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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 weathering of silicate-bearing rocks, but some are formed by hydrothermal activity. When mixed with water, clays form a moldable substance, which when dried or heated loses its plasticity. Clays occur naturally in many soils.

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.

Clay is also the name of

See also: List of minerals

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