ORIGINAL PAPER DFT study of the cation arrangements in the octahedral and tetrahedral sheets of dioctahedral 2:1 phyllosilicates Alfonso Herna ´ ndez-Laguna Æ Elizabeth Escamilla-Roa Æ Vicente Timo ´n Æ Martin T. Dove Æ C. Ignacio Sainz-Dı ´az Received: 7 April 2006 / Accepted: 4 September 2006 / Published online: 25 October 2006 Ó Springer-Verlag 2006 Abstract Quantum mechanical calculations based on the density functional theory (DFT) are used to study the crystal structures of dioctahedral 2:1 phyllosilicates. The isomorphous cation substitution is investigated by exploring different substitutions of octahedral Al 3+ by Mg 2+ or Fe 3+ , tetrahedral substitution of Si 4+ by Al 3+ , and different interlayer cations (IC) (Na + ,K + , Ca 2+ , and Mg 2+ ). Samples with different kinds of layer charges are studied: only tetrahedrally charged, only octahedrally charged, or mixed octahedral/tetrahedral charged. The effect of the relative arrangements of these substitutions on the lattice parameters and total energy is studied. The experimental observation of segregation tendency of Fe 3+ and dispersion tendency of Mg 2+ in the octahedral sheet is reproduced and ex- plained with reference to the relative energies of the octahedral cation arrangements. These energies are higher than those due to the IC/tetrahedral and IC/ octahedral relative arrangements. The tetrahedral and octahedral substitutions that generate charged layers also tend to be dispersed. The octahedral cation exchange potentials change with the IC-charge/ionic radius value. Introduction A wide diversity of 2:1 phyllosilicates exists in nature because these materials can accommodate a wide range of cation compositions in the octahedral and tetrahedral sheets. In dioctahedral 2:1 phyllosilicates, isomorphous substitution of Al 3+ by Fe 3+ or Mg 2+ in the octahedral sheet, or Si 4+ by Al 3+ in the tetrahedral sheet, can result in a net negative charge that is bal- anced by the presence of cations in the interlayer space. Each sheet can have different cation arrange- ments, and different layers can coexist within the same crystal leading to the formation of a mixed-layer structure. Hence the determination of the arrangement of cations within the sheets is a complex problem. Nevertheless, this type of study can be useful to understand natural processes, such as the smectite to illite transformation, and to analyse how cation arrangements affect lattice stability. Understanding the mechanism of the smecite/illite transformation is important for a number of reasons, not least because the illite/smectite distribution can be used as a proxy in the exploration of underground oil deposits (Velde and Espitalie ´ 1989). Furthermore, the stability of the clay barriers for toxic and nuclear waste disposal is closely related to the smectite–illite transformation, changing the cation exchange and water swelling capacity, and thereby increasing the risk of leaks in the barrier A. Herna ´ ndez-Laguna  E. Escamilla-Roa  V. Timo ´n Estacio ´n Experimental del Zaidı´n, Consejo Superior de Investigaciones Cientı ´ficas (CSIC), C/ Profesor Albareda 1, Granada 18008, Spain M. T. Dove Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK C. Ignacio Sainz-Dı´az (&) Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Cientı ´ficas (CSIC)/ Universidad de Granada, Av. Fuentenueva s/n, Granada 18002, Spain e-mail: sainz@lec.ugr.es 123 Phys Chem Minerals (2006) 33:655–666 DOI 10.1007/s00269-006-0120-z