Structures of Layer Silicates

Abstract

THE hydrous layer silicates commonly known as clay minerals are part of the larger family of phyllosilicates. The layer silicates considered here contain continuous two-dimensional tetrahedral sheets of composition T₂O₅, (T = tetrahedral cation, normally Si, Al, or Fe³⁺), in which individual tetrahedra are linked with neighboring tetrahedra by sharing three corners each (the basal oxygens) to form an hexagonal mesh pattern (Fig. 1.1a). The fourth tetrahedral corner (the apical oxygen) points in a direction normal to the sheet and at the same time forms part of an immediately adjacent octahedral sheet in which individual octahedra are linked laterally by sharing octahedral edges (Fig. 1.1b). The common plane of junction between the tetrahedral and octahedral sheets consists of the shared apical oxygens plus unshared OH groups that lie at the center of each tetrahedral six-fold ring at the same z-level as the apical oxygens. F may substitute for OH in some spccies. The octahedral cations normally are Mg, Al, Fe²⁺, and Fe³⁺, but other medium-sized cat:ons such as Li, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn also occur in some species. The smallest structural unit contains three octahedra. If all three octahedra are occupied, i.e. have octahedral cations at their centers, the sheet is classified as trioctahedral. If only two octahedra are occupied and the third octahedron is vacant, the sheet is classified as dioctahedral. The assemblage formed by lirking one tetrahedral sheet with one octahedral sheet is known as a 1 : 1 layer. In such layers the uppermost, unshared plane of anions in the octahedral sheet consists entirely of OH groups. In the years 1930—1950 clay mineral identification involved mainly a combination of X-ray powder diffraction and chemical analysis with some assistance from other techniques, notably differential thermal analysis. In the period 1950—1970 additional procedures have emerged including infrared analysis, electron optical methods and a variety of thermal methods. These procedures are now treated in other monographs sponsored by the Mineralogical Society and in many other publications. Despite the availability of other techniques, X-ray diffraction remains a basic tool for studying minerals and we hope that this monograph will continue to serve, as did the previous editions, both those concerned with the more academic aspects of clay mineralogy and also those, such as geologists, civil engineers and soil scientists, for whom identification and quantitative estimation of the minerals in natural clayey materials is a practical requirement.