Clays, muds, and shales / Глины, шламы и сланцы

The clay minerals are small hydrous layer silicates and are part of the phyllosilicate family. As discussed in Chapter 11, many of the hydrous layer silicates in clays, muds, soils, shales, slates, etc. are coarser than clay (< 2 or < 4 pm). Primarily for this reason I have suggested (Weaver, 1980) using the term physil, which has no size connotation, to refer to the low-temperature ( 5 400°C) hydrous layer silicates. The layer silicates are constructed of planes of atoms forming tetrahedral and octahedral sheets arranged in various combinations (Bailey, 1980a). The tetrahedral sheets are composed of tetrahedra linked with adjacent tetrahedra by sharing oxygen ions at three corners. The shared oxygen (basal oxygens) form a hexagonal pattern. The fourth tetrahedral oxygen (apical oxygen) of all tetrahedra is perpendicular to the sheet and forms part of the adjacent octahedral sheet (Figure 1-1).

The tetrahedral cations are primarily Si and Al, and rarely Fe3+. The octahedral sheet consists of cations (Al, Fe, Mg) that are octahedrally coordinated by shared apical oxygens plus unshared OH’S that lie at the center of the hexagonal hole formed by the basal oxygens.
The smallest structural unit contains three octahedra. If all three octahedra are occupied with cations,the sheet is classified astrioctrahedral. If only two octahedra are occupied and one is vacant,the sheet is classified as dioctahedral. The combination of one tetrahedral sheet and one octahedral sheet is called a 1 :1 layer. The unshared plane of anions in the octahedral sheet consists of OH anions. A 2:l layer consists of an octahedral sheet sandwiched between two tetrahedral sheets. Simplified sketches of the major physil groups are shown in Figure 1-1. <...>