Bentonite

Bentonite is a very widely distributed clay material that is the result of the devitrification and chemical alteration of glassy volcanic ash or tuff. Bentonite should be used as the name of a rock derived from volcanic glass and it is commonly composed of the mineral montmorillonite but less often of beidellite. Its characteristic minerals are completely crystalline and have a micaceous habit, high birefringence and facile cleavage. It contains no gel colloids, and few of its crystal particles are so small as to reach "colloidal size". It shows high adsorptive powers and this property is more dependent upon physical form than upon chemical composition. That is upon the micaceous structure and easy cleavage which give very great surface area and the 'felt-like texture which facilitates permeability.
The clay minerals of the type here described resemble the micas in many ways, but do not seem to possess the marked chemical variability of that group. A large number of analyses of the clay minerals from bentonite indicates that the most widespread of these is montmorillonite with the formula (Mg, Ca)O A1203 5SiO₂, nH₂0.
A reanalysis of the type montmorillonite from Montmorillon, France, gives the same formula, and the mineral has also been recognized in lithium-bearing pegmatites and in fullers' earth.