Bentonite

Bentonites are hydrated aluminosilicate minerals, comprised chiefly of montmorillonite.  Structurally they consist of alumina sheets sandwiched between tetrahedral SiO₄ units.  Water is usually present between the triple layers.

A simplified formula for montmorillonite is:

(OH)₂Al₂Si₄O₁₀ or Al₂O₃.4SiO₂.H₂O

In reality, a proportion of the Al³⁺ are replaced by Mg²⁺ and Fe³⁺ (the plates are therefore positively charged at their edges and negatively charged at their surfaces).  Wherever a Mg²⁺ occurs, the charge imbalance is compensated for by the presence of exchangeable cations – predominantly sodium or calcium, though minor amounts of several other metals will also be present.  As a result, bentonites typically exhibit a cation exchange capacity (CEC) of 70 – 100 meq/gram.

When bentonite is mixed with water, the water molecules will enter between the clay plates, forcing them apart.  While the plates are dispersed, the bentonite slurry becomes quite fluid. However, on standing the particles become oriented with the negative surfaces of the plates being attracted to the positive edges.  Viscosity increases and a gel is formed – a reversible effect known as thixotropy. 

A typical empirical formula for bentonite is:

(Al, Fe_0.67 Mg_0.33) Si₄O₁₀(OH)₂Na,Ca_0.33

For further information please contact RS Minerals.