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Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate and phyllite.

Clay minerals include the following groups:

• Kaolin group which includes the minerals kaolinite, dickite, halloysite and nacrite.^[1]
  • Some sources include the serpentine group due to structural similarities (Bailey 1980).
• Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites for example saponite.^[1]
• Illite group which includes the clay-micas. Illite is the only common mineral.^[1]
• Chlorite group includes a wide variety of similar minerals with considerable chemical variation.^[1]

Knowledge of the nature of clay became better understood in the 1950s with advancements in microscope technology necessary to analyze the infinitesimal nature of clay particles.^[2] Standardization in terminology arose during this period as well^[2] with special attention given to similar words that resulted in confusion such as sheet and plane.^[2]

Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO₄ and AlO₄ tetrahedra. Each tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction (i.e. all of the unshared vertices are on the same side of the sheet). These tetrahedral sheets have the chemical composition (Al,Si)₃O₄.

In clays the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminium or magnesium, coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also form part of one side of the octahedral sheet but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorised depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.

Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na⁺ or K⁺. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.

• Clay
• Expansive clay

1. ^ ^{a b c d} Amethyst Galleries. "The Clay Mineral Group." 2006. February 22, 2007. [1]
2. ^ ^{a b c} Bailey, S. W., 1980, Summary of recommendations of AIPEA nomenclature committee on clay minerals, American Mineralogist Volume 65, pages 1-7. [2]