CLAY MINERAL

any of a group of important hydrous aluminum silicates with a layer (sheetlike) structure and very small particle size. They may contain significant amounts of iron, alkali metals, or alkaline earths. any of a group of important hydrous aluminum silicates with a layer (sheetlike) structure and very small particle size (two microns [0.002 millimetre]). It may contain significant amounts of iron, alkali metals, or alkaline earths. The clay minerals are very similar, and many can be distinguished only by X-ray diffraction, electron microscopy, or differential thermal analysis. A number of classifications that differ in detail have been proposed for these minerals, but it is generally agreed that they can be divided into nine broad groups on the basis of variations in structure and composition. These groups are (1) allophane; (2) kaolinite; (3) halloysite; (4) montmorillonite; (5) illite; (6) chlorite; (7) vermiculite; (8) sepiolite, attapulgite, and palygorskite; and (9) mixed-layer clay minerals. Two structural units describe the structures of most clay minerals. The first of these consists of a layer of silicon-oxygen tetrahedrons arranged in a hexagonal network in two dimensions, each tetrahedron sharing three oxygens with other tetrahedrons. (The oxygen atoms lie approximately in a plane.) The silicon-centred tetrahedrons all point in one direction with the fourth oxygen or hydroxyl group of the tetrahedrons also lying in a plane. The thickness of this undistorted unit is about 4.7 angstroms (1 angstrom is equal to 10-4 microns). The second structural unit characteristic of clay minerals consists of two layers of close-packed oxygen or hydroxyl ions forming octahedral sites in which metals, usually aluminum, magnesium, or iron, are located. Not all of these sites are filled, much depending on the charge of the octahedral ion. The undistorted thickness of this octahedral layer is about 5.05 angstroms. The tetrahedral and octahedral structural units can be joined or stacked in several configurations. Less common clay minerals are either amorphous or have a structure based on double tetrahedral chains similar to that of amphibole (q.v.) minerals. Clay minerals are usually the products of weathering processes. Different environments, particularly different climatic and hydrologic regimes, will produce different clay minerals from the same parent rock. In some cases, clay minerals may be formed as a result of hydrothermal action, which may produce nearly monomineral deposits. Clay minerals occur widely in such sedimentary rocks as mudstones and shales, in marine sediments, and in soils. Clay minerals have considerable economic importance. Such minerals' colloidal properties (i.e., capacity to take up and hold water), for example, make them particularly suitable for use as drilling muds in petroleum extraction. Various clay minerals readily react with organic substances and thus are utilized as catalysts in petroleum refining and as a decolourizing agent in the processing of vegetable and mineral oils. Additional reading Ralph E. Grim, Clay Mineralogy, 2nd ed. (1968); Karl Jasmund, Die silicatischen Tonminerale, 2nd enlarged ed. (1955); Georges Millot, Gologie des Argiles (1963); H. Beutelspacher and H.W. van der Marel, Atlas of Electron Microscopy of Clay Minerals and Their Admixtures (1968); J.A. Gard (ed.), The Electron-Optical Investigation of Clays (1971); Rex W. Grimshaw, The Chemistry and Physics of Clays and Allied Ceramic Materials, 4th ed. rev. (1971); Charles E. Weaver and Lin D. Pollard, The Chemistry of Clay Minerals (1973); John E. Gieseking (ed.), Soil Components, vol. 2, Inorganic Components (1975); Raymond N. Yong and Benno P. Warkentin, Soil Properties and Behaviour (1975); H. Van Olphen and J.J. Fripiat (eds.), Data Handbook for Clay Materials and Other Non-metallic Minerals (1979); G.W. Brindley and G. Brown (eds.), Crystal Structures of Clay Minerals and Their X-ray Identification, new ed. (1980); Erno Nemecz, Clay Minerals (1981; originally published in Hungarian, 1973); Toshio Sudo et al., Electron Micrographs of Clay Minerals (1981); Simonne Caillre and Stphane Hnin, Minralogie des Argiles, 2nd ed. rev., 2 vol. (1982); Friedrich Liebau, Structural Chemistry of Silicates: Structure, Bonding, and Classification (1985); Robert H.S. Robertson, Fuller's Earth: A History of Calcium Montmorillonite (1986); A.C.D. Newman (ed.), Chemistry of Clays and Clay Minerals (1987); S.W. Bailey (ed.), Hydrous Phyllosilicates (Exclusive of Micas) (1988); J.B. Dixon and S.B. Weed (eds.), Minerals in Soil Environments, 2nd ed. (1989); Charles E. Weaver, Clays, Muds, and Shales (1989). Hideomi Kodama