Each type of clay has a different chemical composition.
Geologists define clay as a mineral comprised of alumina silicate with an ultra-fine particle size of four microns of less, unlike silts which have particles greater than four microns in size. Clays are the most abundant of minerals contained in sedimentary rocks. According to Charles Weaver and Lin Pollard, authors of "The Chemistry of Clay Minerals," the three most common types of clay are illite, montmorillonite and kaolinite.
Clay Formation and Structure
According to Nora Foley of the U.S. .Geological Survey, clay minerals form where rocks come into contact with steam, air or water. Weathering rock formations, geothermal fields, riverbeds, sea floors and volcanic deposits are typical environments for clay mineral formation. These types of environments cause clay minerals to form from pre-existing mineral deposits through erosion, weathering or process of chemical transformation called diagenesis. Common to all clays is their layered structure. Each layer or sheet is comprised of horizontally-linked tetrahedral or octahedral units. A tetrahedron is a regular equilateral triangular pyramid and an octahedron is a solid shape made up of eight congruent equilateral triangular faces.
Illite
Illites are comprised of two silica tetragonal sheets pointing inward which sandwich a central octahedral sheet. This type of mineral clay is most commonly found in cool climates. Weathering of silicates such as feldspar in alkaline conditions where high concentrations of aluminum and potassium are present contributes to the formation of illites. The chemical composition of illites is predominantly potassium, aluminum and silicon. The chemical expression for illite is (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]. Illite clays have been used for centuries throughout for therapeutic purposes and is applied topically as well as taken orally.
Montmorillonite
Montmorillonite belongs to the smectite group of clays which are characterized by their composition of two tetrahedral sheets and a central sheet of alumina octahedrons. This type of clay is usually the product of the weathering of bentonite by volcanic ash, however, the mineral can also form in cave environments. Silicon and aluminum are the main chemical components of this type of clay. The chemical formula for it is (Na,Ca)0,3(Al,Mg)2Si4O10(OH)2•n(H2O). Montmorillonite is widely used in the oil drilling industry as an additive to improve the workability of drought-prone soils and to keep drill bits cool.
Kaolinite
Kaolin is a soft, white mineral also referred to as china clay or white clay. This type of clay is the product of the chemical weathering of rocks containing alumino-silicate minerals in a hot, moist climates -- such as rainforests. The structural composition of kaolin is one tetrahedral sheet condensed by oxygen atoms with an alumina octahedral sheet. Oxygen, silicon and aluminum are the main chemical compounds in kaolin, which has a chemical formula of Al2Si2O5(OH)4 . This type of clay is the main ingredient used in porcelain making and is also widely used in the production of paper, cosmetics, medicine, toothpaste and as a food additive.
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