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

Clay is a generic term for an aggregate of hydrous silicate earth particles less than 4 micrometers in diameter. They are 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.

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 layered clay formed by seasonal differences in erosion and organic content so as to make visible annual layers. This type of deposit is common in former glacial lakes from the ice age.

See also: Grain size, List of minerals

Clay is a by-product of the earth’s crust, resulting from the decomposition of aging rocks exposed to the natural elements. Formed from small crystal compounds it is made up of the mineral kaolin, which consists of silica (SiO2) and alumina (Al2O3). These elements, silicon, oxygen and aluminum are the most abundant in the earth’s crust.