Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

s, known as

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

. It is named after Montmorillon in

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

. Montmorillonite, a member of the smectite group, is a 2:1 clay, meaning that it has two

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...

sheets of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

sandwiching a central octahedral sheet of alumina. The particles are plate-shaped with an average diameter around 1

μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...

and a thickness of 0.96 nm; magnification of about 25,000 times, using an electron microscope, is required to "see" individual clay particles. Members of this group include, amongst others, saponite, nontronite, beidellite, and

hectorite Hectorite is a rare soft, greasy, white clay mineral with a chemical formula of . Hectorite was first described in 1941 and named for an occurrence in the United States near Hector (in San Bernardino County, California, 30 miles east of Barstow ...

. Montmorillonite is a subclass of smectite, a 2:1 phyllosilicate mineral characterized as having greater than 50% octahedral charge; its cation exchange capacity is due to isomorphous substitution of Mg for Al in the central alumina plane. The substitution of lower valence cations in such instances leaves the nearby oxygen atoms with a net negative charge that can attract cations. In contrast, beidellite is smectite with greater than 50% tetrahedral charge originating from isomorphous substitution of Al for Si in the silica sheet. The individual crystals of montmorillonite clay are not tightly bound hence water can intervene, causing the clay to swell, hence montmorillonite is a characteristic component of swelling soil. The

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

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 . Potassium, iron, and other cations are common substitutes, and the exact ratio of cations varies with source. It often occurs intermixed with

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorou ...

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...

, illite, cookeite, and

kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahed ...

Cave conditions

Montmorillonite can be concentrated and transformed within

cave A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

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 its formation. Montmorillonite can then transform to palygorskite under dry conditions and to halloysite-10Å ( endellite) in acidic conditions (pH 5 or lower). Halloysite-10Å can further transform into halloysite-7Å by drying.

Uses

Montmorillonite is used in the oil drilling industry as a component of

drilling mud In geotechnical engineering, drilling fluid, also called drilling mud, is used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are als ...

, making the mud slurry

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the in ...

, which helps in keeping the drill bit cool and removing drilled solids. It is also used as a

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

additive to hold soil water in drought-prone soils, used in the construction of earthen dams and

levee A levee (), dike (American English), dyke (Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually earthen and that often runs parallel to the course of a river in its floodplain or along low-lying coastli ...

s, and to prevent the leakage of fluids. It is also used as a component of

foundry sand Molding sand, also known as foundry sand, is a sand that when moistened and compressed or oiled or heated tends to pack well and hold its shape. It is used in the process of sand casting for preparing the mold cavity. Green sand Green sand is an a ...

and as a

desiccant A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids that absorb water. Desiccants ...

to remove

moisture Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

from air and gases. Montmorillonite clays have been extensively used in catalytic processes. Cracking catalysts have used montmorillonite clays for over 60 years. Other acid-based catalysts use acid-treated montmorillonite clays. Similar to many other clays, montmorillonite swells with the addition of water. 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 nonexplosive agents for splitting rock in natural stone quarries in an effort 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 Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-m ...

useful also as an annular seal or plug for water wells and as a protective liner for landfills. Other uses include as an anticaking agent in animal feed, in papermaking to minimize deposit formation, and as a retention and drainage aid component. Montmorillonite has also been used in

cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protec ...

. Sodium montmorillonite is also used as the base of some cat litter products, due to its adsorbent and clumping properties.

Calcined clay products

Montmorillonite can be calcined to produce arcillite, a porous material. This calcined clay is sold as a soil conditioner for

playing fields A pitch or a sports ground is an outdoor playing area for various sports. The term ''pitch'' is most commonly used in British English, while the comparable term in American and Canadian English is playing field or sports field. For most sports ...

and other soil products such as for use as bonsai soil as an alternative to akadama.

Medicine and pharmacology

Montmorillonite is effective as an adsorptive of heavy metals, however the impact this has on human health is unknown. It's assumed that heavy metal adsorption would only be applicable when the clay has direct contact. Hence it will not help when ingested, as it almost certainly doesn't pass through the intestinal mucous membranes. For external use, montmorillonite has been used to treat

contact dermatitis Contact dermatitis is a type of acute or chronic inflammation of the skin caused by exposure to chemical or physical agents. Symptoms of contact dermatitis can include itchy or dry skin, a red rash, bumps, blisters, or swelling. These rashes are ...

Pet food

Montmorillonite clay is added to some dog and cat foods as an anti-caking agent and because it may provide some resistance to environmental toxins, though research on the subject is not yet conclusive. In a fine powder form, it can also be used as a flocculant in ponds. Tossed on the surface as it drops into the water, making the water "clouded", it attracts minute particles in the water and then settles to the bottom, cleaning the water. Koi and goldfish (carp) then actually feed on the "clump" which can aid in the digestion of the fish. It is sold in pond supply shops.

Discovery

Montmorillonite was first described in 1847 for an occurrence in Montmorillon in the department of Vienne,

, more than 50 years before the discovery of

in the US. It is found in many locations worldwide and known by other names. Recently, a new source of Montmorillonite has been explored in

Sulaiman Mountains The Sulaiman Mountains, also known as Kōh-e Sulaymān ( Balochi/Urdu/ fa, ; "Mountains of Solomon") or Da Kasē Ghrūna ( ps, د كسې غرونه; "Mountains of Kasi"), are a north–south extension of the southern Hindu Kush mountain system i ...

Pakistan Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...

References

*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.
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