Bentonite is a type of clay that swells to several times its original size when wet. It is an important industrial material used to bond molding sands, in the preparation of oil-well drilling mud, and to remove the color from oils. Bentonite consists of several clay minerals, of which montmorillonite is the chief mineral. Most bentonite is found in beds from 1 inch to 10 feet (2.5 centimeters to 3 meters) or more in thickness. Bentonite forms from volcanic ash.
