Clay is a naturally occurring material, composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried or fired. Clay deposits are mostly composed of clay minerals (phyllosilicate minerals), minerals which impart plasticity and harden when fired or dried, and variable amounts of water trapped in the mineral structure by polar attraction. Organic matter and materials which do not impart plasticity may also be a part of clay deposits.^[1]

Clay minerals are typically formed over long periods of time by the gradual chemical weathering of rocks (usually silicate-bearing) by low concentrations of carbonic acid and other diluted solvents. These solvents (usually acidic) migrate through the weathering rock after leaching 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, 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 such as large lake and marine deposits.

Definition

Clays are distinguished from other fine-grained soils by various distinctions. Silts, which are fine-grained soils which 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 both silts and clays. 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, silts being coarser), sedimentologists often use 4-5 μm, and colloid chemists use 1 μm.^[1] Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg Limits.

Grouping

Depending upon academic source, there are three or four main groups of clays: kaolinite, montmorillonite-smectite, illite, and chlorite. Chlorites are not always considered a clay, sometimes being classified as a separate group within the phyllosilicates. There are approximately thirty different types of "pure" clays in these categories, but most "natural" clays are mixtures of these different types, along with other weathered minerals.

Varve (or varved clay) is clay with visible annual layers, formed by seasonal differences in erosion and organic content. This type of deposit is common in former glacial lakes.

Quick clay is a unique type of marine clay indigenous to the glaciated terrains of Norway, Canada and Sweden. It is a highly sensitive clay, prone to liquefaction, which has been involved in several deadly landslides.

Historical and modern uses

Clays exhibit plasticity when mixed with water in certain proportions. When dry, clay becomes firm and when fired in a kiln, permanent physical and chemical reactions occur which, amongst other changes, causes the clay to be converted into a ceramic material. It is because of these properties that clay is used for making pottery items, both practical and decorative. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware and porcelain. Early humans discovered the useful properties of clay in prehistoric times, 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 was also used as the very first writing medium. Thousands of years BCE the cuneiform script was written in clay tablets with a blunt reed called a stylus.

Clays sintered in fire were the first form of ceramic, and remains one of the cheapest to produce and most widely used materials to this day. Bricks, cooking pots, art objects, dishware and even musical instruments such as the ocarina can all be shaped from clay before being fired. Clay is also used in many industrial processes, such as paper making, cement production and chemical filtering. Additionally, Clay is often used in the manufacture of pipes for smoking tobacco.

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).^[2]

Recent studies have been carried out to investigate clay's adsorption capacities in various applications, such as the removal of heavy metals from waste water and air purification.

See also

Bentonite Ceramic Clay (industrial plasticine) Clay court Clay minerals Clay pit Geophagy (eating of soils, including eating of clay) Expansive clay Graham Cairns-Smith, proposed the 'clay theory' of abiogenesis

List of minerals London Clay Modelling clay Paperclay Particle size Plasticine Pottery Vertisol

Footnotes

References

• id="CITEREFGuggenheimMartin1995">Guggenheim, Stephen & R. T. Martin (1995), "Definition of clay and clay mineral: Journal report of the AIPEA nomenclature and CMS nomenclature committees", Clays and Clay Minerals 43: 255–256, <[1]
  • Clay mineral nomenclature American Mineralogist.
  • Ehlers, Ernest G. and Blatt, Harvey (1982). 'Petrology, Igneous, Sedimentary, and Metamorphic' San Francisco: W.H. Freeman and Company. ISBN 0-7167-1279-2.

  External links

  • Historical information about the clays of North Staffordshire, UK
  • Information about clays used in the UK pottery industry

  •  • [ e] Topics in geotechnical engineering
  Soils	Clay Silt Sand Gravel Peat
  Soil properties	Hydraulic conductivity Water content Void ratio Bulk density Thixotropy Reynolds' dilatancy Angle of repose Cohesion Porosity Permeability Specific storage
  Soil mechanics	Effective stress Pore water pressure Shear strength Overburden pressure Consolidation Soil compaction Soil classification Shear wave
  Geotechnical investigation	Cone penetration test Standard penetration test Exploration geophysics Monitoring well Borehole
  Laboratory tests	Atterberg limits California bearing ratio Direct shear test Hydrometer Proctor compaction test R-value Sieve analysis Triaxial shear test
  Foundations	Bearing capacity Shallow foundation Deep foundation Dynamic load testing Wave equation analysis Crosshole sonic logging
  Retaining walls	Lateral earth pressure Mechanically stabilized earth Soil nailing Tieback Gabion Slurry wall
  Slope stability	Mass wasting Landslide
  Earthquakes	Soil liquefaction Response spectrum Seismic hazard Ground-structure interaction
  Geosynthetics	Geotextile Geomembranes Geosynthetic clay liner

  A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties.
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  plasticity is a property of a material to undergo a non-reversible change of shape in response to an applied force. For example, a solid piece of metal or plastic being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself.
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  Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, or wood on a volumetric or gravimetric basis.
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  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.
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  Chemical polarity, also known as bond polarity or simply polarity, is a concept in chemistry which describes how equally bonding electrons are shared between atoms.
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  Organic may refer to:

  • Organism, a living entity.
  • Organ (anatomy), of or relating to a bodily organ.

  Life:

  • Organic life
  • Life
  • Biology
  • Organism

  Materials and substances:

  • Organic material

  
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  Weathering is the decomposition of rocks, soils and their minerals through direct contact with the Earth's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not to be confused with erosion, which involves the movement and disintegration of rocks
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  Carbonic acid (ancient name acid of air or aerial acid) has the formula H₂CO₃. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H₂CO₃.
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  A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. The most common solvent in everyday life is water. Most other commonly-used solvents are organic (carbon-containing) chemicals. These are called organic solvents.
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  Leaching is the process of extracting a substance from a solid by dissolving it in a liquid.

  Agriculture

  In agriculture, leaching may refer to the loss of water-soluble plant nutrients from the soil, due to rain and irrigation.
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  Hydrothermal circulation in its most general sense is the circulation of hot water; 'hydros' in the Greek meaning water and 'thermos' meaning heat. Hydrothermal circulation occurs most often in the vicinity of sources of heat within the Earth's crust.
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  Sedimentary rock is one of the three main rock groups (the others being igneous and metamorphic rock). Rock formed from sediments covers 75-80% of the Earth's land area, and includes common types such as chalk, limestone, dolomite, sandstone, conglomerate and shale.
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  Silt is soil or rock derived granular material of a specific grain size. Silt may occur as a soil or alternatively as suspended sediment in a water column of any surface water body. It may also exist as deposition soil at the bottom of a water body.
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  Silt is soil or rock derived granular material of a specific grain size. Silt may occur as a soil or alternatively as suspended sediment in a water column of any surface water body. It may also exist as deposition soil at the bottom of a water body.
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  A geologist is a contributor to the science of geology, studying the physical structure and processes of the Earth and planets of the solar system (see planetary geology).
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  Soil science deals with soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils per se; and these properties in relation to the use and management of soils.
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  1 micrometre =
  SI units
  010⁻⁶ m 010⁻³ mm
  US customary / Imperial units
  010⁻⁶ ft 010⁻⁶ in

  A micrometre (American spelling: micrometer; symbol µm
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  Sedimentology encompasses the study of modern sediments and understanding the processes that deposit them. It also compares these observations to studies of ancient sedimentary rocks.
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  chemist is a scientist trained in the science of chemistry. Chemists study the composition of matter and its small-scale properties such as density and acidity instead of large-scale properties like size and shape.
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  Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering includes investigating existing subsurface conditions and materials; assessing risks posed by site conditions; designing earthworks and
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  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.
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  Kaolinite is a clay mineral with the chemical composition Al₂Si₂O₅(OH)₄. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra.
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  Montmorillonite is a very soft phyllosilicate mineral that typically forms 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
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  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.
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  Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered silicate. Structurally illite is quite similar to muscovite or sericite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminium and interlayer
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  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.
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  The silicate minerals make up the largest and most important class of rock-forming minerals. They are classified based on the structure of their silicate ion group.
  
  Subclasses:

  Nesosilicates or Isosilicates

  Nesosilicates (or orthosilicates
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  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.
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  Erosion is displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion).
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  Deposit may refer to:

  • Deposit account, the liability owed by the bank to its depositor
  • Deposit (geology), material added to a landform
  • Damage deposit taken in relation to rental or an item or property
  • Deposit (town), New York
  • Deposit (village), New York

  
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