Clays

Naturally occurring aluminosilicates which form pastes and gels with water.

Clays are active mineral portions of soils, less than 2 pm in diameter and are predominantly colloidal and crystalline alumino-silicates. They are characteristically plastic and virtually impermeable when wet. They crack when dry. A clay particle is called micelle.
Soil clays act in three ways: as particles of<2 pm diameter, as minerals, and as part of the soil structure.
Clays have a net negative charge which attracts and holds charged cations such as K', Na', Na+, Ca++, Mg++ and H+. The amounts of positive ions held by clays vary with the type of clays. The quantity of cations that can be held or exchanged by a given amount of soil is the cation exchange capacity (CEC) of that soil. The exchangeable cations are not easily lost by leaching until they are exchanged by other cations, generally hydrogen ions (H+). Plant roots can use these firmly-held cations as nutrients.
Clays are referred to by group names like silicate clays and sesquioxide clays. Montmorillonite, mica (illite), vermiculite, chlorite and kaolinite belong to the class of silicate clays, while iron oxides, aluminum oxides, manganese oxides and titanium oxides belong to the class of sesquioxide clays. These clays occur as a major constituent of young soils formed from volcanic ash, and include allophanes and imogolite. Sesquioxide clays do not swell, are not sticky and, therefore, do not behave like silicate clays.
Clay minerals are essentially crystalline and have a layer structure. These are responsible for the plastic properties of clay. Most clays become plastic when mixed with water. The type of clay mineral, particle size and shape, organic matter, soluble salts, adsorbed ions, and the amount and type of non-clay minerals are all known to affect the plastic properties of clay.
Irrigation techniques employed for clayey soils encounter problems which are due to (a) large volume changes caused when clays are wetted and dried, (b) great stickiness and plasticity, and (c) exceptionally high water retention capacity. These characteristics cause inadequate aeration when clay is wet.
The properties of clay minerals are controlled by at least six major factors. These are clay mineral composition, non-clay mineral composition, organic material, soluble salts, exchangeable ions and texture. Generally, the clay mineral composition is the most important factor and sometimes as little as 5% of a particular clay mineral may largely determine the properties of a whole body of clay.
The commercial value of clays is related to their mineral and chemical composition. Major mineral or soluble salt impurities are quartz, mica, carbonates, iron oxides, sulphides and feldspar.
Clays are used for various applications like pottery, insulation, etc.