Bentonite is essentially a high swelling clay mineral whose name was derived from the location of the first commercial deposits mined at Fort Benton, Wyoming U.S.A.

Geologically, bentonites are mainly of volcanic origin and can date to Cretaceous or even Jurassic times. The majority of commercial deposits were formed by the hydrolysis of volcanic rock or ash and are found in every continent except Antarctica.

Mineralogically, bentonite principally consists of montmorillonite in combination with various ancillary minerals. Montmorillonite belongs to the smectite group of clays which also include similar behaving minerals such as beidellite, saponite and hectorite.

The structure of montmorillonite is fundamentally a three layer ‘flake” or “platelet” with an octahedral aluminium hydroxyl sheet sandwiched between two layers of silicon-oxygen tetrahedra. The aluminium atoms however, are partially substituted with either magnesium or iron, creating a charge deficiency within the unit structure. This results in a small negative charge on the basal plane of the silica tetrahedra, which is balanced by adsorption of (exchangeable) cations between adjacent platelets. In naturally occurring montmorillonites, these cations are usually calcium, sodium or magnesium, depending on the weathering agent associated with the original formation of the mineral.

Chemically, montmorillonite can be represented by the formula

(Na,Ca)_0.33 (Al_1.67Mg_0.33)Si₄O₁₀(OH)_2.nH₂O

The physical structure of montmorillonite can be considered as a cluster of platelets, with an individual platelet tending to have a broad basal plane varying in length from 0.2 to 2.0 microns and a thickness of 6 to 10 Angstroms. In practice, the actual crystal morphology and aggregate type can vary considerably according to the origin of the montmorillonite and the nature of the final deposit.

The resultant characteristics and properties of bentonites are complex and often individual to a specific source. Nonetheless, their combined range of physiochemical properties, particularly their fundamental structure, extremely fine particle size and overall anionic charge, make them particularly suitable as micro-particles for Paper Retention, Drainage and Formation systems as well as high-surface area adsorbents in Pitch, De-inking or Effluent Treatment applications.