Montmorillonite is a very soft phyllosilicate mineral that typically forms in microscopic crystals, forming a clay. It is the main constituent of the volcanic ash weathering product, bentonite. Montmorillonite's water content is variable and it increases greatly in volume when it absorbs water. Discovered 1847 in Montmorillon in the Vienne prefecture of France. It is found in many locations world wide.

Chemically it is hydrated sodium calcium aluminum magnesium silicate hydroxide (Na,Ca)_x(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O. The exact ratio of cations varies with source.

It is used in the oil drilling industry as a component of drilling mud, making the water slurry viscous which helps keeping the drill head cool and in removing drill debris. It is also used as a soil additive to hold soil water in drought prone soils, to the construction of earthen dams and levees and to prevent the leakage of fluids. It is also used as a component of foundry sand and as a desiccant to remove moisture from air and gases.

It is also used therapeutically on humans.

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