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Bentonite is an absorbent aluminium phyllosilicate, consisting mostly of montmorillonite.These are three layered Aluminosilicate with molecules of water between layers.
For industrial purposes, two main classes of bentonite exist:

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Sodium bentonite
Sodium bentonite expands when wet, possibly absorbing several times its dry mass in water. Because of its excellent colloidal properties it is often used in drilling mud for oil and gas wells and for geotechnical and environmental investigations.The property of swelling also makes sodium bentonite useful as a sealant, especially for the sealing of subsurface disposal systems for spent nuclear fuel and for quarantining metal pollutants of groundwater. In fact in all bentonites available in the world, the sodium variety has high swelling and gelling properties along with good viscosity and liquid limit.