Clay minerals

Clay minerals: A term reserved for those constituents of a clay, mudstone or shale which give it its plastic properties. Their atomic structure is basically that of the layer-lattice minerals and they generally occur as minute, platy, or more rarely, fibrous, crystals. An important characteristic is their ability to lose or take up water according to the temperature and amount of water present in a system. Some clay minerals contain loosely bonded cations which can easily be exchanged for others, according to the local chemical concentration. Both two- and three-layer types are known. Clay particles range in size from the near-colloidal to those within the resolving power of an ordinary microscope. Clay minerals are produced by the degradation (weathering, hydrothermal processes, etc) of other silicates or by direct precipitation from a solution. Four clay mineral groups are important:

1. The kaolinite group, which includes kaolinite, dickite and nacrite, which are isochemical (A1₄Si₄O₁₀(OH)₈) but not isostructural. They contain no exchangeable cations and are mainly produced by the destruction of alkali feldspars under acidic conditions. Kaolin is the main constituent of china clay. Kaolinite is an important pore-blocking mineral in sandstones.
2. The illite group, which includes illite and the hydromicas (see glauconite). Glauconite is often considered a mica and contains other cations besides K, including Na, Ca, Mg, FeII, and FeIII. Illites are amongst the commonest clay minerals; they develop by the alteration of micas, alkali feldspars etc, under alkaline conditions. A very important pore-blocking clay.
3. The montmorillonite group, which includes montmorillonite, nontronite, bentonite and beidellite. This group is especially notable for the way in which it takes up and loses water (swelling clays), and for its important base exchange properties. Montmorillonites are formed by the alteration of mafic igneous rocks, or other silicates low in K, under alkaline conditions, providing Ca and Mg are present.
4. The chlorite group, heterogeneous group of green clay minerals - mostl formed as alteration products of ferromagnesion minerals in metamorphic rocks or igneous rocks. Important component in mudstones, occasionally pore-blocking in sandstones.