What is "Bentonite"

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 ages, i.e. the period of ammonites and dinosaurs. 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 10 to 20 per cent various mineral impurities such as feldspars, calcite, silica, gypsum, etc. 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. However, the aluminium atoms are partially replaced by the substitution of either magnesium or iron atoms, thereby creating a charge deficiency within the unit structure. This results in a small negative charge on the basal plane of silica tetrahedra which is balanced by absorption of (exchangeable) cations between adjacent platelets. In natural montmorillonites these cations are usually calcium, sodium or magnesium according to the weathering agent associated with the formation of mineral.


Chemically, montmorillonite can be represented by the formula

(Na,Ca)0.33 (Al1.67Mg0.33)Si4O10(OH)2.nH2O where M+ denotes the exchangeable cation.


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, the combined range of physio-chemical properties of extremely fine particle size and exchangeable cation makes many absorbent and rheological applications possible. Moreover, these properties allow many new uses after beneficiation and treatment.

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