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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 particles less than 4 μm (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. Clays 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 small size, flake or layered 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 and a major component of bentonite.

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.

Quick clay is a unique type of marine clay, indigenous to the glaciated terrains of Norway, Canada, and Sweden. It is a highly sensitive clay, prone to liquefaction which has been involved in several deadly landslides.

Historical and modern uses of clay

Clays are heavy in texture yet soft to the touch. Clay is a malleable substance when wet, which means it can be shaped easily with the hands. When dry, it becomes firm and when "fired," or hardened by intense heat, clay becomes permanently solid. A fireplace specifically designed for hardening clay is called a kiln. These properties make clay an ideal subtance for making pottery, stoneware and various other practical items. Early humans discovered the useful properties of clay in prehistoric times, and one of the earliest artifacts ever uncovered is a drinking vessel made of sun-dried clay. Depending on the content of the soil, clay can appear in various colors, from a dull gray to a deep orange-red.

Clays sintered in fire were the first ceramic, and remain one of the cheapest to produce and most widely used materials even in the present day. Bricks, cooking pots, art objects, dishware, spark plug bodies, 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.

See also

• Ceramics
• Clay pit
• Grain size
• List of minerals
• Plasticine
• Pottery
• Clay court

References

• Clay mineral nomenclature American Mineralogist.

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