Also found in: Dictionary, Acronyms, Wikipedia.
(named after the French city Montmorillon in Vienne Department), a clay mineral of the lamellar silicate subclass. It has a variable chemical composition, (Ca, Na)(Mg, Al, Fe)2[(Si, Al)4O10] (OH)2 • nH2O. Its structure is characterized by the symmetrical arrangement of lamellar aggregates (as in pyrophyllite). Interlaminar water molecules and atoms constituting exchange bases (for example, Ca and Na) are distributed between the pyrophyllite sheets. The lamellar aggregates are located at a considerable distance from each other. The mineral forms compact argillaceous masses. Monoclinic crystals rarely occur and can be seen only through an electron microscope; irregular sheets are most commonly observed.
Montmorillonite is white, pink, blue-gray, brown, red, or green, depending on the admixture content. Its hardness on Mohs’ scale is approximately 1, and its density is about 1,800 kg/m3. When wet, the mineral swells considerably as the water penetrates the interlayer spaces.
Montmorillonite is a typical product of aluminosilicate erosion under alkaline environmental conditions. It is the dominant mineral in bentonite and is present in soil, detrital loam, and other sedimentary rocks. Montmorillonite is an extremely valuable mineral. It is an active component of bleaching clay and fuller’s earth, which are used in the petroleum, textile, and soap industries for their adsorptive and saponifying properties.
V. P. PETROV