessive consolidation.
sepiolite-palygorskite group a group name for phyllosilicates with characteristics that are
dominantly fiber-like, but with some plate-like character. The atomic structure has continuous
planes of basal oxygen atoms (thus forming the plate-like character) with the apical oxygen
atoms of the tetrahedra pointing alternatively in opposite directions away from the basal plane.
The tetrahedral apices link to partial octahedral sheets, discontinuous along one direction but
infinite in the other, to form a pattern of octahedral ribbons (thus forming the dominant fiber-like
character). These minerals are classified as modulated phyllosilicates. The width of ribbons may
vary, which leads to different numbers of octahedral cation sites per formula unit (5 for
palygorskite, 8 for sepiolite).
serpentine-kaolin a group name for platy phyllosilicates of 1:1 layer and a layer charge of ~ 0
per formula unit. Generally, the d(001) spacing is approximately 7.1-7.3 Å. The group is further
divided into subgroups that are either trioctahedral (serpentine) or dioctahedral (kaolin), and
these subgroups are further divided into mineral species based on chemical composition. The 1:1
layers are bonded by long hydrogen bonds (~2.9 Å) and possible coulombic interactions between
the octahedral sheets of one layer and the tetrahedral sheet of the adjacent layer. See “group
sesquan see cutan
shale Shale is a mudrock with a high proportion of clay and silt sized particles, that usually
exhibits lamination and fissility. When blocky and non-fissile the mudrock is named mudstone.
shear strength In soil science, shear strength is a measure of the ability of a soil to resist failure
along a surface when subjected to a critical combination of shearing and normal forces. The shear
strength has cohesive (commonly related to clay content) and frictional components (related to
the interaction of angular particles). Pore water pressures also play a role.
sheet For phyllosilicates, a sheet refers to corner-sharing linkages of tetrahedral coordination
polyhedra (i.e., tetrahedral sheet) or edge-sharing linkages of octahedral coordination polyhedra
(i.e., octahedral sheet). For a tetrahedral sheet, three corners of a tetrahedron are shared with
other tetrahedra and the fourth corner may point in any direction. See Guggenheim et al. (2006)
and references therein. Cf., tetrahedral sheet, octahedral sheet, layer
shrinkage a ceramics industry term describing the reduction in size of a solid body relating to
dehydration during initial drying and/or devolatilization during the final firing stage of making
the ceramic body (e.g., bricks, whiteware, sanitary articles, porcelain, and stoneware).
shrinkage limit The shrinkage limit is the boundary water content that separates the semi-solid
state of clay from its solid state in the classification of a fine-grained soil. At this water content,
further drying of the clay does not cause a change in its total volume (i.e., no shrinkage).
However, if the water content is greater than the shrinkage limit, drying causes the clay to shrink.
The shrinkage limit is one of the three Atterberg Limits (i.e., liquid, plastic, and shrinkage
limits). Cf., Atterberg Limits
SI International System (of units); Système International d'Unités
silan see cutan
silane coupling agent a class of reagents characterized by a central silicon atom coordinated
tetrahedrally to four R groups. Such compounds are commonly used to bond an inorganic
substrate, such as clay minerals, to a polymer. In the most reactive form, two or three of the R
groups are chlorine atoms and the remaining are organic groups. The chlorine atoms in these
compounds are very reactive with water and hydrolyze readily to form condensation polymers
with Si-O-Si backbones, or the chlorine atoms can react with hydroxyl groups on clay mineral
surfaces with loss of HCl. A less reactive form has two or three of the R groups as methoxy or
ethoxy groups with the remaining R group being another nonlabile organic group. These silanes
can be hydrolyzed to form similar polymers or reactions with hydroxyls on clay minerals, but
generally require heat and vacuum to drive the condensation reaction. These agents allow the
surface of hydroxyl containing compounds to be converted to hydrophobic or reactive surfaces
depending on the character of the organic R group.
silanol group a surficial SiOH group, which is able to incorporate or dissociate protons. A
ferronol group (FeOH) is an additional surface group that behaves similarly to an aluminol or
silanol group. See aluminol group
silica Silica refers to SiO2 chemical composition only and the term does not connote structure.
The term commonly is used to describe a mineraloid or a glass, as in a silica-rich glass. Thus,
silica does not specifically refer to quartz or opal (but opal has a structure involving small, non-crystalline silica spheres). Cf., mineraloid
silicosis lung fibrosis caused by the inhalation of dust containing crystalline silica (SiO2) in any
of its forms (e.g., quartz, cristobalite, tridymite, coesite, stishovite).
silt A general term for any non-consolidated, clast-rich material that consists of >50% primary
particles with diameters of 4-63 µm (American geologists), 2-63 µm (European geologists), or 2-50 µm (American soil scientists). See siltstone
siltstone A sedimentary rock with primarily silt-size components; siltstone rocks may show
sedimentary structures, such as flow structures and cross-bedding. Siltstones are often, but not
always, chemically cemented. See mud, mudstone, silt
sintering bonding of powdered material by solid-state reactions at temperatures lower than
site energy energy required to separate an ion an infinite distance from its equilibrium position
in a crystal. In calculating an electrostatic site energy, the site energy is the sum of all the
Coulombic and all the repulsive energies between the ion in the site and all neighbors in the unit
skeletal microfabric see microfabric, clay
slake see slaking
slaking the breakdown of large soil or clay material aggregates (typically >2-5 mm) or fine-grained sedimentary rock rich in clay into small particles (<0.25 mm) when rapidly immersed in
water. Slaking occurs when aggregates are not sufficiently strong to withstand internal stresses
caused by rapid water uptake into the pore structure or fabric of the aggregate. Internal stresses
result from differential swelling of clay particles, trapped and escaping air from pores, rapid
release of heat during wetting, and the mechanical action of moving water. Slaking is influenced
by the presence of smectitic clays (either smectite, especially Na-montmorillonite, or
interstratified clay with a smectite component) that shrink when dry and swell when wet.
Organic matter often reduces slaking by binding the particles or by slowing the rate of surface
slip See blunging
smectite a group name for platy phyllosilicates of 2:1 layer and a layer charge of ~ -0.2 to -0.6
per formula unit. Generally for natural samples, the d(001) spacing is approximately 14.4-15.6
Å, although other spacing may occur depending on H2O retention and interlayer occupancy. The
group is further divided into subgroups that are either trioctahedral (according to Bailey, 1980,
this subgroup name is “saponite”) or dioctahedral (subgroup name of “montmorillonite”,
according to Bailey, 1980), and these subgroups are further divided into mineral species based on
chemical composition. Guggenheim et al. (2006) did not give subgroup names. Smectite
minerals have large specific surface areas (10 - 700 m2/g) and exhibit a high expansion (swelling)
capability in the presence of H2O. Smectite and vermiculite minerals are often referred to as
“swelling” or “expandable” clay minerals. Cation-exchange capacity or solvation of polar
molecules is large. Smectite is commonly a primary constituent of bentonite (see bentonite for
respective genesis information) and pelitic sediments (e.g., shales) and occurs in soils. Prior to
circa 1975, the smectite group was called the montmorillonite-saponite group, but this
nomenclature was abandoned because of the confusion between the use of the same name for
both a group and species. Very early (Kerr and Hamilton, 1949), smectite was used as a term for
fuller’s earth (initially), montmorillonite, and certain bentonitic clay deposits. See “group
smectite-to-illite transition the conversion of smectite to illite involving a significant change in
the chemical composition of the 2:1 layer from a smectite-like composition to an illite-like
composition. The resulting net negative charge on the layer must become sufficiently large to
preferentially fix potassium or other large cations and cause dehydration of the interlayer. The
process of conversion may be one of apparent solid-state transformation or mostly in the solid
state or apparent neoformation via a solution. The degree of conversion may be useful to
understand crystallization parameters during burial diagenesis or metamorphism (e.g., Hower et
al. (1976) or hydrothermal activity (e.g., Inoue et al., 1987). Cf., illite/smectite
smoke see suspension
soil plasma In soil science, plasma refers to fine-grained (too fine to resolve individual
constituents by visible-light microscopy) crystalline or amorphous materials in soils, which
includes saprolites. See saprolite, clay groundmass, Syn. pedoplasma
soil, residual in soil science, describing soil formed in place by in-situ weathering of rock parent
material. Cf., clay (sedimentary).
sol or clay solution see suspension
solid emulsion see suspension
solid foam see suspension
solid solution isostructural phases that show a compositional variation whereby one or more
ions can substitute at a specific atomic site in a crystal structure. In fact, the ions can also be
elements or ionic groups, and it is possible that more than one atomic site may be involved. A
common type of “substitutional solid solution series” shows a miscibility between two end
members, with permissible variations in composition between these two end members. For
example, olivine (Mg,Fe)2SiO4, represents a chemical variation (i.e., solid solution) between two
end members, forsterite Mg2SiO4 and fayalite Fe2SiO4, where Mg and Fe can substitute for one
another in two atom sites. The chemical formula may be written as Mg2-xFexSiO4, where x
represents the Fe content, and this value can vary between 0 and 2.0. Other types of solid
solutions include “interstitial solid solution” where atoms may reside in interstices between the
ideal locations of the atoms of a structure and “omission solid solution” where a site is
incompletely filled. “Limited solid solutions” may occur where the compositional variation does
not range entirely between the two end members. In addition, two or more substitutions
(“coupled substitutional solid solution”) may occur over multiple sites so that the requirement of
overall charge neutrality in a phase is satisfied, for example, where Na+ + Si4+ substitute for Ca2+
+ Al3+ in the plagioclase feldspar structures between end members albite, NaAlSi3O8, and
solid solution, coupled substitutional, see solid solution
solid solution, interstital, see solid solution
solid solution, limited, see solid solution
solid solution, omission, see solid solution
solid solution, substitutional, see solid solution
solid-state diffusion a process of mass transfer where atoms, ions or molecules move through a
solid. Cf., absorbent, adsorbate, cation exchange
solidus in a phase diagram, the solidus describes the upper limits where only solid phases exist.
Cf., liquidus, phase diagram
soluan see cutan
soluble salts Unprocessed clay samples commonly contain soluble salts, including chlorides,
sulfates, and carbonates, which should be removed either prior to analysis or prior to drying or
firing. Such salts can inhibit deflocculation, and they may interfere with the determination of
particle size. In addition, they may prevent the preferred orientation of clay-mineral aggregates
in preparation for some X-ray analysis procedures. During drying, soluble salts migrate to the
clay body surface and can interfere with glazing; after firing, these salts can cause efflorescence.
sorbent a generic term for materials displaying a high degree of absorptive and/or adsorptive
physicochemical properties, often useful in consumer or industrial applications.
Southern Bentonite Initially, an industrial or commercial term, designating calcium bentonite
from Mississippi, USA. This material exhibits characteristics of low water adsorption, dilation,
viscosification and high green-sand binding strength. Use of the term has broadened to include
any bentonite having performance characteristics equal to that of high quality calcium bentonite
from Mississippi. Whereas the term “Southern Bentonite”, and its synonyms, continue to have
meaning in the industrial and commercial realm, they are scientifically obsolete and should not
be used in that context. syn: Mississippi Bentonite
space group the symmetry used to describe a three-dimensional crystal structure, including both
translation-free symmetry operators, translational symmetry (i.e., screw axes and glide planes),
and Bravais lattices. There are 230 space groups. In mineralogy, Hermann-Mauguin symbols are
used, whereas in chemistry and spectroscopy, Schoenflies symbolism is used to characterize the
different combinations of symmetry elements.
specific gravity a dimensionless physical parameter used to measure the relative density of
solids, including minerals and soils. Specific gravity is the ratio of the unit weight (or density) of
a solid to the weight (or density) of an equal volume of water at 4 oC.
specific heat capacity see heat capacity
speckled b-fabric see b-fabric
spectroscopy the study of the interaction of radiation with matter
sponge microfabric see microfabric, clay
spray drying in scientific research, a drying technique used primarily to minimize preferential
orientation of clay in X-ray analysis methods. Spray drying involves high dispersion of a solid
(i.e., clay), usually suspended in a water-organic binder, where the binder is subsequently
evaporated. Although effective, the process usually involves large sample sizes (typically
grams). In industrial applications, spray drying is used to economically produce commercial
quantities of dried clay having a controlled aggregate particle size, from a liquid clay suspension,
without the need for mechanical grinding.
spread quick-clay landslide see quick-clay landslide
stability constant see intrinsic stability constant
state function In thermodynamics, a state function is one that is not dependent on the path (i.e.,
history) that the system has undergone and is only dependent on the state of the system as
determined by temperature, pressure, volume, etc. at a particular moment.
Stern layer Helmholtz and later Stern divided the solution side of a diffuse double layer into an
inner part (Stern or Helmholtz layer) and an outer part (Gouy or diffuse layer). In a more simple
model, the ions with a finite size "d" adsorb on a surface and form a so-called outer Helmholtz
plane. In the so-called triple layer model, the Helmholtz plane is subdivided into an inner
Helmholtz plane (iHp, where specifically adsorbed ions such as protons or K on high-charged
clay minerals reside) and an outer Helmholtz plane (oHp, where non-specifically adsorbed ions
reside). Note that for all models the following equation holds:
Sigma_zero + Sigma _iHp + Sigma_oHp + Sigma_diffuse = zero.
where: Sigma_zero denotes the charge density (in Coulomb per square meter) of the solid,
Sigma_iHp denotes the charge density of species adsorbed in the inner Helmholtz plane (usually
specifically adsorbed ions), Sigma_iHp denotes the charge density of species adsorbed in the
outer Helmholtz plane (usually nonspecifically adsorbed species), and Sigma_Diffuse is the
charge density of the diffuse double layer formed in the solution. See diffuse double layer
stipple-speckled b-fabric see b-fabric
Stokes’ law a mathematical expression that describes how a spherical particle, nominally less
than 20 micrometers, settles in a viscous fluid. This equation is used in clay science, although
clay particles are generally flat plates and not spherical, and thus fall at slower velocities than
spheres of equal density as calculated using Stokes’ law. Cf., equivalent spherical diameter
strata plural of stratum; see stratum.
stratum a layer of sediment or sedimentary rock (pl. strata).
stress, effective soil see effective stress, soil
stress path the graphical representation of the loading or unloading processes that a clay has
experienced in a triaxial stress condition. The stress path is usually plotted in the coordinate
system of mean effective stress and shear stress. The mean effective stress is the average of the
vertical and horizontal effective normal stresses, whereas the shear stress is the difference
between the vertical and horizontal effective normal stresses.
strial b-fabric see b-fabric
striated b-fabric see b-fabric
structure the accepted atomic, ionic, and molecular arrangement of atoms of a material. (Quot
Guggenheim et al., 2006)
structured clay A structured clay is a clay that exhibits stiffened or strengthened behavior,
usually caused by microstructure features, such as cementation, aggregation, ageing, or
flocculation. Usually, a structured clay exhibits a higher yield stress, Young’s modulus, and
failure strength than a non-structured clay. Re-sedimentation of the same material in the
laboratory usually destroys the in-situ structure formed in the natural, non-processed clay.
subcell see supercell
subhedral see crystal
sublimation the physical process where a solid phase transitions directly to a gas phase without
going through a liquid phase.
submerged unit weight see unit weight, effective
sulfo-peloid see peloid
supercell A supercell is where, in an atomic structure, an approximately repeating subunit
occurs within a larger cell (the unit cell). The subunit is referred to as a subcell and the larger
cell is referred to as a supercell. In molecular simulations, the definition is different: a subcell is
a unit cell that is expanded by multiple repeats to generate a larger supercell. Calculations are
then made on the supercell. Supercells are often used in molecular simulations to reduce or
eliminate the effect of nearest neighbors in adjacent cells by considering an expanded atomic
structure. Cf., unit cell
superheating an industrial term for the high-temperature heating of porous clays to remove
adsorbed water by volatization, in addition to partial dehydroxylation (structural water) of the
clay minerals present. For most clay minerals, the point where the rapid loss of structural water
first occurs is often referred to as the onset of LVM characteristics. Termination of heating at
this or a somewhat higher temperature commonly generates a material whose porous bulk fabric
remains intact and which does not readily slake in water. Unlike calcination, which implies a
complete dissociation reaction, superheating produces limited dehydroxylation without
destroying plasticity (i.e., without destroying clay-like properties). Thus, LVM clays will still
display a measurable weight loss on ignition. Cf., low volatile matter (LVM); slake; calcine; loss
on ignition; water, adsorbed; water, structural; water, zeolitic
surface complexation There are two kinds of surface complexes, one with no H2O molecules
interposed between it and the mineral surface, termed an inner-sphere complex, and one in which
at least one H2O molecule is interposed, and this is termed an outer-sphere complex (Sposito,
1989). Inner-sphere complexes are chemically bonded; outer-sphere complexes or those that exist
in the diffuse layer are said to by physically adsorbed (Parks, 1990).
surfactant a wetting agent that lowers the surface tension of a liquid or lowers the interfacial
tension between two liquids. Detergents are an important economic group of wetting agents that
can affect clay surfaces.
surface a boundary, planar or near planar, between two phases. The term is often used to
connote planar or near planar interfaces between a condensed phase (solid) and gases, liquids, or
other solids, or between any two phases (e.g., liquid-liquid, gas-gas, liquid-gas). Surfaces
generally have an interfacial energy term and a chemical compositional gradient existing from
the surface, and a discrete molecular entity or multi-component substance may exist on or at the
surface. In clay science, clay surfaces include “external surfaces” where there are broken bonds
at particle edges and “internal surfaces” at the junction between the layer and the interlayer. The
external surfaces include the broken-bond particle edges and the terminating basal surface.
Internal surfaces, although a term commonly used in clay science, may not strictly adhere to the
above definition because the material (or lack of material) in the interlayer is not necessarily a
“phase”, but the atomic arrangement (or its behavior) in the interlayer is sufficiently distinct from
the layer that the term is useful (e,g., the discrete molecular entity or multi-component substance
addressed above). Internal surfaces are readily accessible to the environment outside the particle
(often owing to the small particle size) and may have characteristics consistent with the
permanent layer charge of the particle and interlayer cation size and charge. Environmental
characteristics, such as water activity (e.g., pH, relative humidity), solute concentration, etc., may
influence the behavior of the internal surface. External surfaces are also affected by
environmental characteristics, but often the broken bonds affect the characteristics/structure of
the electrolyte nearest the external surface. Cf., interlayer, layer, phase
surface-controlled growth dissolution ‘surface controlled’ growth or dissolution requires the
rate-determining step in the growth or dissolution reaction to occur at the mineral surface; this
rate-determining step is in the form of an attachment to or a detachment from the surface of a
metal or metal-ligand ‘activated complex.’
suspension A two-phase system with a solid (“dispersed phase”) dispersed in a fluid
(“continuous phase”). In colloid chemistry, a suspension differs from a colloidal suspension (or
“sol”) by having particles >1μm. The term "suspension" is preferred over "clay solution" or
"colloidal solution" to avoid confusion with true solutions, which do not have an interface. Thus,
the presence of an interface between the solid and the liquid phase (in the thermodynamic sense)
is important. The table below lists names for systems with dispersed phases. See blunging.
Table of descriptive names for systems with dispersed phases (after Hiemenz and
Continuous phase Dispersed phase Descriptive names (* recommended)
gas liquid aerosol*, fog, mist
gas solid aerosol*, smoke
liquid gas foam
liquid liquid emulsion
liquid solid suspension*, sol, colloidal solution, gel
solid gas solid foam
solid liquid gel, solid emulsion
solid solid alloy
swelling clay mineral a clay mineral that can sorb large amounts of water and thereby expands
in volume. Both vermiculite and smectite are swelling clay minerals. Cf., swelling clay.
swelling clay a clay that can sorb large amounts of water and thereby expands in volume. The
swelling clay minerals, smectite and vermiculite, if they occur as fine-grained material, are
referred to as “swelling clays”. In industry, bentonites are commonly referred to as “swelling
clay”. Some clays that are referred to as “swelling clays” have been shown to be dominated by
clay minerals characterized by interstratifications of two varieties of phyllosilicate layers, one
being expandable via adsorption of H2O and one not, as in “swelling chlorite” which is
interstratified chlorite and smectite. Syn., expandable clay; Cf., swelling clay mineral
symmetry plane see mirror plane
symmetry is used to describe an object with a systematic repetition of features, and is
particularly useful to describe crystal shapes or atom locations in an object
syngenetic material formed contemporaneously with rocks that are associated or enclose
syntaxy a geometrically fixed intergrowth between two phases. Originally defined as between
two polymorphs only and extended to include an oriented intergrowth between any two phases.
Cf., epitaxy, topotaxy
system a region of space within the universe. Systems are considered in thermodynamic or other
studies to determine how a change in the environment (e.g., temperature changes, pressure
changes, etc.) will affect the system. Systems may be closed by encapsulating in noble metals,
placing a liquid in a sealed beaker, etc. A “closed system” is affected only by receiving energy
from or giving energy to the outside environment. An “open system” differs from a closed
system by an exchange of matter, in addition to energy. An “isolated system” receives neither
matter nor energy across the boundary.
system, closed see system
system, isolated see system
system, open see system
tactoid In the context of polymer/clay nanocomposites, a tactoid is any collection of “primary”
(i.e., the smallest division of the phyllosilicate particle that retains the chemical character of the
compound, either a 1:1 or 2:1 layer), colloidal-size clay particles, which are essentially acting as
tailings see gangue
talc layer inappropriate usage for a 2:1 layer, see layer
talc-pyrophyllite a group name for platy phyllosilicates of 2:1 layer and a layer charge of ~ 0 per
formula unit. Generally, the d(001) spacing is approximately 9.1-9.4 Å. The group is further
divided into subgroups that are either trioctahedral (talc) or dioctahedral (pyrophyllite), and these
subgroups are further divided into mineral species based on chemical composition. The layers
are bonded by weak van der Waals interactions. See “group names”
Tatatila-type montmorillonite A term first used by Schultz (1969) to describe a
montmorillonite with the same chemical characteristics of the Chambers-type montmorillonite
(i.e., a total net layer charge of -0.85 to -1.20 per unit cell [O20(OH)4] with a layer-charge
contribution from tetrahedral substitutions of between -0.15 to -0.50), but with higher
temperatures of dehydroxylation at 710-730 oC rather than at 660-690 oC. Use of this term is
obsolete. See Chambers-type montmorillonite. Terms used in this obsolete classification are:
Wyoming-type, Otay-type, Chambers-type, Tatatila-type, beidellite-type (ideal and non-ideal),
and non-ideal montmorillonite. Current nomenclature for montmorillonite is that it is an Al-rich,
dioctahedral smectite with an ideal structural formula of (Al3.15Mg0.85)Si8O20(OH)4X0.85.nH2O with
layer charge from primarily octahedral substitutions of Mg.
tempered glass glass that has been heat treated in a specific way such that its thermal properties
and mechanical strength are improved. This treatment often involves heating to near the glass
softening point followed by controlled cooling, resulting in a glass that will break into granular
fragments rather than sharp plates. Such glass is also known as “safety glass.”
tenacity resistance to breaking or deforming a crystal
tensile strength the maximum stress developed in a material by a pulling load at the point of
rupture, given as a load per cross sectional (e.g., kg per cm2)
terra cotta unglazed or glazed building blocks of either low- or high-fired clay, typically used as
ornamental features on buildings
terra rosa a red glaze made from hematite
tetragonal see crystal system
tetrahedral sheet A tetrahedral sheet contains continuous two-dimensional corner-sharing
coordination tetrahedra involving three corners and the fourth corner pointing in any direction.
The tetrahedral sheet generally has a composition of T2O5 (T = Si, Al, Fe3+, Be, B...). After
Guggenheim et al. (2006); see also references therein. Cf., octahedral sheet
tetrahedral layer inappropriate usage for a tetrahedral sheet. See tetrahedral sheet
tetrasilicic an invalid term, previously used as a classification of the micas where the number of
silicon atoms per formula unit is four per four tetrahedral sites, see Rieder et al. (1998). Cf.,
mica, true mica, brittle mica, interlayer-deficient mica, group names
thermal diffusion see diffusion. Cf., thermal conductivity
thermal conductivity rate of heat flow through a material. Heat flow is given as a unit of cross
sectional area, per unit of temperature and time along the direction of heat flow.
thermal analysis the recording of the change in temperature and/or mass of a material when
thermal expansion the change in volume of a material with increase in temperature. It is often
expressed as the coefficient of thermal expansion, which is the degree of expansion divided by
the change in temperature.
thermodynamics the study of energy (potential, kinetic and internal energy) and its conversion
to heat (involving work, determining forces, energy transfer, etc.) to characterize systems.
Chemical thermodynamics considers energy changes that occur from reactants to reaction
products or with physical changes of state during a chemical reaction. Thermodynamics
considers macroscopic qualities (temperature, pressure, volume, composition, etc.) and does not
provide mechanistic (atomic theory, atomic structure, molecules, etc.) understanding.
thin section Material, such as a sliced piece of mineral material, rock material, or soil material,
that is mounted on a glass microscope slide, and placed in the optical path of a polarizing
(petrographic) microscope is referred to as a “thin section”. The standard thickness of the
material mounted on the glass is 30 μm. See petrographic microscope.
thixotropy a) In the classic sense, thixotropy refers to a material that exhibits reversible sol-gel-sol-… behavior. For example, ‘chemically modified’ bentonite drilling muds used by the
petroleum industry are thixotropic. The property prevents the granular material (“cuttings”)
produced during drilling from settling out when drilling is stopped (briefly, or for substantial
periods), thus preventing the drill rod from seizing. b) The ‘modern viscometric’ meaning refers
to the increase in shear resistance when the shear rate is increased, and the decrease when the
shear rate is reduced. Most thoroughly dispersed clay materials above some relatively low
concentration exhibit this phenomenon. Cf., dilatancy, Newtonian fluid, quick clay
tonstein A kaolinite-rich rock formed by alteration of glassy volcanic ash or tuff deposited in an
organic-rich aqueous environment, such as those associated with coal-forming environments.
topotaxy a geometrically fixed intergrowth between a reactant and one or more of its products
which resulted from a solid-state transformation. Different authors have suggested that the
product phase(s) should maintain orientation a) of most atom positions or b) of symmetry axes of
the reactant phase. c) The reaction involves conversion throughout a single crystal. Cf., epitaxy,
total charge see point of zero charge
trans-vacant A trans-vacant phyllosilicate is dioctahedral with the vacancy ordered to the site
where the OH,F anions are on opposite octahedral corners (i.e., trans orientation). Trans sites
are located on the mirror plane of an ideal layer in a phyllosilicate. Cf., cis-vacant
triclinic see crystal system
trace element Mineralogical term: an element found in small quantities in a mineral and not
considered essential because it is not required for the structure to exist. A trace element is not
quantitatively defined, but is generally considered to be <1wt. %. b) Medical/biological term: an
element that occurs in animals or plants in small quantities and is required for physiological
health. Cf., essential trace element
transformation The phenomenon by which primary minerals in general, and chain silicates and
phyllosilicates in particular, alter usually by weathering to secondary minerals through the direct
incorporation and reuse of some part of the structure of the parent mineral. Products of
transformation reactions are often pseudomorphous or alteromorphous after the primary reactant
(parent) mineral, and often exhibit regular crystallographic and orientation relations with the
reactant mineral. See neoformation, inheritance
Transition State Theory (TST) If an energetically unstable complex is present, TST states that
a transitional state, or activated complex, occurs. This activated complex is a transitional state
between the reactants and products, and is considered a hypothetical way to develop or explain
the kinetics of, most commonly, a single-step (elementary) chemical reaction. The transitional
state represents an energy barrier that must be overcome for mineral growth or dissolution. Cf.,
transparent describes a material that allows radiant energy to pass through it without significant
adsorption, scatter or reflection. Cf., opaque, translucent
tri,dioctahedral chlorite a species of the chlorite mineral group that would have a trioctahedral
2:1 layer and a dioctahedral interlayer. There are no known chlorite structures of this type.
Bailey (1988) described franklinfurnaceite, which has Ca between the 2:1 layer and the interlayer
and thus is not a true chlorite, as tri,dioctahedral if the Ca is not considered. Cf., dioctahedral
chlorite, di,trioctahedral chlorite, trioctahedral chlorite, dioctahedral sheet, trioctahedral sheet
trimethylphenylammonium organoclay Low-charge smectite (e.g., SWy-1, SWy-2) treated
with trimethylphenylammonium chloride (= phenyltrimethylammonium chloride) or
trimethylammonium chloride yield organoclays that can effectively remove nonionic organic
contaminants from water (Lee et al., 1990; Jaynes and Boyd 1990).
trinuclear complex see binuclear complex
trioctahedral sheet In the ideal case, the smallest structural unit contains three octahedra. If
two such sites are occupied with cations and one site is vacant, then the octahedral sheet is
considered “dioctahedral”. If all three sites are occupied, the sheet is considered “trioctahedral”.
(Quot Guggenheim et al., 2006; see also references therein). A trioctahedral sheet generally
contains predominantly divalent cations. Cf., dioctahedral sheet
trioctahedral chlorite a species of the chlorite mineral group with trioctahedral sheets only.
This is the common form of chlorite. Cf., dioctahedral chlorite, di,trioctahedral chlorite,
triple layer model see Stern layer
trisilicic an invalid term, previously used as a classification of the micas where the number of
silicon atoms per formula unit is three per four tetrahedral sites, see Rieder et al. (1998). Cf.,
mica, true mica, brittle mica, interlayer-deficient mica, group names
true mica a group name for platy phyllosilicates of 2:1 layer and a layer charge of ~ -1.0 per
formula unit. True micas do not show swelling capacity. Rieder et al. (1998) defines the true
micas as having greater than 50% of the interlayer cations as univalent cations. The true mica
group is further divided into subgroups based on the octahedral sheet being either trioctahedral or
dioctahedral. Cf., mica, brittle mica, interlayer-deficient mica, group names
TST see Transition State Theory
turbostratic stacking In phyllosilicates, turbostratic stacking involves highly disordered (non
regular) stacking arrangements of layers where there is no registry from one layer to another,
much like a stack of playing cards lying flat on each other but with no alignment of edges.
Smectite minerals and halloysite commonly have turbostratic stacking. See rotational stacking
turbulent microfabric see microfabric, clay
Udden-Wentworth scale The Udden-Wentworth scale (often referred to as the Wentworth
scale) is a size scale (diameter) for clasts and is used primarily in sedimentology and related
disciplines. The Udden-Wentworth scale considers size only and does not imply composition.
The “clay” term in the scale has the potential to be confusing because “clay” is defined in clay
mineralogy as having specific properties unrelated to particle size alone. Thus, to avoid
confusion, use of “clay size” instead of “clay” is recommended here to delineate size
characteristics of particles only. Further divisions, such as “fine”, “medium”, “coarse”, etc. may
be used also. Pettijohn (1957) discusses the history of the use of size terms, alternative
classification schemes, and modifications to the nomenclature. See clay
Udden-Wentworth scale (after Pettijohn, 1957)
boulders >256 mm
cobbles 64 - 256 mm
pebbles 2 - 64 mm
sand 1/16 to 2 mm
silt 1/256 to 1/16 mm
clay <1/256 mm
underclay a fine-particle sedimentary seat rock composed mainly of clay minerals, that is
generally non-bedded and contains traces of plant roots. Kaolinite-rich underclay deposits are
economically important for ceramics manufacturing. (modified from Huddle and Patterson,
1961) Cf., seat rock, ball clay, flint clay, fire clay
undrained shear the loading and shearing of a clay or soil where the pore fluids are confined.
Thus, during undrained shearing of a clay, the pore fluid pressure may change, but the total clay
volume does not change.
unit cell A unit cell is the smallest repeating parallelipiped (= reduced cell) that contains the
atomic structure of a crystal. The unit cell contains a crystals full symmetry and chemical
composition and is chosen with cell edges (a, b, c) coinciding with any symmetry axes present.
The unit cell is determined by X-ray diffraction, usually from a single crystal. Cf., cell
parameters, unit structure, X-ray diffraction.
unit structure For phyllosilicates, the unit structure is the total assembly of the layer and any
interlayer material. After Guggenheim et al. (2006) and references therein. Cf., layer, interlayer
unit weight, effective In soil science, for saturated soils or soils beneath the groundwater table,
the effective unit weight is the difference between the total unit weight of the soil mass and the
unit weight of water. Thus, the effective unit weight removes the effect of the pore water within
the soil mass. One difficulty in its measurement is the determination of loosely held H2O
between clay layers vs H2O between grains (i.e., pores). Syn., buoyant unit weight, submerged
van der Waals forces van der Waals forces are residual forces between atomic groups or
molecules and are comprised of primarily dispersion and dipole-dipole forces. Dispersion forces,
or London forces, involve the temporary formation of polarity where one side of an atom (or
molecule) may have more electrons at a given moment than the opposing side. Thus, one side is
slightly more negative than the other slightly more positive side (by having a deficiency in
electrons). Neighboring atoms have similar polarity, and a weak bond is formed where opposite
charges between atoms attract each other.
Vegard’s law describes a solid solution series where there is a linear relationship between the
lattice parameters and the chemical composition (as atomic percentage). The term “law” is a
misnomer because a linear relationship often does not exist.
vermiculite a) a group name for platy phyllosilicates of 2:1 layer and a layer charge of ~ -0.6 to
-0.9 per formula unit. Generally for natural samples, the d(001) value is approximately 14.4-15.6
Å, although other spacings may occur depending on H2O retention and interlayer occupancy.
The group is further divided into subgroups that are either trioctahedral or dioctahedral and these
subgroups are further divided into mineral species based on chemical composition. Both
vermiculites and smectites have swelling capabilities. Distinguished from smectite in that Mg-exchanged vermiculite has d(001) of 14.5 Å after glycerol solvation, whereas smectite has
d(001) of 17.7 Å. Vermiculite occurs in soils, and are most common in subtropical and
temperate climates. b) an industrial/commercial commodity obtained from heat-treated
naturally-occurring material composed of hydrous phyllosilicates (e.g., vermiculite, hydrobiotite,
biotite). This material is heated rapidly to high temperature to cause exfoliation by volume
expansion to produce an expanded product of low bulk density. The product involves an
intimate mosaic-like intergrowth of dehydrated or partially dehydrated layers and thus is a
heterogeneous mixture of phases. The product resembles vermiculite (senso stricto) primarily
based on its physical properties, such as density, some adsorptive properties, and chemical
composition. See “group names”; Cf., smectite
vibrational spectroscopy a method of measuring intramolecular or intracrystalline modes of
motion (vibrational modes) by observing the interaction of incident radiation with a sample.
Common techniques used to obtain spectra are infrared and Raman spectroscopy, and inelastic
neutron scattering. Cf., spectroscopy, normal mode
Vickers hardness the measured hardness of a surface as tested using a special diamond
pyramid-shaped indenter under various loads. A Vickers number is reported based on surface
area indented (in mm2) divided by pressure (in Newtons). Cf., hardness, Mohs hardness
virgin clay A general term used in the pelotherapy clay industry to denote an unmodified
(“raw”) clay from a deposit, before maturation with saline or mineral water.
viscosity a measure of the resistance of a fluid to flow when the fluid is placed under stress
vitreous glassy luster Cf., luster
vitrification The process of changing a solid, often crystalline material, into an amorphous
glass-like material by heating the solid to its melting point followed by sufficiently rapid cooling
and solidification so that short-distance atomic ordering resulting in recrystalization does not
occur. As vitrification proceeds, the porosity decreases. Devitrification is the reverse process.
void ratio the ratio of the empty space (void volume) of a soil or geomaterial to the volume of
its solid particles. Cf., porosity
water, adsorbed or (H2O -) H2O molecules attracted to internal or external surfaces of a
phyllosilicate, or other material, and adhered to these surfaces in thicknesses of one or more
molecules. The term “water” (rather than “H2O”) is not precisely used here because “water” is a
(liquid) phase. Elevated temperatures, typically 110 oC for 12 hours, can desorb the adhered
H2O. However, for phyllosilicates, a temperature of 110 oC may not liberate all the adsorbed
water molecules present, and temperatures of as high as 300 oC (in special cases, even higher)
may be needed for some vermiculites and smectites. syn. absorbed water, Cf., water, structural
water, combined see water, structural
water content, soil The degree of soil water content is defined by the fraction, by mass or
weight, of pore water relative to the total dry soil solid particles, usually expressed as a
percentage. In practice, soil water content is measured by heating a wet soil sample at 105 - 110
oC to a constant mass. The weight loss by heating is assumed to be the weight of water, and the
residual weight is the weight of dry soil solids. Syn., moisture content Cf., water saturation,
water, crystallization of see water, structural
water, degree of saturation The degree of water saturation is the ratio of pore water volume to
the total void volume, usually expressed as a percentage. In soil science, values may range from
0 % for a dry soil to 100% for a soil with its void fully occupied by water. Water saturation of a
soil refers to the latter. See also water content
water, hydration of see water, structural
water, hygroscopic H2O adsorbed by soil that is equilibrated with the atmosphere to which it is
exposed at a given temperature and relative humidity, usually 25 oC at 98 % relative humidity.
water, interlayer water (or more precisely, H2O molecules) adsorbed between the (1:1 or 2:1)
layers of a phyllosilicate. The use of “water” is a misnomer because the interlayer H2O is not
equivalent to bulk water (i.e., a phase, a liquid), which involves randomly oriented H2O. Instead,
the interlayer H2O is affected structurally by the adjacent 1:1 or 2:1 layers and by the cations
present in the interlayer. Cf., water, adsorbed
water, lattice see water, structural
water, molecular see water, structural
water, structural or (H2O+) water (more precisely, H2O or OH molecules) that is directly bound
to cations at crystallographic sites in a crystal structure. This H2O or OH is capable of being
driven off at elevated temperature, generally by heating to 1000 oC. For phyllosilicates,
structurally bound “water” is in the form of hydroxyl (OH) groups coordinated to cations, and the
high temperature is required to promote dehydroxylation. Fluorine and certain other anions may
be driven off at these temperatures also. However, hydrates, such as gypsum (CaSO4 .2H2O),
have structural water coordinated to cations as H2O (e.g., water of hydration) that is liberated by
heating to relatively low values, 65 - 95 oC. syn. combined water, molecular water, structurally
bound water, water of crystallization, water of hydration. “Lattice water” is also used, but is not
recommended because a “lattice” refers to a collection of identipoints, see “lattice”. Cf., water,
water, structurally bound see water, structural
water, zeolitic water molecules adsorbed within the cavities/tunnels of the zeolite framework
structure, commonly removed by heating at 350 to 400 oC for about 12 hours. “Zeolitic water”
was a term used by early workers to describe interlayer water of phyllosilicates. However, the
analogy is imperfect (and not in use anymore) because the number of interlayer water molecules
affects the layer-to-layer spacing of a phyllosilicate, whereas the shape and size of zeolitic
tunnels are not significantly affected by the number of H2O molecules present. Cf., water,
structural; water, adsorbed
weathering the physical (mechanical) and/or chemical breakdown of rock, sediment, and soil in
place under the influence of the hydrosphere and/or atmosphere. Biota may influence or control
physical or chemical weathering. See physical weathering, chemical weathering. Cf., erosion
Wentworth scale see Udden-Wentworth scale
western Bentonite An industrial or commercial term, originally used to designate the high
quality sodium bentonite from Wyoming, USA. This material exhibits characteristics of high
water adsorption, dilation, viscosification and dry sand binding strength. Usage of the term has
since broadened to include any bentonite having performance characteristics equal to that of high
quality sodium bentonite from Wyoming. Whereas the term “Western Bentonite”, and its
synonyms, continue to have meaning in the industrial and commercial realm, they are
scientifically obsolete and should not be used in that context. syn: Wyoming Bentonite,
Wyoming Sodium Bentonite, Wyoming-Type Bentonite
whiteware any ceramic that fires to a white or ivory color, commonly used in wall tiles,
Wulff rule The Wulff rule or Gibbs-Curie-Wulff Theorem states that when the surface free
energy of a crystal is minimized at equilibrium conditions, the perpendicular distance from a
given crystal face to the center of the crystal divided by the surface free energy of that face is a
constant for all faces of the crystal.
Wyoming sodium bentonite an obsolete term, see western bentonite
Wyoming bentonite an obsolete term, see western bentonite
Wyoming-type montmorillonite As described by Schultz (1969) based on chemical and
thermal analysis, Wyoming-type montmorillonite is a type of montmorillonite characterized by a
small net negative layer charge of about -0.35 to -0.85 per O20(OH)4, with tetrahedral
substitutions causing from -0.15 to -0.50 of the total layer charge and specific thermal properties.
Current nomenclature for montmorillonite is that it is an Al-rich, dioctahedral smectite with an
ideal structural formula of (Al3.15Mg0.85)Si8O20(OH)4X0.85.nH2O with layer charge from primarily
octahedral substitutions of Mg, whereas beidellite has a net layer charge that occurs from
tetrahedral substitutions of Al, with an ideal structural formula of
Al4.0(Si7.15Al0.85)O20(OH)4X0.85.nH2O. Although the structure of a mineral is important (not given
here), thermal properties are not part of the definition. The Wyoming-type montmorillonite of
Schultz (1969) is best described as an intermediate in the montmorillonite-beidellite series. The
term”Wyoming-type montmorillonite” is obsolete and should not be used.
Wyoming-type bentonite an obsolete tern, see Western bentonite
xenobiotic substances foreign to a body or an environment, for example, clay in the blood
X-ray diffraction The interaction of X-rays (electromagnetic radiation with wavelengths in the
approximate range of 0.1 to 10 Å) with matter produces scattering in all directions. The electrons
in the matter oscillate to the electric field of the impinging X-rays so that the scatter will be in the
same wavelength and frequency of the impinging X-rays. Under certain conditions (defined by
the Bragg equation), parallel rays produced by the scatter reinforce each other to produce a
diffracted wave, and this is referred to as X-ray diffraction. Cf., Bragg equation, unit cell
yielding the phenomenon where a clay undergoes the transition from a purely elastic
deformation stage to an elasto-plastic deformation stage. Yielding marks the onset of plastic
deformation, and the stress at which yielding occurs is called yield stress or yield strength.
Young’s modulus a measure of the response of a homogeneous material to uniaxial tension or
compression, in units of pressure (usually gigapascals), obtained from the slope of the (linear)
relationship of stress versus strain in the elastic regime. Cf., elastic constants
zeolite a family of hydrated aluminosilicate minerals with a three-dimensional Si,Al tetrahedral
framework-type structure with molecular-size channels and cages. Rings of four, six and eight
tetrahedra are common structural subunits of the framework. The “extra-framework” content
includes cations and H2 O; the cations are often exchangeable and the H2 O may be dehydrated or
partially dehydrated. The tetrahedra are occupied by >50% Si, as required by the Lowenstein Al-avoidance rule; in some cases, Be is present. Zeolites are used commercially in ion-exchange,
molecular-sieve, and hydration-dehydration applications.
zero point of charge see point of zero charge
zpc see point of zero charge
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