A montmorillonite-type clay formed by the alteration of volcanic ash. It varies in composition and is usually highly colloidal and plastic. Swelling bentonite is so named because of its capacity to absorb large amounts of water accompanied by an enormous increase in volume. Occurs in thin deposits in the Cretaceous and Tertiary rocks of the 293 Western United States. It is used for making refractory linings, water softening, decolorizing oils, thickening drilling muds, and preparing fine grouting fluids. As a mud flush, bentonite is used at a concentration of about 3 lb/ft₃(48.1 kg/m₃) of water.