The structure of dioctahedral 2:1 phyllosilicates with different interlayer charge was studied theoretically using ab initio calculations. The standard Kohn-Sham self-consistent density functional method was used in the generalized gradient approximation (GGA), with numerical atomic orbitals as the basis set, by means of the SIESTA program. Once the method had been checked satisfactorily, the theoretical study was extended beyond the systems for which there are experimental information, and structural characteristics were predicted. The SIESTA program was shown to be a useful tool in studying the crystallographic properties of 2:1-dioctahedral phyllosilicates. The crystal structures of pyrophyllite, beidellite, and other smectites and illites were simulated. The experimental crystallographic properties of phyllosilicates with high, medium, and low interlayer charge were reproduced. The calculated structures agree with the main experimental structural features of the crystal lattice of these minerals. The effect of cation substitutions in the octahedral and tetrahedral sheets was also studied. The calculated effects are consistent with experimental results. The Mg²⁺ cations were found to have a tendency to be distributed in the octahedral sheet, in contrast to Fe³⁺ ions that tend to be clustered.

A series of bentonite illite/smectite with a range of Fe content (0.08-0.95 atoms per half formula unit) and illite composition (3-97 percent illite) has been analyzed by means of FTIR, ²⁷ Al MAS NMR, and theoretical calculations by reverse Monte Carlo simulations to study cation distribution in the octahedral sheet. FTIR was used to determine the proportion of different cation pairs bound to OH groups utilizing the OH bending bands. The proportions of cation pairs were used to generate Monte Carlo distributions of the octahedral cations in the layers. The calculated distributions were then analyzed in comparison with the experimental ²⁷ Al NMR results. All samples have shown a high dispersion of the Mg cations and a tendency for Fe segregation which increased with progressing illitization. The illite-rich samples showed medium- or long-range ordering of the octahedral cations. The minimum radius for the wipeout sphere of the ²⁷ Al NMR signal around Fe atoms is 7.5 Aa, for our working conditions of 11.7 T.