Bentonite is a clay mineral of the smectite group and is composed mainly of montmorillonite.
The smectites are a group of minerals that swell as they absorb water or organic molecules within the structural layers; they also have considerable cationic exchange properties.
The clay mineral they are composed of in the crystalline state is derived from the devitrification, and consequent chemical change, of glass of magmatic origin, usually tufa or volcanic ash (definition by Ross and Shannon, 1926).
The nature and volcanic origins of bentonite deposits give rise to varieties of the mineral that are often extremely heterogeneous. The bentonites that are thus formed that can be described as sodium, calcium and acid bentonites.
The crystallographic basis of the montmorillonite (bentonite) is typical of phyllosilicates: sheets of AIX octahedrons (X=oxygen or oxydril) between two sheets of SiO4 tetrahedrons.
In the octahedron layer the aluminium may be replaced by magnesium, thus giving rise to an excess negative charge: the negative charge in excess is compensated for by various mono and bivalent cations (Ca, Mg, Na…).
This elementary particle is a lamella: the various lamellas are held together in “packets” by Van der Waals force fig, but they can be “delamellised” and dispersed in water in submicronic particles until a specific superficial area of 800 m2/g is developed.