Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite group, is a 2:1 clay, meaning that it has two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina. The particles are plate-shaped with an average diameter around 1 μm 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 saponite.

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. 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, and the exact ratio of cations varies with source. It often occurs intermixed with chlorite, muscovite, illite, cookeite, and kaolinite.

A sample of montmorillonite
Smectite group
(repeating unit)
Crystal systemMonoclinic
Crystal classPrismatic 2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 5.19 Å, b = 9.02 Å,
c = 12.4 Å; β = 94°; Z = 2
ColorWhite, pale pink, blue, yellow, red, green
Crystal habitcompact masses of lamellar or globular microcrystalline aggregates
Cleavage{001} perfect
Mohs scale hardness1–2
LusterDull, earthy
Specific gravity1.7-2
Optical propertiesBiaxial (-)
Refractive indexnα = 1.485–1.535 nβ = 1.504–1.550 nγ = 1.505–1.550
Birefringenceδ = 0.020
2V angleMeasured: 5° to 30°

Cave conditions

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 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.[5]


Structure of montmorillonite

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, used in 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.

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.[6]

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 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 paper making to minimize deposit formation, and as a retention and drainage aid component. Montmorillonite has also been used in cosmetics.

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.

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 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.[7]

For external use, montmorillonite has been used to treat contact dermatitis.[8]

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.[9]


Montmorillonite was first described in 1847 for an occurrence in Montmorillon in the department of Vienne, France,[3] more than 50 years before the discovery of bentonite in the US. It is found in many locations worldwide and known by other names.

Lipid organization

Montmorillonite is also known to cause micelles (lipid spheres) to assemble together into vesicles. These structures resemble cell membranes on many cells. It can also help nucleotides to assemble into RNA which will end up inside the vesicles. This could have generated highly complex RNA polymers that could reproduce the RNA trapped within the vesicles.[10][11] This process may have played a part in abiogenesis which led to life on Earth.[12] Minerals similar to montmorillonites have also been found on Mars.[13]

See also


  1. ^ "Mineralienatlas - Fossilienatlas". Archived from the original on 23 April 2018. Retrieved 23 April 2018.
  2. ^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1995). "Montmorillonite" (PDF). Handbook of Mineralogy. II (Silica, Silicates). Chantilly, VA, USA: Mineralogical Society of America. ISBN 0962209716. OCLC 895497384. Archived (PDF) from the original on 2012-02-05. Retrieved 2017-06-22.
  3. ^ a b Montmorillonite Archived 2012-05-24 at the Wayback Machine.
  4. ^ Montmorillonite Archived 2011-06-07 at the Wayback Machine. Webmineral
  5. ^ Hill, Carol; Paolo Forti (1997). "Deposition and Stability of asdSilicate Minerals". Cave Minerals of the World (Second ed.). National Speleological Society. p. 177. ISBN 1-879961-07-5.
  6. ^ Lloyd, Lawrie (2011). Handbook of Industrial Catalysts. New York: Springer. pp. 181–182. ISBN 978-0387246826.
  7. ^ Bhattacharyya, KG; Gupta, SS (2008). "Adsorption of a few heavy". Advances in Colloid and Interface Science. 140 (2): 114–31. doi:10.1016/j.cis.2007.12.008. PMID 18319190.
  8. ^ Saary, J; Qureshi, R; Palda, V; Dekoven, J; Pratt, M; Skotnicki-Grant, S; Holness, L (2005). "A systematic review of contact dermatitis treatment and prevention". Journal of the American Academy of Dermatology. 53 (5): 845. doi:10.1016/j.jaad.2005.04.075. PMID 16243136.
  9. ^ "Montmorillonite Clay Benefits, Uses in Cat / Dog Food, Structure & Properties". DurableHealth. Archived from the original on 4 November 2015. Retrieved 12 October 2015.
  10. ^ Clays May Have Aided Formation of Primordial Cells Archived 2016-01-26 at the Wayback Machine Howard Hughes Medical Institute, from EurekAlert!, American Association for the Advancement of Science.
  11. ^ "Clays May Have Aided Formation of Primordial Cells". Archived from the original on 1 June 2013. Retrieved 23 April 2018.
  12. ^ "Clay's matchmaking could have sparked life". Archived from the original on 28 June 2015. Retrieved 23 April 2018.
  13. ^ Clark, Stuart (7 June 2013). "Nasa's Opportunity rover finds Martian water appropriate for the origin of life - Stuart Clark". the Guardian. Archived from the original on 13 February 2017. Retrieved 23 April 2018.
  • 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
Argillaceous minerals

Argillaceous minerals are minerals containing substantial amounts of clay-like components (Greek: ἄργιλλος = clay). Argillaceous components are fine-grained (less than 2 μm) aluminosilicates, and more particularly clay minerals such as kaolinite, montmorillonite-smectite, illite, and chlorite. Claystone and shales are thus predominantly argillaceous. Argillaceous minerals may appear silvery upon optical reflection.The adjective "argillaceous" is also used to define rocks in which clay minerals are a secondary but significant component. For example, argillaceous limestones are limestones consisting predominantly of calcium carbonate, but including 10-40% of clay minerals: such limestones, when soft, are often called marls. Similarly, argillaceous sandstones are sandstones consisting primarily of quartz grains, with the interstitial spaces filled with clay minerals.


Bentonite () is an absorbent aluminium phyllosilicate clay consisting mostly of montmorillonite. It was named by Wilbur C. Knight in 1898 after the Cretaceous Benton Shale near Rock River, Wyoming.The different types of bentonite are each named after the respective dominant element, such as potassium (K), sodium (Na), calcium (Ca), and aluminium (Al). Experts debate a number of nomenclatorial problems with the classification of bentonite clays. Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tonstein, has been used to describe clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite. In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. In addition to montmorillonite and illite another common clay species that is sometimes dominant is kaolinite. Kaolinite-dominated clays are commonly referred to as tonsteins and are typically associated with coal.

CI chondrite

CI chondrites, sometimes C1 chondrites, are a group of rare stony meteorites belonging to the carbonaceous chondrites. Samples have been discovered in France, Canada, India, and Tanzania. Compared to all the meteorites found so far, their chemical composition most closely resembles the elemental distribution in the sun's photosphere.

Clay minerals

Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

Clay minerals form in the presence of water and have been important to life, and many theories of abiogenesis involve them. They are important constituents of soils, and have been useful to humans since ancient times in agriculture and manufacturing.

Expansive clay

Expansive clay is a clay soil that is prone to large volume changes (swelling and shrinking) that are directly related to changes in water content. Soils with a high content of expansive minerals can form deep cracks in drier seasons or years; such soils are called vertisols. Soils with smectite clay minerals, including montmorillonite and bentonite, have the most dramatic shrink-swell capacity.

The mineral make-up of this type of soil is responsible for the moisture retaining capabilities. All clays consist of mineral sheets packaged into layers, and can be classified as either 1:1 or 2:1. These ratios refer to the proportion of tetrahedral sheets to octahedral sheets. Octahedral sheets are sandwiched between two tetrahedral sheets in 2:1 clays, while 1:1 clays have sheets in matched pairs. Expansive clays have an expanding crystal lattice in a 2:1 ratio; however, there are 2:1 non-expansive clays.Mitigation of the effects of expansive clay on structures built in areas with expansive clays is a major challenge in geotechnical engineering. Some areas mitigate foundation cracking by watering around the foundation with a soaker hose during dry conditions. This process can be automated by a timer, or using a soil moisture sensor controller. Even though irrigation is expensive, the cost is small compared to repairing a cracked foundation. Admixtures can be added to expansive clays to reduce the shrink-swell properties, as well.One laboratory test to measure the expansion potential of soil is ASTM D 4829.

Geosynthetic clay liner

Geosynthetic clay liners (GCLs) are factory manufactured hydraulic barriers consisting of a layer of bentonite or other very low-permeability material supported by geotextiles and/or geomembranes, mechanically held together by needling, stitching, or chemical adhesives. Due to environmental laws, any seepage from landfills must be collected and properly disposed of, otherwise contamination of the surrounding ground water could cause major environmental and/or ecological problems. The lower the hydraulic conductivity the more effective the GCL will be at retaining seepage inside of the landfill. Bentonite composed predominantly (>70%) of montmorillonite or other expansive clays, are preferred and most commonly used in GCLs. A general GCL construction would consist of two layers of geosynthetics stitched together enclosing a layer of natural or processed sodium bentonite. Typically, woven and/or non-woven textile geosynthetics are used, however polyethylene or geomembrane layers or geogrid geotextiles materials have also been incorporated into the design or in place of a textile layer to increase strength. GCLs are produced by several large companies in North America, Europe, and Asia. The United States Environmental Protection Agency currently regulates landfill construction and design in the US through several legislations.


Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are aluminium (20.90%), silicon (21.76%) and hydrogen (1.56%). Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite, kaolinite, montmorillonite and other clay minerals. X-ray diffraction studies are required for positive identification. It was first described in 1826 and named after the Belgian geologist Omalius d'Halloy.

Ion exchange

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic "ion exchangers".

Typical ion exchangers are ion-exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers, which exchange positively charged ions (cations), or anion exchangers, which exchange negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously. However, the simultaneous exchange of cations and anions can be more efficiently performed in mixed beds, which contain a mixture of anion- and cation-exchange resins, or passing the treated solution through several different ion-exchange materials.

Ion exchanges can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure. This can be dependent on the size of the ions, their charge, or their structure. Typical examples of ions that can bind to ion exchangers are:

H+ (proton) and OH− (hydroxide).

Singly charged monatomic ions like Na+, K+, and Cl−.

Doubly charged monatomic ions like Ca2+ and Mg2+.

Polyatomic inorganic ions like SO42− and PO43−.

Organic bases, usually molecules containing the amine functional group −NR2H+.

Organic acids, often molecules containing −COO− (carboxylic acid) functional groups.

Biomolecules that can be ionized: amino acids, peptides, proteins, etc.Along with absorption and adsorption, ion exchange is a form of sorption.

Ion exchange is a reversible process, and the ion exchanger can be regenerated or loaded with desirable ions by washing with an excess of these ions.

James Ferris

James "Jim" P. Ferris (1932 – March 4, 2016) was an American chemist. He is known for his contributions to the understanding of the origins of life on Earth, specifically by demonstrating a successful mechanism of clay-catalyzed polymerization of RNA, providing further evidence for the RNA World Hypothesis. Additionally, his work in atmospheric photochemistry has illuminated many of the chemical processes which occur in the atmospheres of Jupiter and Saturn's moon, Titan.

List of countries by bentonite production

Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite. There are a few types of bentonites and their names depend on the dominant elements, such as potassium, sodium, calcium, and aluminium. As noted in several places in the geologic literature, there are some nomenclatorial problems with the classification of bentonite clays.

Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tonstein, have been used for clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium bentonite and calcium bentonite.

In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. Other common clay species, and sometimes dominant, are montmorillinite and kaolinite. Kaolinite dominated clays are commonly referred to as tonsteins and are typically associated with coal.


Heilerde-Gesellschaft Luvos Just GmbH & Co. KG is a German manufacturer of medicinal clay (Heilerde, "healing earth")-based products for both internal and external application. Four different fineness grades of loess in both capsule and powder form are available from the company, as well as cosmetics products. The Luvos purified loess consists mainly of montmorillonite.

Mawrth Vallis

Mawrth Vallis (Welsh: [maurθ]) (Mawrth means "Mars" in Welsh) is a valley on Mars, located in the Oxia Palus quadrangle at 22.3°N, 343.5°E with an elevation approximately two kilometers below datum. Situated between the southern highlands and northern lowlands, the valley is a channel formed by massive flooding which occurred in Mars’ ancient past. It is an ancient water outflow channel with light-colored clay-rich rocks.

Prior to the selection of Gale Crater for the Mars Science Laboratory (MSL) Curiosity rover mission, Mawrth Vallis was considered as a potential landing site because of the detection of a stratigraphic section rich in clay minerals. Clay minerals have implications for past aqueous environments as well as the potential to preserve biosignatures, making them ideal targets for the search for life on Mars. Although Mawrth Vallis was not chosen as a landing target, there is still interest in understanding the mineralogy and stratigraphy of the area. Until a rover mission is committed to exploring Mawrth Vallis, orbiters remain the only source of information. These orbiters consist of a number of spectrometers that contribute to our knowledge of Mawrth Vallis and the rest of the Martian surface.

Medicinal clay

The use of medicinal clay in folk medicine goes back to prehistoric times. Indigenous peoples around the world still use clay widely, which is related to geophagy. The first recorded use of medicinal clay goes back to ancient Mesopotamia.

A wide variety of clays are used for medicinal purposes—primarily for external applications, such as the clay baths in health spas (mud therapy). Among the clays most commonly used are kaolin and the smectite clays such as bentonite, montmorillonite, and Fuller's earth.


Nontronite is the iron(III) rich member of the smectite group of clay minerals. Nontronites typically have a chemical composition consisting of more than ~30% Fe2O3 and less than ~12% Al2O3 (ignited basis). Nontronite has very few economic deposits like montmorillonite Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations.

A typical structural formula for nontronite is Ca.5(Si7Al.8Fe.2)(Fe3.5Al.4Mg.1)O20(OH)4. The dioctahedral sheet of nontronite is composed mainly of trivalent iron (Fe3+) cations, although some substitution by trivalent aluminium (Al3+) and divalent magnesium (Mg2+) does occur. The tetrahedral sheet is composed mainly of silicon (Si4+), but can have substantial (about 1 in 8) substitution of either Fe3+ or Al3+, or combinations of these two cations. Thus, nontronite typically is characterised by having most (usually greater than 60%) of the layer charge located in the tetrahedral sheet. The layer charge is typically balanced by divalent calcium (Ca2+) or magnesium (Mg2+).

Nontronite forms from the weathering of biotite and basalts, precipitation of iron and silicon rich hydrothermal fluids and in deep sea hydrothermal vents. Some evidence suggests that microorganisms may play an important role in their formation. Microorganisms are also involved in reduction of structural iron in nontronite when soils undergo anoxia, and the reduced form of the clay appears to be highly reactive towards certain pollutants, perhaps contributing to the destruction of these compounds in the environment.The only known commercially viable and operational nontronite mine is located in Canterbury, New Zealand. The mine is operated by Palmer Resources and the finished products are used internationally in industrial applications (pulp & paper, surface coating) and in cosmetics marketed as New Zealand Glacial Clay.


A protocell (or protobiont) is a self-organized, endogenously ordered, spherical collection of lipids proposed as a stepping-stone toward the origin of life. A central question in evolution is how simple protocells first arose and how they could differ in reproductive output, thus enabling the accumulation of novel biological emergences over time, i.e. biological evolution. Although a functional protocell has not yet been achieved in a laboratory setting, the goal to understand the process appears well within reach.

Shrink–swell capacity

The shrink–swell index of clay refers to the extent certain clay minerals will expand when wet and retract when dry. Soil with a high shrink–swell capacity is problematic and is known as shrink–swell soil, or expansive soil. The amount of certain clay minerals that are present, such as montmorillonite and smectite, directly affects the shrink-swell capacity of soil. This ability to drastically change volume can cause damage to existing structures, such as cracks in foundations or the walls of swimming pools.


Tonstein (from the German "Ton", meaning clay, plus "Stein", meaning rock) is a hard, compact sedimentary rock that is composed mainly of kaolinite or, less commonly, other clay minerals such as montmorillonite and illite. The clays often are cemented by iron oxide minerals, carbonaceous matter, or chlorite. Tonsteins form from volcanic ash deposited in swamps.Tonsteins occur as distinctive, thin, and laterally extensive layers in coal seams throughout the world. They are often used as key beds to correlate the strata in which they are found. The regional persistence of tonsteins and relict phenocrysts indicate that they formed as the result of the diagenetic alteration of volcanic ash falls in an acidic (low pH) and low-salinity environment, consistent with a freshwater swamp. In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces a bentonite layer.The induration of tonsteins is in contrast to kaolin claystones that can be mined for kaolin clay, such as the ball clays found at Bovey Tracey which formed by the erosion of a nearby kaolinised granite. These deposits are generally softer, white, and plastic.


In both the World Reference Base for Soil Resources (WRB) and the USDA soil taxonomy, a Vertisol (Vertosol in the Australian Soil Classification) is a soil in which there is a high content of expansive clay minerals, many of them known as montmorillonite, that form deep cracks in drier seasons or years. In a phenomenon known as argillipedoturbation, alternate shrinking and swelling causes self-ploughing, where the soil material consistently mixes itself, causing some Vertisols to have an extremely deep A horizon and no B horizon. (A soil with no B horizon is called an A/C soil). This heaving of the underlying material to the surface often creates a microrelief known as gilgai.

Vertisols typically form from highly basic rocks, such as basalt, in climates that are seasonally humid or subject to erratic droughts and floods, or that impeded drainage. Depending on the parent material and the climate, they can range from grey or red to the more familiar deep black (known as "black earths" in Australia, "black gumbo" in East Texas, and "black cotton" soils in East Africa).

Vertisols are found between 50°N and 45°S of the equator. Major areas where Vertisols are dominant are eastern Australia (especially inland Queensland and New South Wales), the Deccan Plateau of India, and parts of southern Sudan, Ethiopia, Kenya, and Chad (the Gezira), and the lower Paraná River in South America. Other areas where Vertisols are dominant include southern Texas and adjacent Mexico, central India, northeast Nigeria, Thrace, New Caledonia and parts of eastern China.

The natural vegetation of Vertisols is grassland, savanna, or grassy woodland. The heavy texture and unstable behaviour of the soil makes it difficult for many tree species to grow, and forest is uncommon.

The shrinking and swelling of Vertisols can damage buildings and roads, leading to extensive subsidence. Vertisols are generally used for grazing of cattle or sheep. It is not unknown for livestock to be injured through falling into cracks in dry periods. Conversely, many wild and domestic ungulates do not like to move on this soil when inundated. However, the shrink-swell activity allows rapid recovery from compaction.

When irrigation is available, crops such as cotton, wheat, sorghum and rice can be grown. Vertisols are especially suitable for rice because they are almost impermeable when saturated. Rainfed farming is very difficult because Vertisols can be worked only under a very narrow range of moisture conditions: they are very hard when dry and very sticky when wet. However, in Australia, Vertisols are highly regarded, because they are among the few soils that are not acutely deficient in available phosphorus. Some, known as "crusty Vertisols", have a thin, hard crust when dry that can persist for two to three years before they have crumbled enough to permit seeding.

In the USA soil taxonomy, Vertisols are subdivided into:

Aquerts: Vertisols which are subdued aquic conditions for some time in most years and show redoximorphic features are grouped as Aquerts. Because of the high clay content, the permeability is slowed down and aquic conditions are likely to occur. In general, when precipitation exceeds evapotranspiration, ponding may occur. Under wet soil moisture conditions, iron and manganese are mobilized and reduced. The manganese may be partly responsible for the dark color of the soil profile.

Cryerts: They have a cryic soil temperature regime. Cryerts are most extensive in the grassland and forest-grassland transitions zones of the Canadian Prairies and at similar latitudes in Russia.

Xererts: They have a thermic, mesic, or frigid soil temperature regime. They show cracks that are open at least 60 consecutive days during the summer, but are closed at least 60 consecutive days during winter. Xererts are most extensive in the eastern Mediterranean and parts of California.

Torrerts: They have cracks that are closed for less than 60 consecutive days when the soil temperature at 50 cm is above 8 °C. These soils are not extensive in the U.S., and occur mostly in west Texas, New Mexico, Arizona, and South Dakota, but are the most extensive suborder of Vertisols in Australia.

Usterts: They have cracks that are open for at least 90 cumulative days per year. Globally, this suborder is the most extensive of the Vertisols order, encompassing the Vertisols of the tropics and monsoonal climates in Australia, India, and Africa. In the U.S. the Usterts are common in Texas, Montana, Hawaii, and California.

Uderts: They have cracks that are open less than 90 cumulative days per year and less than 60 consecutive days during the summer. In some areas, cracks open only in drought years. Uderts are of small extent globally, being most abundant in Uruguay and eastern Argentina, but also found in parts of Queensland and the "Black Belt" of Mississippi and Alabama.

Zeolite facies

Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.

The zeolite facies is generally considered to be transitional between diagenetic processes which turn sediments into sedimentary rocks, and prehnite-pumpellyite facies, which is a hallmark of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres. The zeolite and prehnite-pumpellyite facies are considered burial metamorphism as the processes of orogenic regional metamorphism are not required.

Zeolite facies is most often experienced by pelitic sediments; rocks rich in aluminium, silica, potassium and sodium, but generally low in iron, magnesium and calcium. Zeolite facies metamorphism usually results in the production of low temperature clay minerals into higher temperature polymorphs such as kaolinite and vermiculite.

Mineral assemblages include kaolinite and montmorillonite with laumontite, wairakite, prehnite, calcite and chlorite. Phengite and adularia occur in potassium rich rocks. Minerals in this series include zeolites, albite, and quartz.

This occurs by dehydration of the clays during compaction, and heating due to blanketing of the sediments by continued deposition of sediments above. Zeolite facies is considered to start with temperatures of approximately 50 - 150 °C and some burial is required, usually 1 - 5 km.

Zeolite facies tends to correlate in clay-rich sediments with the onset of a bedding plane foliation, parallel with the bedding of the rocks, caused by alignment of platy clay minerals in a horizontal orientation which reduces their free energy state.

Generally plutonic and volcanic rocks are not greatly affected by zeolite facies metamorphism, although vesicular basalts and the like will have their vesicles filled with zeolite minerals, forming amygdaloidal texture. Tuff can also become zeolitized, as is seen in the Obispo formation on the California coast.

Pyrophyllite series
Smectites and vermiculite family


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