Midland Provincial Park, Alberta, Canada. Bentonite is an absorbent aluminum phyllosilicate, a generally impure clay consisting mostly of montmorillonite. There are a few types of bentonites and their names depend on the dominant elements. Bentonite usually forms from weathering of ancient volcanic ash. It looks harmless, like gray popcorn, but any amount of moisture causes bentonite to swell and become very greasy and slippery. The term bentonite has also been used for clay beds of uncertain origin.