Mixed layering of illite-smectite: Results from high-resolution transmission electron microscopy and lattice-energy calculations

Mixed layering of illite-smectite was studied both experimentally, by using high-resolution transmission electron microscopy (HRTEM) and analytical el ectron microscopy (AEM), and theoretically, by using lattice-energy calcula tions. Samples from a hydrothermal origin show the transformation of smectite to i llite with different ordering types in the illite-smectite layer sequences. Ordering ranges from complete disordered (Reichweite, R = 0 type) in the l ess transformed samples to increased local order, with IS and IIS sequences (R = 1 and R = 2, respectively; I = illite, S = smectite) in more illitize d samples. Lattice-energy calculations are used to determine the structure of the illi te-smectite sequence, which corresponds to the minimum energy. The unit lay ers are: O0.5TI'TO0.5 (O, T, and I', respectively, denote the octahedral an d tetrahedral sheets, and the interlayer. The 0.5 signifies half of the oct ahedral cations.) For example, the arrangements of the perfectly ordered... ISIS... and...IISIIS... sequences are respectively... O,(TI'T),O,(TI'T),... and... O-M(TI'T)(l)O-l(TI'T)(l)O-M(TI'T)(S)... (the subscripts I, S, and M , respectively, refer to compositions of illite, smectite, and midway betwe en at 0.5). Such arrangements produce a polar model for TOT layers, which d isplay a T,O,T, structure in the case of IS adjacent layers. Furthermore, t he lattice energies of...ISIS... and...IISIIS... are found to be nearly equ al to the corresponding sums of the lattice energies of illite and smectite . This result indicates that interstratified illite-smectite and the two-ph ase assemblage of illite + smectite have similar stabilities. On the basis of the above model, the solid-state transformation of one smec tite layer to one illite layer, which produces mixed-layer sequences, invol ves the transformation of an O0.5Tl'TO0.5 unit of smectite into the same co rresponding unit of illite.