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[1] 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.

OxfordClay Weymouth
Oxford Clay (Jurassic) exposed near Weymouth, England


Kaolinite - USGS bws00008
Hexagonal sheets of a clay mineral (kaolinite) (SEM image, ×1340 magnification)

Clays form flat hexagonal sheets similar to the micas. Clay minerals are common weathering products (including weathering of feldspar) and low-temperature hydrothermal alteration products. Clay minerals are very common in soils, in fine-grained sedimentary rocks such as shale, mudstone, and siltstone and in fine-grained metamorphic slate and phyllite.

Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so may require special analytical techniques for their identification and study. These include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, Raman spectroscopy, and SEM-EDS or automated mineralogy processes. These methods can be augmented by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.


Given the requirement of water, clay minerals are relatively rare in the Solar System, though they occur extensively on Earth where water has interacted with other minerals and organic matter. Clay minerals have been detected at several locations on Mars,[2] including Echus Chasma, Mawrth Vallis, the Memnonia quadrangle and the Elysium quadrangle. Spectrography has confirmed their presence on asteroids including the dwarf planet Ceres[3] and Tempel 1[4] as well as Jupiter's moon Europa.[5]


Clay minerals can be classified as 1:1 or 2:1, this originates because they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in the structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite and montmorillonite.

Clay minerals include the following groups:

Mixed layer clay variations exist for most of the above groups. Ordering is described as random or regular ordering, and is further described by the term reichweite, which is German for range or reach. Literature articles will refer to a R1 ordered illite-smectite, for example. This type would be ordered in an ISISIS fashion. R0 on the other hand describes random ordering, and other advanced ordering types are also found (R3, etc.). Mixed layer clay minerals which are perfect R1 types often get their own names. R1 ordered chlorite-smectite is known as corrensite, R1 illite-smectite is rectorite.[10]


Knowledge of the nature of clay became better understood in the 1930s with advancements in x-ray diffraction technology necessary to analyze the molecular nature of clay particles.[11] Standardization in terminology arose during this period as well[11] with special attention given to similar words that resulted in confusion such as sheet and plane.[11]


Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO4 tetrahedra and/or AlO4 octahedra. The sheet units have the chemical composition (Al,Si)3O4. Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction; i.e. all of the unshared vertices are on the same side of the sheet.

In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminium or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.

Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrugated or twisted, causing ditrigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite.

Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.

Biomedical applications of clays

As most of the clays are made from minerals, they are highly biocompatible and have interesting biological properties. Due to disc-shape and charged surfaces, clay interact with a range of macromolecules such as drugs, protein, polymers, DNA, etc. Some of the applications of clays include drug delivery, tissue engineering, and bioprinting.

See also


  1. ^ Kerr, P.F. (1952). "Formation and Occurrence of Clay Minerals". Clays and Clay Minerals. 1 (1): 19–32. doi:10.1346/CCMN.1952.0010104.
  2. ^ Clays On Mars: More Plentiful Than Expected Science Daily. Dec. 20, 2012
  3. ^ Rivkin, A.S.; Volquardsen, E.L.; Clark, B.E. (2006). "The surface composition of Ceres: Discovery of carbonates and iron-rich clays" (PDF). Icarus. 185 (2): 563–567. doi:10.1016/j.icarus.2006.08.022.
  4. ^ Napier, W.M.; Wickramasinghe, J.T.; Wickramasinghe, N.C. (2007). "The origin of life in comets". International Journal of Astrobiology. 6 (04). doi:10.1017/S1473550407003941. ISSN 1473-5504.
  5. ^ Greicius, Tony (26 May 2015). "Clay-Like Minerals Found on Icy Crust of Europa".
  6. ^ a b c d Amethyst Galleries. "The Clay Mineral Group." 2006. February 22, 2007 Archived December 27, 2005, at the Wayback Machine.
  7. ^ Agle, DC; Brown, Dwayne (March 12, 2013). "NASA Rover Finds Conditions Once Suited for Ancient Life on Mars". NASA. Retrieved March 12, 2013.
  8. ^ Wall, Mike (March 12, 2013). "Mars Could Once Have Supported Life: What You Need to Know". Retrieved March 12, 2013.
  9. ^ Chang, Kenneth (March 12, 2013). "Mars Could Once Have Supported Life, NASA Says". The New York Times. Retrieved March 12, 2013.
  10. ^ Moore, D. and R.C. Reynolds, Jr., 1997, X-Ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd ed.: Oxford University Press, New York
  11. ^ a b c Bailey, S. W. (1980). "Summary of recommendations of AIPEA nomenclature committee on clay minerals". American Mineralogist. 65: 1–7.
Argillaceous minerals

Argillaceous minerals may appear silvery upon optical reflection and 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.

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.

Chlorite group

The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn.

Clinochlore: (Mg5Al)(AlSi3)O10(OH)8

Chamosite: (Fe5Al)(AlSi3)O10(OH)8

Nimite: (Ni5Al)(AlSi3)O10(OH)8

Pennantite: (Mn,Al)6(Si,Al)4O10(OH)8In addition, zinc, lithium, and calcium species are known. The great range in composition results in considerable variation in physical, optical, and X-ray properties. Similarly, the range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal rocks and deeply buried sediments.

The name chlorite is from the Greek chloros (χλωρός), meaning "green", in reference to its color. They do not contain the element chlorine, also named from the same Greek root.


Clay is a finely-grained natural rock or soil material that combines one or more clay minerals with possible traces of quartz (SiO2), metal oxides (Al2O3 , MgO etc.) and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to particle size and geometry as well as water content, and become hard, brittle and non–plastic upon drying or firing. Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red.

Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, however, some overlap in particle size and other physical properties. The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 µm (clays being finer than silts), sedimentologists often use 4–5 μm, and colloid chemists use 1 μm. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits. ISO 14688 grades clay particles as being smaller than 2 μm and silt particles as being larger.

Mixtures of sand, silt and less than 40% clay are called loam. Loam makes good soil and is used as a building material.

Clay chemistry

Clay chemistry is an applied subdiscipline of chemistry which studies the chemical structures, properties and reactions of or involving clays and clay minerals. It is a multidisciplinary field, involving concepts and knowledge from inorganic and structural chemistry, physical chemistry, materials chemistry, analytical chemistry, organic chemistry, mineralogy, geology and others.

The study of the chemistry (and physics) of clays and clay minerals is of great academic and industrial relevance as they are among the most widely used industrial minerals, being employed as raw materials (ceramics, pottery, etc.), adsorbents, catalysts, additives, mineral charges, medicines, building materials and others.

The unique properties of clay minerals including: nanometric scale layered construction, presence of fixed and interchangeable charges, possibility of adsorbing and hosting (intercalating) molecules, ability of forming stable colloidal dispersions, possibility of tailored surface and interlayer chemical modification and others, make the study of clay chemistry a very important and extremely varied field of research.

Many distinct fields and knowledge areas are impacted by the phisico-chemical behavior of clay minerals, from environmental sciences to chemical process engineering, from pottery to nuclear waste management.

Their cation exchange capacity (CEC) is of great importance in the balance of the most common cations in soil (Na+, K+, NH4+, Ca2+, Mg2+) and pH control, with direct impact on the soil fertility. It also plays an important role in the fate of most Ca2+ arriving from land (river water) into the seas.

The ability to change and control the CEC of clay minerals offers a valuable tool in the development of selective adsorbants with applications as varied as chemical sensors or pollution cleaning substances for contaminated water, for example.

The understanding of the reactions of clay minerals with water (intercalation, adsorption, colloidal dispersion, etc.) are indispensable for the ceramic industry (plasticity and flow control of ceramic raw mixtures, for example). Those interactions also influence a great number of mechanical properties of soils, being carefully studied by building and construction engineering specialists.

The interactions of clay minerals with organic substances in the soil also plays a vital role in the fixation of nutrients and fertility, as well as in the fixation or leaching of pesticides and other contaminants. Some clay minerals (Kaolinite) are used as carrier material for fungicides and insecticides.

The weathering of many rock types produce clay minerals as one of its last products. The understanding of these geochemical processes is also important for the understanding of geological evolution of landscapes and macroscopic properties of rocks and sediments. Presence of clay minerals in Mars, detected by the Mars Reconnaissance Orbiter in 2009 was another strong evidence of the existence of water on the planet in previous geological eras.

The possibility to disperse nanometric scaled clay mineral particles into a matrix of polymer, with the formation of an inorganic-organic nanocomposite has prompted a large resurgence in the study of these minerals from the late 1990s.

In addition, study of clay chemistry is also of great relevance to the chemical industry, as many clay minerals are used as catalysts, catalyst precursors or catalyst substrates in a number of chemical processes, like automotive catalysts and oil cracking catalysts.

Columbus (crater)

Columbus Crater is a crater in the Terra Sirenum of Mars, located at 29.8° south latitude and 166.1° west longitude. It is 119 km in diameter and was named after Christopher Columbus, Italian explorer (1451–1506). The discovery of sulfates and clay minerals in sediments within Columbus Crater are strong evidence that a lake once existed in the crater. Research with an orbiting near-infrared spectrometer, which reveals the types of minerals present based on the wavelengths of light they absorb, found evidence of layers of both clay and sulfates in Columbus crater. This is exactly what would appear if a large lake had slowly evaporated. Moreover, because some layers contained gypsum, a sulfate which forms in relatively fresh water, life could have formed in the crater.


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.

Huygens (crater)

Huygens is an impact crater on Mars named in honour of the Dutch astronomer, mathematician and physicist Christiaan Huygens.

The crater is approximately 467.25 km (290.34 mi) in diameter and can be found at 304.42°W 13.88°S, in the Iapygia quadrangle.

Scientists were delighted to see branched channels in pictures taken with spacecraft that were sent in orbit around Mars. The existence of these channels is strong evidence that much water once flowed on the surface of the planet. Simple organisms may have once lived where water once was. An excellent group of these channels is shown in the picture below from the rim of Huygens taken with THEMIS.

Carbonates (calcium or iron carbonates) were discovered in a crater on the rim of Huygens. The impact on the rim exposed material that had been dug up from the impact that created Huygens. These minerals represent evidence that Mars once had a thicker carbon dioxide atmosphere with abundant moisture. Carbonates of these kinds only form when there is a lot of water. They were found with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter. Earlier, the instrument had detected clay minerals. The carbonates were found near the clay minerals. Both of these minerals form in wet environments. It is supposed that billions of years ago Mars was much warmer and wetter. At that time, carbonates would have formed from water and the carbon dioxide-rich atmosphere. Later the deposits of carbonate would have been buried. The double impact has now exposed the minerals. Earth has vast carbonate deposits in the form of limestone.


Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina octahedron (O) – silica tetrahedron (T) layers. The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are responsible for the absence of swelling. Structurally, illite is quite similar to muscovite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminium and interlayer potassium. The chemical formula is given as (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)], but there is considerable ion (isomorphic) substitution. It occurs as aggregates of small monoclinic grey to white crystals. Due to the small size, positive identification usually requires x-ray diffraction or SEM-EDS (automated mineralogy) analysis. Illite occurs as an altered product of muscovite and feldspar in weathering and hydrothermal environments; it may be a component of sericite. It is common in sediments, soils, and argillaceous sedimentary rocks as well as in some low grade metamorphic rocks. The iron rich member of the illite group, glauconite, in sediments can be differentiated by x-ray analysis.The cation-exchange capacity (CEC) of illite is smaller than that of smectite but higher than that of kaolinite, typically around 20 – 30 meq/100 g.

Illite was first described for occurrences in the Maquoketa shale in Calhoun County, Illinois, US, in 1937. The name was derived from its type location in Illinois.Illite is also called hydromica or hydromuscovite. Brammallite is a sodium rich analogue. Avalite is a chromium bearing variety which has been described form Mt. Avala, Belgrade, Serbia.


Kaolinite () is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O5(OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO4) linked through oxygen atoms to one octahedral sheet of alumina (AlO6) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.The name "kaolin" is derived from "Gaoling" (Chinese: 高嶺; pinyin: Gāolǐng; literally: "High Ridge"), a Chinese village near Jingdezhen in southeastern China's Jiangxi Province. The name entered English in 1727 from the French version of the word: kaolin, following François Xavier d'Entrecolles's reports on the making of Jingdezhen porcelain.Kaolinite has a low shrink–swell capacity and a low cation-exchange capacity (1–15 meq/100 g). It is a soft, earthy, usually white, mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminium silicate minerals like feldspar. In many parts of the world it is colored pink-orange-red by iron oxide, giving it a distinct rust hue. Lighter concentrations yield white, yellow, or light orange colors. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, United States. Commercial grades of kaolin are supplied and transported as dry powder, semi-dry noodle or as liquid slurry.

Matijevic Hill

Matijevic Hill, named after American NASA engineer Jacob "Jake" Matijevic (1947 - 2012), is a hill located on "Cape York", itself on the western rim of Endeavour Crater lying within the Margaritifer Sinus quadrangle (MC-19) region of the planet Mars. It was discovered by the Opportunity rover, and named by NASA on September 28, 2012. The "approximate" site coordinates are: 2.22923°S 5.35068°W / -2.22923; -5.35068.

The hill includes a rock outcrop called Kirkwood, where Opportunity found a concentration of small spherical features. It also includes an area where clay minerals have been detected from orbiter observations.

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.

McLaughlin (Martian crater)

McLaughlin Crater is an old crater in the Oxia Palus quadrangle of Mars, located at 21.9°N 337.63°E / 21.9; 337.63. It is 90.92 km (56.50 mi) in diameter and 2.2 km (1.4 mi) deep. The crater was named after Dean B. McLaughlin, an American astronomer (1901-1965). The Mars Reconnaissance Orbiter has found evidence that the water came from beneath the surface between 3.7 billion and 4 billion years ago and remained long enough to make carbonate-related clay minerals found in layers. McLaughlin Crater, one of the deepest craters on Mars, contains Mg-Fe clays and carbonates that probably formed in a groundwater-fed alkaline lake. This type of lake could have had a massive biosphere of microscopic organisms.


Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.Mudstone can be separated into these categories:

Siltstone — more than half of the composition is silt-sized particles.

Claystone — more than half of the composition is clay-sized particles.

Mudstone — hardened mud; a mix of silt and clay sized particles. Mudstone can include:

Shale — exhibits lamination or fissility.

Argillite — has undergone low-grade metamorphism.


Nacrite Al2Si2O5(OH)4 is a clay mineral that is polymorphous (or polytypic) with kaolinite. It crystallizes in the monoclinic system. X-ray diffraction analysis is required for positive identification.

Nacrite was first described in 1807 for an occurrence in Saxony, Germany. The name is from nacre in reference to the mother of pearl luster of nacrite masses.


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) substitiution 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 notronite 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.

Orson Welles (crater)

Orson Welles is an impact crater in the Coprates quadrangle of Mars, located at 0.2° S and 45.9° W. It is 124.5 kilometers in diameter and was named after Orson Welles (1915–1985), an American radio and motion picture actor and director. He is famous for, among other things, his radio broadcast of The War of the Worlds by H. G. Wells in which Martians invade Earth. The layers and the clay minerals found in Orson Welles Crater are evidence that it once contained a lake.


Palygorskite or attapulgite is a magnesium aluminium phyllosilicate with formula (Mg,Al)2Si4O10(OH)·4(H2O) that occurs in a type of clay soil common to the Southeastern United States. It is one of the types of fuller's earth. Some smaller deposits of this mineral can be found in Mexico, where its use is tied to the manufacture of Maya blue in pre-Columbian times.


Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.

Stokes (Martian crater)

Stokes is an impact crater on Mars, located on the Martian Northern plains at 55.9°N latitude and 188.8°W longitude. It measures approximately 63 kilometers in diameter and was named after Irish-born physicist George Gabriel Stokes (1819–1903). The crater's name was officially adopted by IAU's Working Group for Planetary System Nomenclature in 1973.It is distinctive for its dark-toned sand dunes, which have been formed by the planet's strong winds. Research released in July 2010 showed that is one of at least nine craters in the northern lowlands that contains hydrated minerals. They are clay minerals, also called phyllosilicates.

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