Structures and Structure Modifications of Layer Silicates

Abstract

The basic concepts in the field of layer silicate structures were introduced and developed in the work of Pauling (1930a,b), Bragg (1937), and Belov (1947, 1949, 1950, 1951) and were based on crystallochemical analyses of experimental results. The simplest and clearest way of representing these structures is in terms of their structural polyhedra, i.e., the tetrahedra and octahedra defined by the large O, OH, and F anions lying at their corners, with the small cations lying inside. The tetrahedra in most cases contain Si atoms, but in some cases these may be partly replaced by Al atoms or, more rarely, by Fe atoms. The octahedra may contain Al, Fe, Cr, Mg, Zn, Li, or other atoms. In the structures of layer silicates, the tetrahedra are connected together through the corners of their faces, and the octahedra through their side edges, to form two-dimensional networks (Fig. 1) in which the centers of the polyhedra are arranged in a hexagonal pattern (Figs. 2–4). The tetrahedral and octahedral networks, which are of similar dimensions, are connected together into layers, and in any given mineral group all the layers are formed from a given combination of these networks.