Abstract: Aqueous Al passes from octahedral to tetrahedral coordination over a narrow pH interval, or threshold. This interval is 5.5–6.5 at 25°C and shifts to lower pH as temperature increases. The concentration of aqueous tetrahedrally coordinated Al is a quasi-step function of the solution pH, and, by the mass-action law, so should be the amount of tetrahedral Al incorporated by a silicate that crystallizes from the aqueous solution. Qualitative support for this prediction (which applies to quartz, opal-CT, kaolin-group minerals, pyrophyllite, micas, chlorites, and other low-temperature silicates) comes from the very topology of equilibrium activity diagrams and from several pairs of associated waters and authigenic silicates from weathering, hydrothermal, and diagenetic environments. The uptake of tetrahedral Al also depends on the aqueous concentrations of monovalent cations and silica, and on the mineral's structural constraints.
Solid solution of tetrahedral Al in halloysite in turn produces the characteristic bent or tubular crystals of this mineral. This genetic link between aqueous chemistry (mainly pH), tetrahedral-Al uptake by a low-temperature silicate, and the mineral's crystal morphology may operate also in other silicates.