Crystallochemical and Structural Classification of Layer Silicates and Sepiolite.

Abstract

The complex intergrated system of classification and identification of phyllosilicates and their structures has been outlined in this work. The identification of mineral species is developed. This is facilitated by the new projection system of chemical composition, as given by a crystallochemical formula, onto a field with orthogonal axes chosen for octachedral divalent cations, R2+, and Si (X, Y, respectively), and oblique axes for octahedral trivalent cations, R3+ and vacancies, (V, Z, respectively). The stoichiometry of compositions is controlled, at any point of the projection field by the crystallochemical formulae, valid equally well for di- and tri-octachedral species.A notation common for all the silicate structures is based on the known Ramsdell symbols, which are combined with those related to the Subfamily and MDO group within homo-, meso-, hetero-octahedral Families. The symbols used have physical meaning as they correspond with the number of layers in the unit cell, symmetry system, and structural XZ and YZ projections.