bentonite

Bentonite is a natural clay formed from volcanic ash and dominated by montmorillonite, a swelling aluminosilicate mineral in the smectite group. It expands when hydrated and has high plasticity and absorbent properties. The name derives from Fort Benton, Wyoming, where the clay was first described in the late 19th century. Commercial deposits are found worldwide, with major producers in the United States, Europe, and Asia.

Bentonite’s structure features layers with isomorphic substitutions that give it a net negative charge and a high cation exchange capacity. The exchangeable cations, typically sodium or calcium, influence swelling behavior: sodium-rich bentonites swell more than calcium-rich varieties and can form gels in water. When dispersed in water, the platelets separate and the material becomes a viscous, high-surface-area colloid with strong binding and sealing properties.

Natural bentonite is mined from sedimentary beds and processed by drying, grinding, and milling to produce powder, granules, or pellets. Some material is activated to sodium bentonite to enhance swelling. The product is used in various forms depending on application, including standard drilling-grade bentonite and specialized grades for industrial uses.

Major applications include drilling fluids for oil and gas wells, where the clay helps control filtration and provides lubrication; binding agents for foundry sands; binders in iron ore pelletization; and sealants for liners, ponds, and landfills. Bentonite also serves as an adsorbent and thickener in cosmetics, pharmaceuticals, and environmental cleanup, and is used in cat litter and as a soil conditioner. Safety considerations center on dust control, as inhalation of bentonite dust can be hazardous.