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Gay Head cliffs in Martha's Vineyard consist almost entirely of clay. Gay head cliffs MV.JPG
Gay Head cliffs in Martha's Vineyard consist almost entirely of clay.

Clay is a type of fine-grained natural soil material containing clay minerals. [1] Clays develop plasticity when wet, due to a molecular film of water surrounding the clay particles, but become hard, brittle and non–plastic upon drying or firing. [2] [3] [4] Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide. [5] [6]


Clay is the oldest known ceramic material. Prehistoric humans discovered the useful properties of clay and used it for making pottery. Some of the earliest pottery shards have been dated to around 14,000 BC, [7] and clay tablets were the first known writing medium. [8] Clay is used in many modern industrial processes, such as paper making, cement production, and chemical filtering. Between one-half and two-thirds of the world's population still live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.

Clay is a very common substance. Shale, formed largely from clay, is the most common sedimentary rock. [9] 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. Mixtures of sand, silt and less than 40% clay are called loam. Soils high in swelling clays, which are clay minerals that readily expand in volume when they absorb water, are a major challenge in civil engineering. [1]

Quaternary clay in Estonia Clay-ss-2005.jpg
Quaternary clay in Estonia


The defining mechanical property of clay is its plasticity when wet and its ability to harden when dried or fired. Clays show a broad range of water content within which they are highly plastic, from a minimum water content (called the plasticity limit) where the clay is just moist enough to mould, to a maximum water content (called the liquid limit) where the moulded clay is just dry enough to hold its shape. [10] The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%. [11] High-quality clay is also tough, as measured by the amount of mechanical work required to roll a sample of clay flat. Its toughness reflects a high degree of internal cohesion. [10]

Electron micrograph of smectite clay - magnification 23,500x Clay magnified.jpg
Electron micrograph of smectite clay – magnification 23,500×

Clay has a high content of clay minerals that give it its plasticity. Clay minerals are hydrous aluminium phyllosilicate minerals, composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen and hydroxyl ions. These plates are tough but flexible, and in moist clay, they adhere to each other. The resulting aggregates give clay the cohesion that makes it plastic. [12] In kaolinite clay, the bonding between plates is provided by a film of water molecules that hydrogen bond the plates together. The bonds are weak enough to allow the plates to slip past each other when the clay is being moulded, but strong enough to hold the plates in place and allow the moulded clay to retain its shape after it is moulded. When the clay is dried, most of the water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile. If the clay is moistened again, it will once more become plastic. When the clay is fired to the earthenware stage, a dehydration reaction removes additional water from the clay, causing clay plates to irreversibly adhere to each other via stronger covalent bonding, which strengthens the material. The clay mineral, kaolin, is transformed into a non-clay material, metakaolin, which remains rigid and hard if moistened again. Further firing through the stoneware and porcelain stages further recrystallizes the metakaolin into yet stronger minerals such as mullite. [4]

The tiny size and plate form of clay particles gives clay minerals a high surface area. In some clay minerals, the plates carry a negative electrical charge that is balanced by a surrounding layer of positive ions (cations), such as sodium, potassium, or calcium. If the clay is mixed with a solution containing other cations, these can swap places with the cations in the layer around the clay particles, which gives clays a high capacity for ion exchange. [12] The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility. [13]

Clay is a common component of sedimentary rock. Shale is formed largely from clay and is the most common of sedimentary rocks. [9] However, most clay deposits are impure. 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. [2] 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.

Some clay minerals (such as smectite) are described as swelling clay minerals, because they have a great capacity to take up water, and they increase greatly in volume when they do so. When dried, they shrink back to their original volume. This produces distinctive textures, such as mudcracks or "popcorn" texture, in clay deposits. Soils containing swelling clay minerals (such as bentonite) pose a considerable challenge for civil engineering, because swelling clay can break foundations of buildings and ruin road beds. [1]


Italian and African-American clay miners in mine Shaft, 1910. Italian and African-American Clay Miners in Mine Shaft.jpg
Italian and African-American clay miners in mine Shaft, 1910.

Clay minerals most commonly form by prolonged chemical weathering of silicate-bearing rocks. They can also form locally from hydrothermal activity. [14] Chemical weathering takes place largely by acid hydrolysis due to low concentrations of carbonic acid, dissolved in rainwater or released by plant roots. The acid breaks bonds between aluminium and oxygen, releasing other metal ions and silica (as a gel of orthosilicic acid).) [15]

The clay minerals formed depend on the composition of the source rock and the climate. Acid weathering of feldspar-rich rock, such as granite, in warm climates tends to produce kaolin. Weathering of the same kind of rock under alkaline conditions produces illite. Smectite forms by weathering of igneous rock under alkaline conditions, while gibbsite forms by intense weathering of other clay minerals. [16]

There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in soil and remain at the site of formation. Secondary clays are clays that have been transported from their original location by water erosion and deposited in a new sedimentary deposit. [17] Secondary clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins. [14]


The main groups of clays include kaolinite, montmorillonite-smectite, and illite. Chlorite, vermiculite, [18] talc, and pyrophyllite [19] are sometimes also classified as clay minerals. There are approximately 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals. [20] Clay minerals in clays are most easily identified using X-ray diffraction rather than chemical or physical tests. [21]

Varve (or varved clay) is clay with visible annual layers that are formed by seasonal deposition of those layers and are marked by differences in erosion and organic content. This type of deposit is common in former glacial lakes. When fine sediments are delivered into the calm waters of these glacial lake basins away from the shoreline, they settle to the lake bed. The resulting seasonal layering is preserved in an even distribution of clay sediment banding. [14]

Quick clay is a unique type of marine clay indigenous to the glaciated terrains of Norway, North America, Northern Ireland, and Sweden. [22] It is a highly sensitive clay, prone to liquefaction, and has been involved in several deadly landslides. [23]

Historical and modern uses

Clay layers in a construction site in Auckland City, New Zealand. Dry clay is normally much more stable than sand in excavations. Clay In A Construction Site.jpg
Clay layers in a construction site in Auckland City, New Zealand. Dry clay is normally much more stable than sand in excavations.
Bottle stopper made of fired clay, 14th century Diosgyor - 2015.02.07 (145).JPG
Bottle stopper made of fired clay, 14th century

Clays are used for making pottery, both utilitarian and decorative, and construction products, such as bricks, walls, and floor tiles. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware, and porcelain. Prehistoric humans discovered the useful properties of clay. Some of the earliest pottery shards recovered are from central Honshu, Japan. They are associated with the Jōmon culture, and recovered deposits have been dated to around 14,000 BC. [7] Cooking pots, art objects, dishware, smoking pipes, and even musical instruments such as the ocarina can all be shaped from clay before being fired.

Clay tablets were the first known writing medium. [8] Scribes wrote by inscribing them with cuneiform script using a blunt reed called a stylus. Purpose-made clay balls were used as sling ammunition. [24] Clay is used in many industrial processes, such as paper making, cement production, and chemical filtering. [25] Bentonite clay is widely used as a mold binder in the manufacture of sand castings. [26] [27]

Clay, being relatively impermeable to water, is also used where natural seals are needed, such as in the cores of dams, or as a barrier in landfills against toxic seepage (lining the landfill, preferably in combination with geotextiles). [28] Studies in the early 21st century have investigated clay's absorption capacities in various applications, such as the removal of heavy metals from waste water and air purification. [29] [30]

Medical use

Clay bath near lake Aheme in Benin. Bain d'argile a Gogotinkpon au Benin 01.jpg
Clay bath near lake Ahémé in Benin.

Traditional uses of clay as medicine goes back to prehistoric times. An example is Armenian bole, which is used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons. [31] Kaolin clay and attapulgite have been used as anti-diarrheal medicines. [32]

As a building material

Clay building in southern Estonia WMEE-exp2019-(113).jpg
Clay building in southern Estonia

Clay as the defining ingredient of loam is one of the oldest building materials on Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood. [33] Between one-half and two-thirds of the world's population, in both traditional societies as well as developed countries, still live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.[ citation needed ] Also a primary ingredient in many natural building techniques, clay is used to create adobe, cob, cordwood, and rammed earth structures and building elements such as wattle and daub, clay plaster, clay render case, clay floors and clay paints and ceramic building material. Clay was used as a mortar in brick chimneys and stone walls where protected from water.

Diamond clay tailings at the Lomonosov mine, Northwestern Russia Saponite Tailings, Lomonosov Deposit .png
Diamond clay tailings at the Lomonosov mine, Northwestern Russia

The forecasted mass of clay minerals to be discharged into the tailings of ore processing makes up millions of tons. Worryingly, when macro- and micro-components are found in non-hazardous concentrations, fewer efforts are put into the environmental management of the tailings, though technogenic sediments offer prospects for reuse and valorization beyond their traditional disposal. Saponite is a demonstrative example of the tailings constituent that is often left unfairly mistreated. Electrochemical separation helps to obtain modified saponite-containing products with high smectite-group minerals concentrations, lower mineral particles size, more compact structure, and greater surface area. These characteristics open possibilities for the manufacture of high-quality ceramics and heavy-metal sorbents from saponite-containing products. [35] Furthermore, tail grinding occurs during the preparation of the raw material for ceramics; this waste reprocessing is of high importance for the use of clay pulp as a neutralizing agent, as fine particles are required for the reaction. Experiments on the Histosol deacidification with the alkaline clay slurry demonstrated that neutralization with the average pH level of 7.1 is reached at 30% of the pulp added and an experimental site with perennial grasses proved the efficacy of the technique. [34]

See also


  1. 1 2 3 Olive et al. 1989.
  2. 1 2 Guggenheim & Martin 1995, pp. 255–256.
  3. Science Learning Hub 2010.
  4. 1 2 Breuer 2012.
  5. Klein & Hurlbut 1993, pp. 512-514.
  6. Nesse 2000, pp. 252-257.
  7. 1 2 Scarre 2005, p. 238.
  8. 1 2 Ebert 2011, p. 64.
  9. 1 2 Boggs 2006, p. 140.
  10. 1 2 Moreno-Maroto & Alonso-Azcárate 2018.
  11. White 1949.
  12. 1 2 Bergaya & Lagaly 2006, pp. 1-18.
  13. Hodges 2010.
  14. 1 2 3 Foley 1999.
  15. Leeder 2011, pp. 5-11.
  16. Leeder 2011, pp. 10-11.
  17. Murray 2002.
  18. Nesse 2000, p. 253.
  19. Klein & Hurlbut 1993, pp. 514-515.
  20. Klein & Hurlbut 1993, p. 512.
  21. Nesse 2000, p. 256.
  22. Rankka et al. 2004.
  23. Natural Resources Canada 2005.
  24. Forouzan et al. 2012.
  25. Nesse 2000, p. 257.
  26. Boylu 2011.
  27. Eisenhour & Brown 2009.
  28. Koçkar, Akgün & Aktürk 2005.
  29. García-Sanchez, Alvarez-Ayuso & Rodriguez-Martin 2002.
  30. Churchman et al. 2006.
  31. Diamond 1999.
  32. Dadu et al. 2015.
  33. Grim 2016.
  34. 1 2 Pashkevich, M.A.; Alekseenko, A.V. (2020). "Reutilization Prospects of Diamond Clay Tailings at the Lomonosov Mine, Northwestern Russia". Minerals. 10 (6): 517. doi: 10.3390/min10060517 . CC-BY-icon-80x15.png  This article incorporates text available under the CC BY 4.0 license.
  35. Chanturiya, V.A.; Minenko, V.G.; Makarov, D.V. (2018). "Advanced Techniques of Saponite Recovery from Diamond Processing Plant Water and Areas of Saponite Application". Minerals. 8 (12): 549. doi: 10.3390/min8120549 . CC-BY-icon-80x15.png  This article incorporates text available under the CC BY 4.0 license.

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Shale Fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock, formed from mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Sediment Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.

Silt Classification of soil or sediment

Silt is granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspension with water and soil in a body of water such as a river. It may also exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury feel when dry, and a slippery feel when wet. Silt can be visually observed with a hand lens, exhibiting a sparkly appearance. It also can be felt by the tongue as granular when placed on the front teeth.

Bentonite A smectite clay consisting mostly of montmorillonite

Bentonite is an absorbent swelling clay consisting mostly of montmorillonite. It usually forms from weathering of volcanic ash in seawater, which converts the volcanic glass present in the ash to clay minerals. Bentonite beds are white or pale blue or green in fresh exposures, turning to a cream color and then yellow, red, or brown as the exposure is weathered further.

Clay mineral Hydrous aluminium phyllosilicates

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.

Soil mechanics Branch of soil physics and applied mechanics that describes the behavior of soils

Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as geophysical engineering, coastal engineering, agricultural engineering, hydrology and soil physics.

Montmorillonite Member of the smectite group of swelling 2:1 clay mineral

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, amongst others, saponite, nontronite, beidellite, and hectorite.

Palygorskite A magnesium aluminium phyllosilicate mineral

Palygorskite or attapulgite is a magnesium aluminium phyllosilicate with the chemical formula (Mg,Al)
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Mudstone Fine grained sedimentary rock whose original constituents were clays or muds

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from shale by its lack of fissility.

Saponite Trioctahedral (Mg2+, Fe2+) calcium smectite, phyllosilicate mineral

Saponite is a trioctahedral mineral of the smectite group. Its chemical formula is Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2·n(H2O). It is soluble in sulfuric acid. It was first described in 1840 by Svanberg. Varieties of saponite are griffithite, bowlingite and sobotkite.

Illite Group of related non-expanding clay minerals

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,H
, 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 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.

Mudrock Class of fine grained siliciclastic sedimentary rocks

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Clastic rock Sedimentary rocks made of mineral or rock fragments

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Geotechnical investigation

Geotechnical investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions. This type of investigation is called a site investigation. Additionally, geotechnical investigations are also used to measure the thermal resistivity of soils or backfill materials required for underground transmission lines, oil and gas pipelines, radioactive waste disposal, and solar thermal storage facilities. A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves soil sampling and laboratory tests of the soil samples retrieved.

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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.

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