1. n. [Geology]
A type of smectite clay mineral that tends to swell when exposed to water. Montmorillonite forms through the alteration of silicate minerals in alkaline conditions in basic igneous rocks, such as volcanic ash that can accumulate in the oceans. Montmorillonite is a component of bentonite commonly used in drilling fluids.
2. n. [Drilling Fluids]
A hydratable, dispersible clay mineral of the smectite group. Montmorillonite is a three-layer, expanding clay with a large surface area and high cation-exchange capacity. Na+ and Ca+2 are the typical exchangeable cations. Sodium montmorillonite, also called sodium bentonite, is a premium clay mud additive. Natural deposits are found in Wyoming, North Dakota, South Dakota and Utah, USA. Calcium montmorillonite is a low-yield bentonite that is more widely distributed and used in many commercial applications, including drilling fluid.