Mixed Layering of Illite-Smectite: Results from High-Resolution Transmission Electron Microscopy and Lattice-Energy Calculations

Juan Olives, Marc Amouric and Régis Perbost
CRMC2-CNRS, Campus de Luminy, Case 913, 13288 Marseille cedex 09, France
E-mail of corresponding author: olives@crmc2.univ-mrs.fr

Abstract: Mixed layering of illite-smectite was studied both experimentally, by using high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM), and theoretically, by using lattice-energy calculations.

Samples from a hydrothermal origin show the transformation of smectite to illite with different ordering types in the illite-smectite layer sequences. Ordering ranges from complete disordered (Reichweite, R = 0 type) in the less transformed samples to increased local order, with IS and IIS sequences (R = 1 and R = 2, respectively; I = illite, S = smectite) in more illitized samples.

Lattice-energy calculations are used to determine the structure of the illite-smectite sequence, which corresponds to the minimum energy. The unit layers are: O0.5TI′TO0.5 (O, T, and I′, respectively, denote the octahedral and tetrahedral sheets, and the interlayer. The 0.5 signifies half of the octahedral cations.) For example, the arrangements of the perfectly ordered … ISIS … and … IISIIS … sequences are respectively … OM(TI′T)1OM(TI′T)S … and … OM(TI′T)1O1(TI′T)1OM(TI′T)S … (the subscripts I, S, and M, respectively, refer to compositions of illite, smectite, and midway between at 0.5). Such arrangements produce a polar model for TOT layers, which display a T1OMTS structure in the case of IS adjacent layers. Furthermore, the lattice energies of … ISIS … and … IISIIS … are found to be nearly equal to the corresponding sums of the lattice energies of illite and smectite. This result indicates that interstratified illite-smectite and the two-phase assemblage of illite + smectite have similar stabilities.

On the basis of the above model, the solid-state transformation of one smectite layer to one illite layer, which produces mixed-layer sequences, involves the transformation of an O0.5TI′TO0.5 unit of smectite into the same corresponding unit of illite.

Key Words: HRTEM-AEM • Illite-Smectite • Lattice-Energy Calculations • Mixed Layering • Polar 2:1 Layers

Clays and Clay Minerals; April 2000 v. 48; no. 2; p. 282-289; DOI: 10.1346/CCMN.2000.0480215
© 2000, The Clay Minerals Society
Clay Minerals Society (www.clays.org)