Clay is a type of fine-grained naturalsoil material containingclay minerals[1] (hydrous aluminium phyllosilicates, e.g.kaolinite,Al2Si2O5(OH)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 ofiron oxide.[2][3]
Clays developplasticity when wet but can be hardened throughfiring.[4][5][6] Clay is the longest-knownceramic material. Prehistoric humans discovered the useful properties of clay and used it for makingpottery. Some of the earliest pottery shards have beendated to around 14,000 BCE,[7] andclay tablets were the first known writing medium.[8] Clay is used in many modern industrial processes, such aspaper making,cement production, and chemicalfiltering. Between one-half and two-thirds of the world's population live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.[citation needed] In agriculture, clay content is a major factor in determining landarability. Clay soils are generally less suitable for crops due to poor natural drainage; however, clay soils are more fertile, due to highercation-exchange capacity.[9][10]
Clay is a very common substance.Shale, formed largely from clay, is the most common sedimentary rock.[11] 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 ofsand, silt and less than 40% clay are calledloam. Soils high inswelling clays (expansive clay), which are clay minerals that readily expand in volume when they absorb water, are a major challenge incivil engineering.[1]
A 23,500 times magnified electron micrograph ofsmectite clay
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 theplastic 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.[12] The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%.[13] 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.[12]
Clay has a high content of clay minerals that give it its plasticity. Clay minerals arehydrousaluminiumphyllosilicate minerals, composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen andhydroxide 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.[14] Inkaolinite clay, the bonding between plates is provided by a film of water molecules thathydrogen 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 form direct hydrogen bonds with each other, making the dried clay rigid but still fragile. If the clay is moistened again, it will once more become plastic. When the clay is fired to theearthenware stage, adehydration reaction removes additional water from the clay, causing clay plates to irreversibly adhere to each other via strongercovalent bonding, which strengthens the material. The clay mineral kaolinite is transformed into a non-clay material,metakaolin, which remains rigid and hard if moistened again. Further firing through thestoneware andporcelain stages further recrystallizes the metakaolin into yet stronger minerals such asmullite.[6]
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 forion exchange.[14] The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility.[15]
Clay is a common component ofsedimentary rock. Shale is formed largely from clay and is the most common of sedimentary rocks.[11] 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 andsoil scientists usually consider the separation to occur at a particle size of 2μm (clays being finer than silts),sedimentologists often use 4–5 μm, andcolloidchemists use 1 μm.[4] Clay-size particles and clay minerals are not the same, despite a degree of overlap in their respective definitions.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 assmectite) 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 asmudcracks or "popcorn" texture, in clay deposits. Soils containing swelling clay minerals (such asbentonite) pose a considerable challenge for civil engineering, because swelling clay can break foundations of buildings and ruin road beds.[1]
Clay is generally considered undesirable for agriculture, although some amount of clay is a necessary component of good soil. Compared to other soils, clay soils are less suitable for crops due to their tendency to retain water, and require artificialdrainage andtillage to make suitable for planting. However, clay soils are often more fertile and can hold onto nutrients better due to their highercation-exchange capacity, allowing more land to remain in production rather than being leftfallow. As clay tends to retain nutrients for longer before leaching them, this also means plants may require more fertilizer in clay soils.[9][10]
Italian and African-American clay miners in mine shaft, 1910
Clay minerals most commonly form by prolonged chemicalweathering of silicate-bearing rocks. They can also form locally fromhydrothermal activity.[16] Chemical weathering takes place largely by acid hydrolysis due to low concentrations ofcarbonic 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 oforthosilicic acid).)[17]
The clay minerals formed depend on the composition of the source rock and the climate. Acid weathering offeldspar-rich rock, such asgranite, in warm climates tends to produce kaolin. Weathering of the same kind of rock under alkaline conditions producesillite.Smectite forms by weathering ofigneous rock under alkaline conditions, whilegibbsite forms by intense weathering of other clay minerals.[18]
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 anddeposited in a newsedimentary deposit.[19] Secondary clay deposits are typically associated with very low energydepositional environments such as large lakes and marine basins.[16]
The main groups of clays includekaolinite,montmorillonite-smectite, andillite.Chlorite,vermiculite,[20]talc, andpyrophyllite[21] 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.[22] Clay minerals in clays are most easily identified usingX-ray diffraction rather than chemical or physical tests.[23]
Varve (orvarved clay) is clay with visible annual layers that are formed by seasonal deposition of those layers and are marked by differences inerosion and organic content. This type of deposit is common in formerglacial 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.[16]
Quick clay is a unique type ofmarine clay indigenous to the glaciated terrains of Norway, North America, Northern Ireland, and Sweden.[24] It is a highly sensitive clay, prone toliquefaction, and has been involved in several deadlylandslides.[25]
Clay layers in a construction site inAuckland, New Zealand. Dry clay is normally much more stable than sand in excavations.A 14th-centurybottle stopper made offired clay
Modelling clay is used in art and handicraft forsculpting. Clays are used for makingpottery, 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 centralHonshu,Japan. They are associated with theJōmon culture, and recovered deposits have beendated to around 14,000 BCE.[7] Cooking pots, art objects, dishware,smoking pipes, and evenmusical instruments such as theocarina can all be shaped from clay before being fired.
Ancient peoples inMesopotamia adopted clay tablets as the first known writing medium.[8] Clay was chosen due to the local material being easy to work with and widely available.[26] Scribes wrote on the tablets by inscribing them with a script known ascuneiform, using a bluntreed called astylus, which effectively produced the wedge shaped markings of their writing. After being written on, clay tablets could be reworked into fresh tablets and reused if needed, or fired to make them permanent records. Nowadays, clay is added as a filler tographite, in pencil lead, to change the hardness and blackness of thepencil. Purpose-made clay balls were used assling ammunition.[27] Clay is used in many industrial processes, such aspaper making,cement production, and chemicalfiltering.[28]Bentonite clay is widely used as a mold binder in the manufacture ofsand castings.[29][30]
Traditional uses ofclay as medicine go back to prehistoric times. An example isArmenian bole, which is used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons.[32]Kaolin clay andattapulgite have been used as anti-diarrheal medicines.[33]
Clay as the defining ingredient of loam is one of the oldestbuilding materials onEarth, among other ancient, naturally occurring geologic materials such as stone and organic materials like wood.[34]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 their load-bearing structure.[citation needed] Also a primary ingredient in manynatural building techniques, clay is used to createadobe,cob,cordwood, and structures and building elements such aswattle and daub, clay plaster, clay render case, clay floors and claypaints andceramic building material. Clay was used as amortar in brickchimneys and stone walls where protected from water.
Clay, relativelyimpermeable to water, is also used wherenatural seals are needed, such as in pond linings, the cores ofdams, or as a barrier inlandfills against toxic seepage (lining the landfill, preferably in combination withgeotextiles).[35] Studies in the early 21st century have investigated clay'sabsorption capacities in various applications, such as the removal ofheavy metals from waste water and air purification.[36][37]
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