Claystone. Credit: Mineral Information Institute.

Clay is any soil material with a particle size of less than 2–4 micrometers in diameter, i.e., finer-grained than silt or sand; an earthy particulate which becomes plastic when wet, including mud (which is used in oil drilling). Clays are used as catalysts in petroleum refining, for making molds for casting and, when molded and fired, for ceramics, porcelain, and pottery, bricks and tiles. They are also used in making cement and rubber, and as ion-exchange agents for softening hard water.


Clay rocks, including mudstones and shales, are microcrystalline rocks composed mainly of clay-size particles. Their mineralogical composition is highly variable, but they usually contain a high proportion of clay minerals, hydrated aluminum and magnesium silicates, including bentonite, chlorite, diaspore (hydrated aluminum oxide), illite (hydrated mica), kaolinite, and meerschaum.