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.