ORIGINAL PAPER C. I. Sainz-Diaz  E. J. Palin A. Herna´ ndez-Laguna  M. T. Dove Octahedral cation ordering of illite and smectite. Theoretical exchange potential determination and Monte Carlo simulations Received: 2 August 2002 / Accepted: 23 April 2003 Abstract The distributions of Al 3+ /Mg 2+ and Al 3+ / Fe 3+ were studied in the octahedral sheet of illites and smectites. Cation exchange interaction parameters J i , as first, second, third and fourth neighbours were calcu- lated 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 octa- hedral 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 octahe- dral compositions Al/Mg 1/1 and 3/1. In the Al/Fe system an order/disorder phase transition was also de- tected 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 experimen- tal results for synthetic Fe/Al smectites. Keywords Cation ordering  Illite  Smectite  Monte Carlo simulations Introduction A great diversity of 2:1 phyllosilicates exists in nature because they can present a wide range of cation com- position in the octahedral and tetrahedral sheets. In dioctahedral 2:1 phyllosilicates, isomorphous substitu- tion of Al 3+ by Mg 2+ in the octahedral sheet or Si 4+ by Al 3+ in the tetrahedral sheet results in a net negative charge that is balanced by the presence of cations in the interlayer space. Determination of the distribution of cations within the sheets is a complex problem, especially in the octahedral sheet, to which we refer in this paper. This type of study can be useful to understand natural processes, such as smectite to illite transformation, and to analyze how cation distribution affects lattice stability. Also, the industrial applications of smectite due to its valuable catalytic and adsorptive properties (e.g. as a barrier in nuclear waste and pollutant disposal repositories) make it of great interest to establish a firm theoretical understanding of their structure and behaviour. The phenomenon of cation ordering in aluminosili- cate minerals has been one of the important aspects of mineral behaviour for a long time, particularly since it can have a significant effect on the thermodynamic properties. The cation distribution for the octahedral sheet of phyllosilicates has been studied experimentally. Besson et al. (1987) found from IR studies that octa- hedral cation distribution is not random and Al 3+ and Fe 3+ tend to segregate from each other. Using 27 Al NMR on montmorillonite, Morris et al. (1990) found that Fe was either segregated from Al in the octahedral sheet or present in a phase different from smectite. Grauby et al. (1991, 1993) synthesized smectites with different proportions of Al 3+ , Mg 2+ and Fe 3+ . They studied how these cations arrange in the octahedral sheet using IR and found that Al 3+ and Fe 3+ tend to mix rather than to segregate, Mg 2+ and Fe 3+ to segregate within the same layer and Mg 2+ and Al 3+ to segregate, creating dioctahedral and trioctahedral layers. Schroeder Phys Chem Minerals (2003) 30: 382–392 Ó Springer-Verlag 2003 DOI 10.1007/s00269-003-0324-4 C. I. Sainz-Diaz (&)  A. Herna´ndez-Laguna Estacio´n Experimental del Zaidı´n (CSIC), C/ Profesor Albareda, 1. 18008-Granada (Spain) Fax: +34–958129600 e-mail: sainz@lec.ugr.es E. J. Palin  M. T. Dove Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK