Bentonite is commonly referred to as clay containing at least 70% of the mineral montmorillonite. Montmorillonite (hereinafter referred to as MM) is a highly dispersed layered aluminosilicate, the specific surface area of which can reach 800 m²/g. The crystal lattice of MM consists of silicate layers formed by two tetrahedral silica-oxygen meshes connected by an octahedral alumino-oxygen mesh by fusion with it through common apical oxygen atoms. Moreover, a characteristic feature of MM is that in the octahedral layer a part of trivalent aluminum atoms is replaced by atoms of divalent cations due to which silicate layers acquire excessive negative charge, which is compensated by cations located between the layers.

Such structure of MM particles causes its high hydrophilicity and adsorption properties. When interacting with water, interlayer cations are hydrated, which causes an increase in the distance between the layers of MM. This phenomenon is called swelling, in which bentonite can increase its volume several times. If swelling occurs in a limited volume, bentonite forms a dense watertight mass. In the free state, however, if the amount of water is further increased, the MM aggregates separate and a highly dispersed stable suspension is formed. Tactoids can swell up to disintegration into individual particles, and the suspension becomes practically a semi-colloid with thixotropic properties. In the hydrated state MM acquires the ability to substitute and exchange interlayer charge-compensating cations, as the force of their interaction with silicate layers weakens.

Due to the properties noted above, bentonite has found wide application in more than 200 different fields and applications. It is used as a binder in molding mixtures and iron ore pellets, for obtaining optimal rheological properties and reducing the filtration index of drilling fluids in well drilling, horizontal directional drilling and tunneling.

Besides, bentonite is effectively used in agriculture – in production of mixed fodder, as animal bedding, mineral-food and regulatory additives, for soil amelioration, as well as for clarification of wines and juices. Bentonite is used to produce the best quality clumping cat litter.

Bentonites with montmorillonite containing mainly exchangeable sodium cations have the best technological properties. Deposits of natural sodium bentonites are found in Transcaucasia and Kazakhstan. Unlike them, almost all Russian bentonites are calcium-magnesium.