Naturally occurring sediments that are produced by chemical actions resulting during the weathering of rocks. Clays are composed of hydrated aluminum silicates, such as Kaolinite, Illite, Palygorskite, Attapulgite, Bentonite, and Montmorillonite. Small amounts of other minerals can change the color (white, yellow, brown or red) and texture of the clays. Clays may include all earths that form a paste with water and harden when heated.

Source: AAT 

http://vocab.getty.edu/page/aat/300010439