Clay is a type of fine-grained natural
soil material containing
clay mineral
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay minerals ...
s (hydrous aluminium phyllosilicates, e.g. kaolin,
Al2 Si2 O5(
OH)
4).
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
Dismissal (also called firing) is the termination of employment by an employer against the will of the employee. Though such a decision can be made by an employer for a variety of reasons, ranging from an economic downturn to performance-related ...
. 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
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of which ...
.
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
Date or dates may refer to:
* Date (fruit), the fruit of the date palm (''Phoenix dactylifera'')
Social activity
* Dating, a form of courtship involving social activity, with the aim of assessing a potential partner
**Group dating
*Play date, ...
to around 14,000 BC, and clay tablets were the first known writing medium. Clay is used in many modern industrial processes, such as
paper making,
cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
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. 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.
Properties
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
The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit.
Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic ...
) 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. The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%. 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.
Clay has a high content of clay minerals that give it its plasticity. Clay minerals are
hydrous
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was underst ...
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
phyllosilicate minerals, composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen and
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
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. 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
Earthenware is glazed or unglazed nonvitreous pottery that has normally been fired below . Basic earthenware, often called terracotta, absorbs liquids such as water. However, earthenware can be made impervious to liquids by coating it with a ...
stage, a
dehydration reaction removes additional water from the clay, causing clay plates to irreversibly adhere to each other via stronger
covalent bonding
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
, 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
Mullite or porcelainite is a rare silicate mineral formed during contact metamorphism of clay minerals. It can form two stoichiometric forms: 3 Al2 O32 SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge-balancing cations present. As a result, t ...
.
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 (
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s), 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
Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...
. The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility.
Clay is a common component of
sedimentary rock.
Shale is formed largely from clay and is the most common of sedimentary rocks. 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.
Geologist
A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, alth ...
s and
soil scientists usually consider the separation to occur at a particle size of 2
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
(clays being finer than silts),
sedimentologist
Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. Sedimentologists apply their understanding of mo ...
s often use 4–5 μm, and
colloid
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
chemist
A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe th ...
s use 1 μm.
Geotechnical engineers
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective engineering problems. It al ...
distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils'
Atterberg limits.
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Is ...
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
Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-mon ...
) pose a considerable challenge for civil engineering, because swelling clay can break foundations of buildings and ruin road beds.
Formation
Clay minerals most commonly form by prolonged chemical
weathering of silicate-bearing rocks. They can also form locally from
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water ( Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with t ...
activity. 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).)
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.
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. Secondary clay deposits are typically associated with very low energy
depositional environments
In geology, depositional environment or sedimentary 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 b ...
such as large lakes and marine basins.
Varieties
The main groups of clays include
kaolin
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral s ...
ite,
montmorillonite-
smectite, and
illite.
Chlorite
The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
,
vermiculite,
talc, and
pyrophyllite
Pyrophyllite is a phyllosilicate mineral composed of aluminium silicate hydroxide: Al2Si4O10(OH)2. It occurs in two forms (habits): crystalline folia and compact masses; distinct crystals are not known.
The folia have a pronounced pearly luste ...
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. Clay minerals in clays are most easily identified using
X-ray diffraction rather than chemical or physical tests.
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.
Quick clay
Quick clay, also known as Leda clay and Champlain Sea clay in Canada, is any of several distinctively sensitive glaciomarine clays found in Canada, Norway, Russia, Sweden, Finland, the United States and other locations around the world. The clay ...
is a unique type of
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.
Marine clay is a particle of soil that is dedic ...
indigenous to the glaciated terrains of
Norway,
North America,
Northern Ireland, and
Sweden. It is a highly sensitive clay, prone to
liquefaction, and has been involved in several deadly
landslides.
Uses
Modelling clay
Modelling clay or modelling compound is any of a group of malleable substances used in building and sculpting. The material compositions and production processes vary considerably.
Ceramic clay
Ceramic clays are water-based substances made f ...
is used in art and handicraft for
sculpting
Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable s ...
.
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
, historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separat ...
,
Japan. They are associated with the
Jōmon culture, and recovered deposits have been
dated
Date or dates may refer to:
* Date (fruit), the fruit of the date palm (''Phoenix dactylifera'')
Social activity
* Dating, a form of courtship involving social activity, with the aim of assessing a potential partner
**Group dating
*Play date, ...
to around 14,000 BC. 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. Scribes wrote by inscribing them with
cuneiform
Cuneiform is a logo-syllabic writing system, script that was used to write several languages of the Ancient Near East, Ancient Middle East. The script was in active use from the early Bronze Age until the beginning of the Common Era. It is nam ...
script using a blunt
reed
Reed or Reeds may refer to:
Science, technology, biology, and medicine
* Reed bird (disambiguation)
* Reed pen, writing implement in use since ancient times
* Reed (plant), one of several tall, grass-like wetland plants of the order Poales
* R ...
called a
stylus
A stylus (plural styli or styluses) is a writing utensil or a small tool for some other form of marking or shaping, for example, in pottery. It can also be a computer accessory that is used to assist in navigating or providing more Accuracy an ...
. Purpose-made clay balls were used as
sling ammunition. Clay is used in many industrial processes, such as
paper making,
cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
production, and chemical
filtering.
Bentonite
Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-mon ...
clay is widely used as a mold binder in the manufacture of
sand castings.
Medicine
Traditional uses of
clay as medicine goes back to prehistoric times. An example is
Armenian bole
Armenian bole, also known as bolus armenus or bole armoniac, is an earthy clay, usually red, native to Armenia but also found in other places. The term Armenian was later referred to a specific quality of the clay. Originally used in medication, i ...
, which is used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons.
Kaolin clay
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral s ...
and
attapulgite
Palygorskite or attapulgite is a magnesium aluminium phyllosilicate with the chemical formula ) that occurs in a type of clay soil common to the Southeastern United States. It is one of the types of fuller's earth. Some smaller deposits of this ...
have been used as anti-diarrheal medicines.
Construction
Clay as the defining ingredient of
loam is one of the oldest
building material
Building material is material used for construction. Many naturally occurring substances, such as clay, rocks, sand, wood, and even twigs and leaves, have been used to construct buildings. Apart from naturally occurring materials, many man-ma ...
s on
Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood. Also a primary ingredient in many
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 renew ...
techniques, clay is used to create
adobe
Adobe ( ; ) is a building material made from earth and organic materials. is Spanish for ''mudbrick''. In some English-speaking regions of Spanish heritage, such as the Southwestern United States, the term is used to refer to any kind of e ...
,
cob,
cordwood
Cordwood construction (also called cordwood masonry or cordwood building, alternatively stackwall or stovewood) is a term used for a natural building method in which short logs are piled crosswise to build a wall, using mortar or cob to perm ...
, and structures and building elements such as
wattle and daub
Wattle and daub is a composite building method used for making walls and buildings, in which a woven lattice of wooden strips called wattle is daubed with a sticky material usually made of some combination of wet soil, clay, sand, animal dung ...
, clay plaster, clay render case, clay floors and clay
paints
Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...
and
ceramic building material
Ceramic building material, often abbreviated to CBM, is an umbrella term used in archaeology to cover all building materials made from baked clay. It is particularly, but not exclusively, used in relation to Ancient Rome, Roman building materials. ...
. Clay was used as a
mortar in brick
chimneys
A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typ ...
and stone walls where protected from water.
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.
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.
Clay, relatively
impermeable
Permeability, permeable, and semipermeable may refer to:
Chemistry
*Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion
*Vascular permeability, the movement of fluids and molecules betwe ...
to water, is also used where
natural seals are needed, such as in the cores of
dam
A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, a ...
s, or as a barrier in
landfill
A landfill site, also known as a tip, dump, rubbish dump, garbage dump, or dumping ground, is a site for the disposal of waste materials. Landfill is the oldest and most common form of waste disposal, although the systematic burial of the waste ...
s against toxic seepage (lining the landfill, preferably in combination with
geotextile
Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. Typically made from polypropylene or polyester, geotextile fabrics come in two basic forms: woven (r ...
s). Studies in the early 21st century have investigated clay's
absorption
Absorption may refer to:
Chemistry and biology
*Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which s ...
capacities in various applications, such as the removal of
heavy metals
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Heavy metals are generally defined as ...
from waste water and air purification.
See also
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Notes
References
Clay mineral nomenclature''American Mineralogist''.
*
*
*
*
*
*
*
*
* Ehlers, Ernest G. and Blatt, Harvey (1982). 'Petrology, Igneous, Sedimentary, and Metamorphic'
San Francisco: W.H. Freeman and Company. .
*
*
*
*
*
*
* Hillier S. (2003) "Clay Mineralogy." pp 139–142 In Middleton G.V., Church M.J., Coniglio M., Hardie L.A. and Longstaffe F.J. (Editors) ''Encyclopedia of Sediments and Sedimentary Rocks''. Kluwer Academic Publishers, Dordrecht.
*
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External links
The Clay Minerals Group of the Mineralogical SocietyInformation about clays used in the UK pottery industryThe Clay Minerals Society
{{Authority control
Types of soil
Sculpture materials
Natural materials
Sedimentology
Sediments
Phyllosilicates
Soil-based building materials