Abstract
The distributions of Al3+/Mg2+ and Al3+/ Fe3+ were studied in the octahedral sheet of illites and smectites. Cation exchange interaction parameters J i , 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 J i parameters were similar in all Al/Mg samples. From these J i values, a strong trend to form AlMg pairs was observed in the Al/Mg system. In the Al/Fe system, the values of J i are very small, indicating no preference for Al/Fe mixing. From these J i 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.
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Acknowledgments.
The authors are grateful to J. Gale for permitting us to use the GULP program, V. Botella for fruitful discussions, J. Ruiz-García for his help and for the financial support of NERC (M.T. Dove), EPSRC (E. Palin), the Royal Society, UK and the MEC, Spain (C.I. Sainz-Díaz), and the PB97-1205 and BTE2000-1146-CO2-01 MCYT projects and Acciones Integradas UK/Spain joint research programme.
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Sainz-Diaz, C., Palin, E., Hernández-Laguna, A. et al. Octahedral cation ordering of illite and smectite. Theoretical exchange potential determination and Monte Carlo simulations. Phys Chem Minerals 30, 382–392 (2003). https://doi.org/10.1007/s00269-003-0324-4
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DOI: https://doi.org/10.1007/s00269-003-0324-4