Английская Википедия:Clay

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Gay Head Cliffs in Martha's Vineyard consist almost entirely of clay.
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A quaternary clay in Estonia

Clay is a type of fine-grained natural soil material containing clay mineralsШаблон:Sfn (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Clays develop plasticity when wet but can be hardened through firing.Шаблон:SfnШаблон:SfnШаблон:Sfn Most pure clay minerals are white or light-colored, but natural clays show a variety of colors from impurities, such as a reddish or brownish color from small amounts of iron oxide.Шаблон:SfnШаблон:Sfn

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,Шаблон:Sfn and clay tablets were the first known writing medium.Шаблон:Sfn 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 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.Шаблон:Sfn 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 (expansive clay), which are clay minerals that readily expand in volume when they absorb water, are a major challenge in civil engineering.Шаблон:Sfn

Properties

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A 23,500 times magnified electron micrograph of smectite 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 the plastic 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.Шаблон:Sfn The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%.Шаблон:Sfn 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.Шаблон:Sfn

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 hydroxide 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.Шаблон:Sfn 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 kaolinite 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.Шаблон:Sfn

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.Шаблон:Sfn The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility.Шаблон:Sfn

Clay is a common component of sedimentary rock. Shale is formed largely from clay and is the most common of sedimentary rocks.Шаблон:Sfn 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.Шаблон:Sfn 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 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.Шаблон:Sfn

Formation

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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.Шаблон:Sfn 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).)Шаблон:Sfn

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.Шаблон:Sfn

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.Шаблон:Sfn Secondary clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins.Шаблон:Sfn

Varieties

The main groups of clays include kaolinite, montmorillonite-smectite, and illite. Chlorite, vermiculite,Шаблон:Sfn talc, and pyrophylliteШаблон:Sfn 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.Шаблон:Sfn Clay minerals in clays are most easily identified using X-ray diffraction rather than chemical or physical tests.Шаблон:Sfn

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.Шаблон:Sfn

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

Uses

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Clay layers in a construction site in Auckland, New Zealand. Dry clay is normally much more stable than sand in excavations.
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A 14th century bottle stopper made of fired clay

Modelling clay is used in art and handicraft for sculpting. 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.Шаблон:Sfn Cooking pots, art objects, dishware, smoking pipes, and even musical instruments such as the ocarina can all be shaped from clay before being fired.

Ancient peoples in Mesopotamia adopted clay tablets as the first known writing medium.Шаблон:Sfn Clay was chosen due to the local material being easy to work with and widely available.[1] Scribes wrote on the tablets by inscribing them with a script known as cuneiform, using a blunt reed called a stylus, 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. Purpose-made clay balls were used as sling ammunition.Шаблон:Sfn Clay is used in many industrial processes, such as paper making, cement production, and chemical filtering.Шаблон:Sfn Bentonite clay is widely used as a mold binder in the manufacture of sand castings.Шаблон:SfnШаблон:Sfn Шаблон:External media

Файл:Bain d'argile à Gogotinkpon au Bénin 01.jpg
Clay bath near lake Ahémé in Benin

Medicine

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.Шаблон:Sfn Kaolin clay and attapulgite have been used as anti-diarrheal medicines.Шаблон:Sfn

Construction

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A clay building in South 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.Шаблон:Sfn Шаблон:Cn span Also a primary ingredient in many natural building techniques, clay is used to create adobe, cob, cordwood, and 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.

Clay, relatively impermeable to water, is also used where natural seals are needed, such as in pond linings, the cores of dams, or as a barrier in landfills against toxic seepage (lining the landfill, preferably in combination with geotextiles).Шаблон:Sfn 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.Шаблон:SfnШаблон:Sfn

See also

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Notes

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References

External links

Шаблон:Wiktionary Шаблон:Commons category Шаблон:EB1911 poster Шаблон:Wikiquote

Шаблон:Soil type Шаблон:Geotechnical engineering Шаблон:Authority control

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  1. ↑ Шаблон:Cite web
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