Bentonite is a soft clay substance composed essentially of clay minerals of the smectite (montmorillonite) group and is formed from in-situ chemical weathering of volcanic materials such as tuff or glass, volcanic ash , other igneous rocks, or from rocks of sedimentary origin. The nature of the weathering environment and the original composition of the volcanic material will generally determine the type of bentonite that forms.

The colour of bentonite ranges from white to light olive green, cream, yellow, earthy red, brown and sometimes sky blue when fresh but yellowing rapidly with exposure to air. When wet it is highly plastic and slippery. Bentonite feels and appears greasy or waxy. 

There are two basic types of bentonite depending on whether they contain sodium or calcium in the crystal lattice.

1. swelling bentonites - sodium bentonite contains sodium in the platy molecular structure; has strong swelling properties (ability to swell to many times its dry volume when wet) and possesses a high dry-bonding strength
2. non-swelling bentonites - calcium bentonite contains calcium in its structure; has a far lower capacity to swell when wet and usually exhibits greater adsorptive characteristics.

Uses of Bentonite

Swelling bentonite has many uses, due to its high plasticity and gelling property, its ability to take on large quantities of water and its capacity to swell to many times its dry volume. The applications for swelling bentonite include:

• well-drilling mud, where several pounds of bentonite are added per cubic foot of water to form a gel which lubricates the drill bit, helps to keep the walls of the borehole intact, reduces loss of drilling fluid and, when circulated during drilling, helps to bring the cuttings to the surface.
• pelletizing and binding agent for iron ore prior to smelting
• binder and digestive aid in animal feed
• sealing agent for reservoirs, lagoons and other engineering projects
• plasticizer in ceramic clay bodies
• binder of foundry sand
• carrier of insecticide and pesticide
• agent in water and effluent purification
• absorbent material for cat litter

The use as a binder in foundry molds is still considered a major application for which the swelling variety is preferred, although non-swelling bentonite is used for some foundry purposes.

Some non-swelling bentonites, depending on their chemistry and molecular structure, can be altered by acid leaching to render them useful as catalysts for petroleum cracking or for bleaching animal litter such as cat litter.

Mining of Bentonite
To be economically mineable, bentonite must be close to the surface. Material overlying the bentonite must first be removed by bulldozer, dragline or bucket excavator. The surface of the bentonite bed must be carefully scraped to remove impurities. Depending on the thickness and grade variation, the bentonite may be scraped off layer by layer or excavated with bucket loaders. The bentonite is processed by drying (sun-dried and kiln-dried),  grinding and bagging. It may be also be stored for bulk loading onto hopper trucks or rail cars.

Mining Areas

Bentonite is mined throughout the world but is abundant in Wyoming, USA and Greece.