The term Bentonite was first used for a clay found in about 1890 in upper cretaceous tuff near Fort Benton, Montana. The main constituent, which is the determinant factor in the clay's properties, is the clay mineral montmorillonite ,generated frequently from the alteration of volcanic ash, consisting predominantly of smectite minerals,
Bentonite deposits are normally exploited by quarrying. Extracted bentonite is distinctly solid, even with a moisture content of approximately 30%. The material is initially crushed and, if necessary, activated with the addition of soda ash (Na2CO3). Bentonite is subsequently dried (air and/or forced drying) to reach a moisture content of approximately 15%. According to the final application, bentonite is either sieved (granular form) or milled (into powder and super fine powder form). For special applications, bentonite is purified by removing the associated gangue minerals, or treated with acids to produce acid-activated bentonite (bleaching earths), or treated with organics to produce organoclays.