What is a bentonite in geology ?

Bentonite : definition

Bentonite is a clayey rock composed of a mixture in varying proportions of different clay minerals of the smectite group, montmorillonite, nontronite, beidellite and saponite being the most important.

Bentonites (or "bentonite clays") have absorbent properties widely used in industry, they are also essential for the drilling industry. Related uses exist in ceramics, pharmaceuticals, cosmetics and the rubber industry.