Bentonite is essentially a clay with predominance (75-85%) of the clay mineral montmorillonite (a complex hydrate of aluminium, magnesium and silicon) and containing an exchangeable base — sodium or calcium. Depending on whether sodium or calcium is the dominant exchangeable base, bentonite is called sodium bentonite or calcium bentonite. The
former is also called swelling type or (sometimes) true bentonite and the latter non-swelling type or pascalite or (sometimes) sub-bentonite. But transitional types between these two are also found in nature. Bentonite is believed to have been formed by the alteration of volcanic ash deposits mostly of Upper Cretaceous age.
In India, the record of the earliest mining of bentonite dates back to pre-independence period when it was mined in Kashmir. But now, the most important mining and processing centre is in the region comprising Kheda, Sabarkantha, Bharuch, Jamnagar, Amreli, Bhavnagar and Kutch districts of Gujarat from where 50,000-120,000 tons of bentonite are mined annually. Some mining is also carried out in the Barmer district of Rajasthan. Its
occurrence is reported in Bhagalpur district of Bihar, but there is no significant mining/processing activity.
The important industrial uses of bentonite are:
- Oil well drilling
- Refining of oils and fats
- Refractories and abrasive wheels
- Pelletizing of iron ore
- Cosmetics and pharmaceuticals
- Detergents and soaps
- Insecticide and fungicide
- Paints and polishes
- Paper, oil cloth and linoleum
- Animal and poultry feed