Bentonite

Bentonite is an absorbent, swelling clay consisting mostly of montmorillonite. It usually forms by weathering of volcanic ash in seawater, which converts the volcanic glass in the ash into clay minerals. Bentonite beds are white, pale blue, or green in fresh exposures, turning to a cream colour and then to yellow, red, or brown as the exposure is weathered further.

Bentonite is a clay commonly formed by the alteration of volcanic ash, consisting predominantly of smectite minerals, usually montmorillonite. Other smectite group minerals include hectorite, saponite, beidellite, and nontronite. Smectites are clay minerals, i.e., they consist of individual crystallites, the majority of which are in the largest dimension. Smectite crystallites themselves are three-layer clay minerals. They consist of two tetrahedral layers and one octahedral layer.