Crystal-Chemical Composition of Dicoctahedral Smectites: An Energy-Based Assessment of Empirical Relations

The experimental characterization of various dioctahedral smectites reveals weak correlations between the ratio of cis- and trans-vacant layer structures, fraction of charge in the tetrahedral sheet, and iron content. These correlations are demonstrated by two independent sample sets examined in different laboratories and, hence, are considered to be significant. For these relations to be rationalized and corroborated, accurate relative energies and ion exchange energies of a set of model minerals were determined by quantum chemical density functional calculations. Accordingly, one may trace these empirical correlations mainly back to energy relations and rationalize some deviating smectites by their particular history of formation.