## 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

**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: O_{0.5}TI′TO_{0.5} (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 … O_{M}(TI′T)_{1}O_{M}(TI′T)_{S} … and … O_{M}(TI′T)_{1}O_{1}(TI′T)_{1}O_{M}(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 T_{1}O_{M}T_{S} 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 O_{0.5}TI′TO_{0.5} unit of smectite into the same corresponding unit of illite.