Clay Mineral | Definition, Structure, Composition, Uses, Types ...

General features

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The structure of clay minerals has been determined largely by X-ray diffraction methods. The essential features of hydrous-layer silicates were revealed by various scientists including Charles Mauguin, Linus C. Pauling, W.W. Jackson, J. West, and John W. Gruner through the late 1920s to mid-1930s. These features are continuous two-dimensional tetrahedral sheets of composition Si2O5, with SiO4 tetrahedrons (Figure 1) linked by the sharing of three corners of each tetrahedron to form a hexagonal mesh pattern (Figure 2A). Frequently, silicon atoms of the tetrahedrons are partially substituted for by aluminum and, to a lesser extent, ferric iron. The apical oxygen at the fourth corner of the tetrahedrons, which is usually directed normal to the sheet, forms part of an adjacent octahedral sheet in which octahedrons are linked by sharing edges (Figure 3). The junction plane between tetrahedral and octahedral sheets consists of the shared apical oxygen atoms of the tetrahedrons and unshared hydroxyls that lie at the centre of each hexagonal ring of tetrahedrons and at the same level as the shared apical oxygen atoms (Figure 4). Common cations that coordinate the octahedral sheets are Al, Mg, Fe3+, and Fe2+; occasionally Li, V, Cr, Mn, Ni, Cu, and Zn substitute in considerable amounts. If divalent cations (M2+) are in the octahedral sheets, the composition is M2+/3 (OH)2O4 and all the octahedrons are occupied. If there are trivalent cations (M3+), the composition is M3+/2 (OH)2O4 and two-thirds of the octahedrons are occupied, with the absence of the third octahedron. The former type of octahedral sheet is called trioctahedral, and the latter dioctahedral. If all the anion groups are hydroxyl ions in the compositions of octahedral sheets, the resulting sheets may be expressed by M2+(OH)2 and M3+(OH)3, respectively. Such sheets, called hydroxide sheets, occur singly, alternating with silicate layers in some clay minerals. Brucite, Mg(OH)2, and gibbsite, Al(OH)3, are typical examples of minerals having similar structures. There are two major types for the structural “backbones” of clay minerals called silicate layers. The unit silicate layer formed by aligning one octahedral sheet to one tetrahedral sheet is referred to as a 1:1 silicate layer, and the exposed surface of the octahedral sheet consists of hydroxyls. In another type, the unit silicate layer consists of one octahedral sheet sandwiched by two tetrahedral sheets that are oriented in opposite directions and is termed a 2:1 silicate layer (Figure 5). These structural features, however, are limited to idealized geometric arrangements.

Real structures of clay minerals contain substantial crystal strains and distortions, which produce irregularities such as deformed octahedrons and tetrahedrons rather than polyhedrons with equilateral triangle faces, ditrigonal symmetry modified from the ideal hexagonal surface symmetry, and puckered surfaces instead of the flat planes made up by the basal oxygen atoms of the tetrahedral sheet. One of the major causes of such distortions is dimensional “misfits” between the tetrahedral and octahedral sheets. If the tetrahedral sheet contains only silicon in the cationic site and has an ideal hexagonal symmetry, the longer unit dimension within the basal plane is 9.15 Å, which lies between the corresponding dimensions 8.6 Å of gibbsite and 9.4 Å of brucite. To fit the tetrahedral sheet into the dimension of the octahedral sheet, alternate SiO4 tetrahedrons rotate (up to a theoretical maximum of 30°) in opposite directions to distort the ideal hexagonal array into a doubly triangular (ditrigonal) array (Figure 2B). By this distortion mechanism, tetrahedral and octahedral sheets of a wide range of compositions resulting from ionic substitutions can link together and maintain silicate layers. Among ionic substitutions, those between ions of distinctly different sizes most significantly affect geometric configurations of silicate layers.

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Another significant feature of layer silicates, owing to their similarity in sheet structures and hexagonal or near-hexagonal symmetry, is that the structures allow various ways to stack up atomic planes, sheets, and layers, which may be explained by crystallographic operations such as translation or shifting and rotation, thereby distinguishing them from polymorphs (e.g., diamond-graphite and calcite-aragonite). The former involves one-dimensional variations, but the latter generally three-dimensional ones. The variety of structures resulting from different stacking sequences of a fixed chemical composition are termed polytypes. If such a variety is caused by ionic substitutions that are minor but consistent, they are called polytypoids.