For the town in the United States, see Clay, New York.

Clay is a generic term for an aggregate of hydrous silicate particles less than 4 micrometers in diameter. It consists of small crystals of the minerals silica (SiO₂) and alumina (Al₂O₃). These elements, silicon, oxygen and aluminium are the most abundant in the earth's crust. Clay is generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid, but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as silt by their oblong shape, affinity for water and high plasticity index.

There are three main groups of clays: Kaolinite-Serpentine, Illite, and Smectite. Altogether, there are about thirty different types of "pure" clays in these categories, but most natural clays are mixtures of these different types, as well as other weathered minerals.

Montmorillonite, with a chemical formula of (Na,Ca)_0.33(Al,Mg)₂Si₄O₁₀(OH)₂^.nH₂O, is typically formed as a weathering product of low silica rocks. Montmorillonite is a member of the smectite group.

Clays hardened by fire were the first ceramic, and remain one of the cheapest and most widely used materials to produce even in the present day. Bricks, cooking pots, art objects, spark plug tips, and even musical instruments such as the ocarina are all made with clay. Clay is also used in many industrial processes, such as paper making, concrete production, and chemical filtering.

Varve (or varved clay) is clay with visible annual layers, formed by seasonal differences in erosion and organic content. This type of deposit is common in former glacial lakes from the ice age.

See also

• Grain size
• List of minerals
• Pottery
• Plasticine
• Henry Clay