Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central octahedral sheet. The particles are plate-shaped with an average diameter of approximately one micrometre. Members of this group include saponite.
It is the main constituent of the volcanic ash weathering product, bentonite.
The water content of montmorillonite is variable and it increases greatly in volume when it absorbs water. Chemically it is hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O. Potassium, iron, and other cations are common substitutes, the exact ratio of cations varies with source. It often occurs intermixed with chlorite, muscovite, illite, cookeite and kaolinite.
Montmorillonite can be concentrated and transformed within cave environments. The natural weathering of the cave can leave behind concentrations of aluminosilicates which were contained within the bedrock. Montmorillonite can form slowly in solutions of aluminosilicates. High HCO3 concentrations and long periods of time can aid in the formation. Montmorillonite can then transform to palygorskite under dry conditions and to endellite in acidic conditions (pH 5 or lower). Endellite can further transform into halloysite by drying.
Montmorillonite is used in the oil drilling industry as a component of drilling mud, making the mud slurry viscous which helps in keeping the drill bit cool and removing drilled solids. 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.
Similar to many other clays, montmorillonite swells with the addition of water. However, some montmorillonites expand considerably more than other clays due to water penetrating the interlayer molecular spaces and concomitant adsorption. The amount of expansion is due largely to the type of exchangeable cation contained in the sample. The presence of sodium as the predominant exchangeable cation can result in the clay swelling to several times its original volume. Hence, sodium montmorillonite has come to be used as the major constituent in non-explosive agents for splitting rock in natural stone quarries in order to limit the amount of waste, or for the demolition of concrete structures where the use of explosive charges is unacceptable.
This swelling property makes montmorillonite-containing bentonite useful also as an annular seal or plug for water wells and as a protective liner for landfills. Other uses include as an anti-caking agent in animal feed, in paper making to minimize deposit formation and as a retention and drainage aid component. Montmorillonite has also been used in cosmetics. Montmorillonite is known for its adsorbent qualities and has been used successfully in scientific trials to eliminate atrazine from water.
Calcined clay products
Montmorillonite can be (calcined) to produce arcillite, a porous, calcined clay sold as a soil conditioner for playing fields and other soil products such as for use as bonsai soil as an alternative to akadama.
Use in medicine and pharmacology
Montmorillonite clay is widely used in medicine and pharmacology.
For internal use, montmorillonite is effective in the treatment of irritable bowel syndrome. It is also used for the prevention of aflatoxicosis, and in the treatment of constipation. Also, a modified version inhibits intestinal absorption of cholesterol (nanotechnology research), and absorbs uric acid. It is used in agriculture to improve growth performance of piglets and fish. It ameliorates hyperthyroidism of rats and mice. Montmorillonite is also used in other related research.
Montmorillonite is proven to be effective in use as an adsorptive of heavy metals, toxins, and hazardous chemicals.
Antibacterial effects of montmorillonite are well demonstrated. 
For external use, montmorillonite has also shown its effectiveness. It has also shown itself useful for tissue engineering.
Montmorillonite is widely used in pharmacology for a variety of application, such as stabilization of suspensions and emulsions, viscosizing, adhesion to the skin, and tablet making. It is also used as drug carrier, or as part of a drug delivery system, such as for controlled drug release; including for gene delivery, and for drug targeting to specific tissues. It is also used for stability enhancement in drug and nutrient application. There are also other similar uses.
Montmorillonite is also used in the production of pharmaceuticals, e.g. as a catalyst. 
Montmorillonite was discovered in 1847 in Montmorillon in the Vienne prefecture of France, more than 50 years before the discovery of bentonite in the US. It is found in many locations world wide and known by other names.
Montmorillonite is also known to cause micelles (lipid spheres) to assemble together into vesicles. These are structures that resemble cell membranes on many cells. It can also help nucleotides to assemble into RNA which will end up inside the vesicles and, under the right conditions, will replicate themselves. This process may have led to the origin of life on Earth. 
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42. ^ FT-IR and FT-Raman spectroscopic studies of adsorption of isoniazid by montmorillonite and saponite. Akyuz, S., Akyuz, T. 2008 Vibrational Spectroscopy 48 (2), pp. 229-232
43. ^ Fe-exchanged montmorillonite K10––the first heterogeneous catalyst for acylation of sulfonamides with carboxylic acid anhydrides. Tetrahedron Letters, Volume 45, Issue 24, 7 June 2004, Pages 4805-4807 Devendrapratap U Singh, Pankajkumar R Singh, Shriniwas D Samant
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45. ^ Montmorillonite KSF clay as an efficient catalyst for the synthesis of 1,4-dioxo-3,4-dihydrophthalazine-2(1H)-carboxamides and -carbothioamides under solvent-free conditions using microwave irradiation. Davood Habibi, Omid Marvi Catalysis Communications, ISSN: 15667367, Vol: 8, Issue: 2, Date: February, 2007, Pages: 127-130
46. ^ Clays May Have Aided Formation of Primordial Cells
47. ^ Clay's matchmaking could have sparked life
* Papke, Keith G. Montmorillonite, Bentonite and Fuller’s Earth Deposits in Nevada, Nevada Bureau of Mines Bulletin 76, Mackay School of Mines, University of Nevada-Reno, 1970.
* Mineral Galleries
* Mineral web
List of minerals