Octahedral cation ordering of illite and smectite. Theoretical exchange potential determination and Monte Carlo simulations

The distributions of Al3+/Mg2+ and Al3+/ Fe3+ were studied in the octahedral sheet of illites and smectites. Cation exchange interaction parameters Ji, as first, second, third and fourth neighbours were calculated by means of empirical interatomic potentials. Several compositions with different interlayer cations and tetrahedral charge were studied in both Al/Mg and Al/Fe systems. The values of Ji parameters were similar in all Al/Mg samples. From these Ji values, a strong trend to form AlMg pairs was observed in the Al/Mg system. In the Al/Fe system, the values of Ji are very small, indicating no preference for Al/Fe mixing. From these Ji parameters, Monte Carlo simulations of octahedral cation ordering were performed. In the Al/Mg system, an order/disorder phase transition was observed obtaining a fully ordered distribution without presence of an MgMg pair, according to experimental data. Similar phase transitions were observed for the octahedral compositions Al/Mg 1/1 and 3/1. In the Al/Fe system an order/disorder phase transition was also detected but at very low temperature for illite and smectite. Complete Al/Fe mixing is observed in the most stable ordered distribution. This is consistent with experimental results for synthetic Fe/Al smectites.