Bentonite is a highly colloidal clay mineral decomposition of volcanic ash, having the ability to absorb large quantities of water and to expand to several times its normal volume. Bentonite, composed of a number of layers, each approximately 10 angstroms thick, and each consisting of two tetrahedral and one octahedral unit. This structure is responsible for three important characteristics, the ability to absorb large amounts of moisture, the ability to exchange ions and thixotropic gel formation. Bentonite is naturally occurring hydrated Alumina Silicate, there are two main varieties, calcium and sodium, where sodium Bentonite is the more commercially important. Calcium Bentonite deposits are much more common than sodium Bentonite deposits. However, calcium Bentonite can be changed to sodium Bentonite by the ion exchange process. The swelling type or sodium Bentonite, which has single water layer particles containing Na+ as the exchangeable ion. The other has double water layer particles with Ca++ as the exchangeable ion. It is called calcium Bentonite or non-swelling type.The clay-water characteristics are of primary importance. The chemical composition of the Bentonite is a consideration only because changes in physical properties accompany changes in chemical composition. Sodium Bentonite swells or expands to a greater degree than its calcium equivalent. Small percentage of cation exchange sites are occupied by another ion, such as calcium, a state is developed in the clay which promotes both optimum swelling and fastest hydration. Sodium Bentonite has higher swelling capacity, this property develops higher viscosity in suspensions, greater state of disaggregation, formation of smaller clay particles or greater colloidal properties.

The property of water absorption is makes bentonite suitable for various industries and the types of montmorrilonite decide the final usage and the process of the same.