Bentonite clay is composed dominantly of the clay mineral smectite. Its physical properties and characteristics are dictated by this clay mineral (Wright, 1968). In the majority of cases, bentonites are formed by the alteration of volcanic ash and rocks after intense contact to water.
Minerals of the smectite group form platelets of a few microns in width and depth but only a few Angstrom in thickness. Individual platelets are composed of three layers, an octahedral layer comprising of Mg, Fe, Al and OH-groups, sandwiched between two tetrahedral layers with Si-O tetrahedrons. Through isomorphic substitution, of divalent cations by monovalent cations (tri-octahedral smectites) or trivalent cations by bivalent cations (di-octahedral smectites), in the octahedral layer as well as Si by Al in the tetrahedral layer, a layer charge is generated. This layer charge is counterbalanced by mono- or divalent cations in the interlayer space which link the individual platelets to stacks. Due to the weak charge of the crystalline layers, the interlayer cations, predominately Na, Mg and Ca, are exchangeable. One of the most abundant smectite minerals is the di-octahedral montmorillonite where part of the Al contained in the octahedral sheets is replaced by Mg and Fe, and another electrical charge generated in the tetrahedral layer by the replacement of Si by Al.
The most important property of smectite clay minerals is the behaviour in contact with water. The weak layer charge allows the interlayer cations to adsorb water molecules to form a hydration shell. This inner-crystalline swelling can be observed by an increase in volume of the clay minerals which can be many times the original volume. Monovalent cations present, such as Na-ions in the interlayer space of the minerals, can hydrate to the extent that individual platelets are separated from each other. The colloidal properties of such dispersions are controlled by the electrical forces generated within the clay mineral platelets and any dissolved and adsorbed electrolytes. In a fully exfoliated state the specific surface area of smectite minerals can be as high as 750m²/g. Free swelling, the resulting high surface area and a negative layer charge provide desired gelling and sealing properties, binding behaviour, and preferred adsorption of cationic molecules.
Bentonites having a dominant occupation of sodium ions in the natural state are rare and are isolated to specific global locations, e.g. Wyoming in the USA, compared to the more abundant Ca-/Mg-bentonites. Ca-/Mg- bentonites have the ability to be modified by exchanging the divalent Ca-/Mg- cations with Na-ions by treatment with soda ash. During treatment Ca and Mg is precipitated after being replaced by Na-ions from the readily soluble Na-carbonate. After modification, these bentonites behave as Na bentonites with high swelling and gelling behaviour.