Clay

Clay

Overview

Clay is a general term including many combinations of one or more clay minerals
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...

 with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.


Clay minerals are typically formed over long periods of time by the gradual chemical weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

 of rocks, usually silicate-bearing, by low concentrations of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 and other diluted solvents.
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Encyclopedia

Clay is a general term including many combinations of one or more clay minerals
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...

 with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.

Formation



Clay minerals are typically formed over long periods of time by the gradual chemical weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

 of rocks, usually silicate-bearing, by low concentrations of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 and other diluted solvents. These solvents, usually acidic, migrate through the weathering rock after leaching
Leaching (pedology)
In pedology, leaching is the loss of mineral and organic solutes due to percolation. It is a mechanism of soil formation. It is distinct from the soil forming process of eluviation, which is the loss of mineral and organic colloids. Leached and elluviated materials tend to be lost from topsoil and...

 through upper weathered layers. In addition to the weathering process, some clay minerals are formed by hydrothermal activity. Clay deposits may be formed in place as residual deposits in soil, but thick deposits usually are formed as the result of a secondary sedimentary deposition process after they have been eroded and transported from their original location of formation. Clay deposits are typically associated with very low energy depositional environments
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...

 such as large lakes and marine deposits.

Primary clays, also known as kaolins
Kaolinite
Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...

, are located at the site of formation. Secondary clay deposits have been moved by erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 and water from their primary location.

Definition



Clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

s, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays, but there is some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include silts and also clay . The distinction between silt and clay varies by discipline. Geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

s and soil scientists usually consider the separation to occur at a particle size of 2 µm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 (clays being finer than silts), sedimentologists often use 4-5 μm, and colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...

 chemist
Chemist
A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...

s use 1 μm. Geotechnical engineers
Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...

 distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg Limits
Atterberg Limits
The Atterberg limits are a basic measure of the nature of a fine-grained soil. Depending on the water content of the soil, it may appear in four states: solid, semi-solid, plastic and liquid. In each state the consistency and behavior of a soil is different and thus so are its engineering properties...

. ISO 14688 grades clay particles as being smaller than 2 μm and silts larger.

Grouping


Depending on the academic source, there are three or four main groups of clays: kaolinite, montmorillonite
Montmorillonite
Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central...

-smectite, illite
Illite
Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron layers. The interlayer space is mainly occupied by poorly hydrated potassium cations...

, and chlorite
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....

. Chlorites are not always considered a clay, sometimes being classified as a separate group within the phyllosilicates. There are approximately 30 different types of "pure" clays in these categories, but most "natural" clays are mixtures of these different types, along with other weathered minerals.

Varve
Varve
A varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...

 (or varved clay) is clay with visible annual layers, formed by seasonal differences in erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 and organic content. This type of deposit
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...

 is common in former glacial lake
Glacial lake
A glacial lake is a lake with origins in a melted glacier. Near the end of the last glacial period, roughly 10,000 years ago, glaciers began to retreat. A retreating glacier often left behind large deposits of ice in hollows between drumlins or hills. As the ice age ended, these melted to create...

s. When glacial lakes are formed there is very little movement of the water that makes the lake, and these eroded soils settle on the lake bed. This allows such an even distribution on the different layers of clay.

Quick clay
Quick clay
Quick clay, also known as Leda clay and Champlain Sea clay in Canada, is a unique form of highly sensitive marine clay, with the tendency to change from a relatively stiff condition to a liquid mass when it is disturbed. Undisturbed quick clay resembles a water-saturated gel...

 is a unique type of marine clay
Marine clay
Marine clay is a type of clay found in coastal regions around the world. In the northern, deglaciated regions, it can sometimes be quick clay, which is notorious for being involved in landslides....

 indigenous to the glaciated terrains of Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, Northern Ireland
Northern Ireland
Northern Ireland is one of the four countries of the United Kingdom. Situated in the north-east of the island of Ireland, it shares a border with the Republic of Ireland to the south and west...

, and Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

. It is a highly sensitive clay, prone to liquefaction
Liquefaction
Liquefaction may refer to:* Liquefaction, the general process of becoming liquid* Soil liquefaction, the process by which sediments become suspended* Liquefaction of gases in physics, chemistry, and thermal engineering* Liquefactive necrosis in pathology...

, which has been involved in several deadly landslide
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...

s.

Historical and modern uses



Clays exhibit plasticity
Plasticity (physics)
In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...

 when mixed with water in certain proportions. When dry, clay becomes firm and when fired in a kiln
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...

, permanent physical and chemical changes occur. These reactions, among other changes, cause the clay to be converted into a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 material. Because of these properties, clay is used for making pottery
Pottery
Pottery is the material from which the potteryware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery . Pottery also refers to the art or craft of the potter or the manufacture of pottery...

 items, both utilitarian and decorative. Different types of clay, when used with different mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s and firing conditions, are used to produce earthenware
Earthenware
Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects.-Types of earthenware:Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball clay, 28% kaolin, 32% quartz, and 15%...

, stoneware
Stoneware
Stoneware is a vitreous or semi-vitreous ceramic ware with a fine texture. Stoneware is made from clay that is then fired in a kiln, whether by an artisan to make homeware, or in an industrial kiln for mass-produced or specialty products...

, and porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

. Prehistoric humans discovered the useful properties of clay, and one of the earliest artifacts ever uncovered is a drinking vessel made of sun-dried clay. Depending on the content of the soil, clay can appear in various colors, from a dull gray to a deep orange-red.

Clay tablets were used as the first known writing medium, inscribed with cuneiform
Cuneiform
Cuneiform can refer to:*Cuneiform script, an ancient writing system originating in Mesopotamia in the 4th millennium BC*Cuneiform , three bones in the human foot*Cuneiform Records, a music record label...

 script through the use of a blunt reed
Reed (plant)
Reed is a generic polyphyletic botanical term used to describe numerous tall, grass-like plants of wet places, which are the namesake vegetation of reed beds...

 called a stylus
Stylus
A stylus is a writing utensil, or a small tool for some other form of marking or shaping, for example in pottery. The word is also used for a computer accessory . It usually refers to a narrow elongated staff, similar to a modern ballpoint pen. Many styli are heavily curved to be held more easily...

.

Clays sintered
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...

 in fire were the first form of ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

. Brick
Brick
A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

s, cooking pots, art objects, dishware, and even musical instrument
Musical instrument
A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...

s such as the ocarina
Ocarina
The ocarina is an ancient flute-like wind instrument. Variations do exist, but a typical ocarina is an enclosed space with four to twelve finger holes and a mouthpiece that projects from the body...

 can all be shaped from clay before being fired. Clay is also used in many industrial processes, such as paper
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

 making, cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

 production, and chemical filter
Filter (chemistry)
In chemistry and common usage, a filter is a device that is designed to physically block certain objects or substances while letting others through. Filters are often used to remove solid substances suspended in fluids, for example to remove air pollution, to make water drinkable, and to prepare...

ing. Clay is also often used in the manufacture of pipes for smoking tobacco
Tobacco
Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...

. Until the late 20th century bentonite
Bentonite
Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

 clay was widely used as a mold binder in the manufacture of sand casting
Sand casting
Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material.It is relatively cheap and sufficiently refractory even for steel foundry use. A suitable bonding agent is mixed or occurs with the sand...

s.

Clay, being relatively impermeable
Permeability (fluid)
Permeability in fluid mechanics and the earth sciences is a measure of the ability of a porous material to allow fluids to pass through it.- Units :...

 to water, is also used where natural seals are needed, such as in the cores of dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

s, or as a barrier in landfill
Landfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...

s against toxic seepage (lining the landfill, preferably in combination with geotextile
Geotextile
Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain...

s).

Recent studies have investigated clay's absorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....

 capacities in various applications, such as the removal of heavy metals
Heavy metals
A heavy metal is a member of a loosely-defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight,...

 from waste water and air purification.

Medical and agricultural uses


A traditional use of clay as medicine
Medicinal clay
The use of medicinal clay in folk medicine goes back to prehistoric times. The 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...

 goes back to prehistoric times. An example is Armenian bole
Armenian bole
Armenian bole, also known as bolus armenus or bole armoniac, is an earthy clay, usually red, native to Armenia. It is red due to the presence of iron oxide; the clay also contains hydrous silicates of aluminum and possibly magnesium.-Uses:...

, which is used to soothe an upset stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

, similar to the way parrot
Parrot
Parrots, also known as psittacines , are birds of the roughly 372 species in 86 genera that make up the order Psittaciformes, found in most tropical and subtropical regions. The order is subdivided into three families: the Psittacidae , the Cacatuidae and the Strigopidae...

s (and later, humans) in South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

 originally used it. Kaolin clay and attapulgite have been used as anti-diarrheal medicines.

Clay as a building material


Clay is one of the oldest building materials on Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood. Between one-half and two-thirds of the world's population, in traditional societies as well as developed countries, still live or work in a building made with clay as an essential part of its load-bearing structure. Also a primary ingredient in many natural building
Natural building
A natural building involves a range of building systems and materials that place major emphasis on sustainability. Ways of achieving sustainability through natural building focus on durability and the use of minimally processed, plentiful or renewable resources, as well as those that, while...

 techniques, clay is used to create adobe
Adobe
Adobe is a natural building material made from sand, clay, water, and some kind of fibrous or organic material , which the builders shape into bricks using frames and dry in the sun. Adobe buildings are similar to cob and mudbrick buildings. Adobe structures are extremely durable, and account for...

, cob, cordwood, and rammed earth
Rammed earth
Rammed earth, also known as taipa , tapial , and pisé , is a technique for building walls using the raw materials of earth, chalk, lime and gravel. It is an ancient building method that has seen a revival in recent years as people seek more sustainable building materials and natural building methods...

 structures

See also



  • Bentonite
    Bentonite
    Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

  • Ceramic
    Ceramic
    A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

  • Clay (industrial plasticine)
    Clay (industrial plasticine)
    Industrial plasticine is a modeling material which is mainly used by automotive design studios. It was developed as an industrial version of plasticine or hobby clay....

  • Clay animation
    Clay animation
    Clay animation or claymation is one of many forms of stop motion animation. Each animated piece, either character or background, is "deformable"—made of a malleable substance, usually Plasticine clay....

  • Clay court
    Clay court
    A clay court is one of the four different types of tennis court. Clay courts are made of crushed shale, stone or brick. The red clay is slower than the green, or Har-Tru "American" clay. The French Open uses clay courts, making it unique among the Grand Slam tournaments.Clay courts are more common...

  • Clay pit
    Clay pit
    A clay pit is a quarry or mine for the extraction of clay, which is generally used for manufacturing pottery, bricks or Portland cement.The brickyard or brickworks is often located alongside the clay pit to reduce the transport costs of the raw material. These days pottery producers are often not...

  • Geophagy
    Geophagy
    Geophagy is the practice of eating earthy or soil-like substances such as clay, and chalk. It exists in animals in the wild and also in humans, most often in rural or preindustrial societies among children and pregnant women...

     (eating of soils, including eating of clay)
  • Expansive clay
    Expansive clay
    Expansive clay is a clay that is prone to large volume changes that are directly related to changes in water content.The mineral make-up of this type of soil is responsible for the moisture retaining capabilities. Soils with smectite clay minerals, including montmorillonite, have the most dramatic...

  • Graham Cairns-Smith
    Graham Cairns-Smith
    Alexander Graham Cairns-Smith is an organic chemist and molecular biologist at the University of Glasgow, most famous for his controversial 1985 book, Seven Clues to the Origin of Life...

    , proposed the 'clay theory' of abiogenesis
    Abiogenesis
    Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...


  • London Clay
    London Clay
    The London Clay Formation is a marine geological formation of Ypresian age which crops out in the southeast of England. The London Clay is well known for the fossils it contains. The fossils from the Lower Eocene indicate a moderately warm climate, the flora being tropical or subtropical...

  • Modelling clay
    Modelling clay
    You can use modelling clay to create items with it. The material compositions and production processes vary considerably. -Ceramic clay:...

  • Paperclay
    Paperclay
    Paper clay is any clay body to which processed cellulose fiber has been added...

  • Particle size
  • Plasticine
    Plasticine
    Plasticine, a brand of modelling clay, is a putty-like modelling material made from calcium salts, petroleum jelly and aliphatic acids. The name is a registered trademark of Flair Leisure Products plc...

  • Pottery
    Pottery
    Pottery is the material from which the potteryware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery . Pottery also refers to the art or craft of the potter or the manufacture of pottery...

  • Vertisol
    Vertisol
    In both the FAO and USA soil taxonomy, a vertisol is a soil in which there is a high content of expansive clay known as montmorillonite that forms deep cracks in drier seasons or years. Alternate shrinking and swelling causes self-mulching, where the soil material consistently mixes itself, causing...




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