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Kaolinite ( ) is a
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 minera ...
, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered
silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...
, with one tetrahedral sheet of silica () linked through
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
s to one octahedral sheet of alumina () octahedra. Rocks that are rich in kaolinite are known as kaolin () or
china China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
clay. Kaolin is occasionally referred to by the antiquated term lithomarge, from the Ancient Greek ''litho-'' and Latin ''marga'', meaning 'stone of
marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part ...
'. Presently the name lithomarge can refer to a compacted, massive form of kaolin. The name ''kaolin'' is derived from Gaoling (), a Chinese village near Jingdezhen in southeastern China's
Jiangxi Province Jiangxi (; ; formerly romanized as Kiangsi or Chianghsi) is a landlocked province in the east of the People's Republic of China. Its major cities include Nanchang and Jiujiang. Spanning from the banks of the Yangtze river in the north into h ...
. The name entered English in 1727 from the French version of the word: , following
François Xavier d'Entrecolles François Xavier d'Entrecolles (1664 in Lyon – 1741 in Beijing; Chinese name: 殷弘绪, Yin Hongxu) was a French Jesuit priest, who learned the Chinese technique of manufacturing porcelain through his investigations in China at Jingdezhen with ...
's reports on the making of
Jingdezhen porcelain Jingdezhen porcelain () is Chinese porcelain produced in or near Jingdezhen in Jiangxi province in southern China. Jingdezhen may have produced pottery as early as the sixth century CE, though it is named after the reign name of Emperor Zhenz ...
. Kaolinite has a low
shrink–swell capacity The shrink–swell capacity of soils refers to the extent certain clay minerals will expand when wet and retract when dry. Soil with a high shrink–swell capacity is problematic and is known as shrink–swell soil, or expansive soil. The amount o ...
and a low
cation-exchange capacity Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules (cations), but allow these to exchange with ot ...
(1–15 meq/100 g). It is a soft, earthy, usually white, mineral (dioctahedral phyllosilicate
clay Clay is a type of fine-grained natural soil material containing clay minerals (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 pa ...
), produced by the chemical weathering of
aluminium silicate Aluminium silicate (or aluminum silicate) is a name commonly applied to chemical compounds which are derived from aluminium oxide, Al2O3 and silicon dioxide, SiO2 which may be anhydrous or hydrated, naturally occurring as minerals or synthetic ...
minerals like
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
. In many parts of the world it is colored pink-orange-red by
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 wh ...
, giving it a distinct
rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
hue. Lower concentrations yield white, yellow, or light orange colors. Alternating layers are sometimes found, as at
Providence Canyon State Park Providence Canyon State Outdoor Recreation Area is a Georgia state park located in Stewart County in southwest Georgia, United States. The park contains Providence Canyon, which is sometimes called Georgia's "Little Grand Canyon". It is cons ...
in Georgia, United States. Commercial grades of kaolin are supplied and transported as dry powder, chips, or liquid
slurry A slurry is a mixture of denser solids suspended in liquid, usually water. The most common use of slurry is as a means of transporting solids or separating minerals, the liquid being a carrier that is pumped on a device such as a centrifugal p ...
.


Chemistry


Notation

The
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
for kaolinite as used in
mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proce ...
is , however, in
ceramics A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
applications the formula is typically written in terms of oxides, thus the formula for kaolinite is .


Structure

Compared with other clay minerals, kaolinite is chemically and structurally simple. It is described as a 1:1 or ''TO'' clay mineral because its crystals consist of stacked ''TO'' layers. Each ''TO'' layer consists of a tetrahedral (''T'') sheet composed of silicon and oxygen ions bonded to an octahedral (''O'') sheet composed of oxygen, aluminum, and hydroxyl ions. The ''T'' sheet is so called because each silicon ion is surrounded by four oxygen ions forming a tetrahedron. The ''O'' sheet is so called because each aluminum ion is surrounded by six oxygen or hydroxyl ions arranged at the corners of an octahedron. The two sheets in each layer are strongly bonded together via shared oxygen ions, while layers are bonded via
hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
between oxygen on the outer face of the ''T'' sheet of one layer and hydroxyl on the outer face of the ''O'' sheet of the next layer. File:Mica T.png, View of the structure of the tetrahedral (''T'') sheet of kaolinite File:Mica dO.png, View of the structure of the octahedral (''O'') sheet of kaolinite File:Kaolinite crystal structure.png, Kaolinite crystal structure looking along the layers A kaolinite layer has no net electrical charge and so there are no large cations (such as calcium, sodium, or potassium) between layers as with most other clay minerals. This accounts for kaolinite's relatively low ion exchange capacity. The close hydrogen bonding between layers also hinders water molecules from infiltrating between layers, accounting for kaolinite's nonswelling character. When moistened, the tiny platelike crystals of kaolinite acquire a layer of water molecules that cause crystals to adhere to each other and give kaolin clay its cohesiveness. The bonds are weak enough to allow the plates to slip past each other when the clay is being molded, but strong enough to hold the plates in place and allow the molded clay to retain its shape. 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.


Structural transformations

Kaolinite group clays undergo a series of phase transformations upon thermal treatment in air at atmospheric pressure.


Milling

High-energy milling of the kaolinite results in the formation of a mechanochemically amorphized phase similar to metakaolin, although the properties of this solid are quite different. The high-energy milling process is highly inefficient and consumes a large amount of energy.


Drying

Below , exposure to dry air will slowly remove liquid water from the kaolin. The end-state for this transformation is referred to as "leather dry". Between 100 °C and about , any remaining liquid water is expelled from kaolinite. The end state for this transformation is referred to as "bone dry". Throughout this temperature range, the expulsion of water is reversible: if the kaolin is exposed to liquid water, it will be reabsorbed and disintegrate into its fine particulate form. Subsequent transformations are ''not'' reversible, and represent permanent chemical changes.


Metakaolin

Endothermic dehydration of kaolinite begins at 550–600 °C producing disordered metakaolin, but continuous
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...
loss is observed up to . Although historically there was much disagreement concerning the nature of the metakaolin phase, extensive research has led to a general consensus that metakaolin is not a simple mixture of amorphous silica () and alumina (), but rather a complex amorphous structure that retains some longer-range order (but not strictly crystalline) due to stacking of its hexagonal layers.
Al2Si2O5(OH)4 -> Al2Si2O7 + 2 H2O


Spinel

Further heating to 925–950 °C converts metakaolin to an aluminium-silicon
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
which is sometimes also referred to as a gamma-alumina type structure:
2 Al2Si2O7 -> Si3Al4O12 + SiO2


Platelet mullite

Upon calcination above 1050 °C, the spinel phase nucleates and transforms to platelet mullite and highly crystalline
cristobalite Cristobalite is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the ...
:
3 Si3Al4O12 -> 2(3 Al2O3. 2 SiO2) + 5 SiO2


Needle mullite

Finally, at 1400 °C the "needle" form of mullite appears, offering substantial increases in structural strength and heat resistance. This is a structural but not chemical transformation. See
stoneware Stoneware is a rather broad term for pottery or other ceramics fired at a relatively high temperature. A modern technical definition is a vitreous or semi-vitreous ceramic made primarily from stoneware clay or non-refractory fire clay. Whether vi ...
for more information on this form.


Occurrence

Georgia red clay Kaolinite is one of the most common minerals; it is mined, as kaolin, in
Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federal constitutional monarchy consists of thirteen states and three federal territories, separated by the South China Sea into two regions: Peninsular Malaysia and Borneo's East Mal ...
,
Pakistan Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...
,
Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making ...
,
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
,
Bulgaria Bulgaria (; bg, България, Bǎlgariya), officially the Republic of Bulgaria,, ) is a country in Southeast Europe. It is situated on the eastern flank of the Balkans, and is bordered by Romania to the north, Serbia and North Macedo ...
,
Bangladesh Bangladesh (}, ), officially the People's Republic of Bangladesh, is a country in South Asia. It is the eighth-most populous country in the world, with a population exceeding 165 million people in an area of . Bangladesh is among the mo ...
,
France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
, the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...
,
Iran Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...
,
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
,
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
,
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
,
South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korea, Korean Peninsula and sharing a Korean Demilitarized Zone, land border with North Korea. Its western border is formed ...
, the
People's Republic of China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
, the
Czech Republic The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The ...
,
Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = '' Plus ultra'' (Latin)(English: "Further Beyond") , national_anthem = (English: "Royal March") , ...
,
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...
,
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands ...
and the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
. Mantles of kaolinitic
saprolite Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds. Deeply weathered profiles are widesprea ...
are common in Western and Northern Europe. The ages of these mantles are
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
to Early Cenozoic. Kaolinite clay occurs in abundance in
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
s that have formed from the chemical
weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...
of rocks in hot, moist
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
s—for example in
tropical rainforest Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equator ...
areas. Comparing soils along a gradient towards progressively cooler or drier climates, the proportion of kaolinite decreases, while the proportion of other clay minerals such as illite (in cooler climates) or smectite (in drier climates) increases. Such climatically related differences in clay mineral content are often used to infer changes in climates in the geological past, where ancient soils have been buried and preserved. In the '' Institut National pour l'Étude Agronomique au Congo Belge'' (INEAC) classification system, soils in which the clay fraction is predominantly kaolinite are called ''kaolisol'' (from kaolin and soil). In the US, the main kaolin deposits are found in central
Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to the ...
, on a stretch of the Atlantic Seaboard fall line between Augusta and Macon. This area of thirteen counties is called the "white gold" belt; Sandersville is known as the "Kaolin Capital of the World" due to its abundance of kaolin. In the late 1800s, an active kaolin surface-mining industry existed in the extreme southeast corner of Pennsylvania, near the towns of Landenberg and
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 octahedra ...
, and in what is present-day White Clay Creek Preserve. The product was brought by train to
Newark, Delaware Newark ( )Not as in Newark, New Jersey. is a small city in New Castle County, Delaware, United States. It is located west-southwest of Wilmington. According to the 2010 Census, the population of the city is 31,454. Newark is home to the Un ...
, on the Newark-Pomeroy line, along which can still be seen many open-pit clay mines. The deposits were formed between the late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
and early
Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
, about 100 to 45 million years ago, in sediments derived from weathered
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
and metakaolin rocks. Kaolin production in the US during 2011 was 5.5 million tons. During the
Paleocene–Eocene Thermal Maximum The Paleocene–Eocene thermal maximum (PETM), alternatively (ETM1), and formerly known as the "Initial Eocene" or "", was a time period with a more than 5–8 °C global average temperature rise across the event. This climate event o ...
sediments deposited in the Esplugafreda area of
Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = '' Plus ultra'' (Latin)(English: "Further Beyond") , national_anthem = (English: "Royal March") , ...
were enriched with kaolinite from a detrital source due to
denudation Denudation is the geological processes in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interc ...
.


Synthesis and genesis

Difficulties are encountered when trying to explain kaolinite formation under atmospheric conditions by extrapolation of thermodynamic data from the more successful high-temperature syntheses. La Iglesia and Van Oosterwijk-Gastuche (1978) thought that the conditions under which kaolinite will nucleate can be deduced from stability diagrams, based as they are on dissolution data. Because of a lack of convincing results in their own experiments, La Iglesia and Van Oosterwijk-Gastuche (1978) had to conclude, however, that there were other, still unknown, factors involved in the low-temperature nucleation of kaolinite. Because of the observed very slow crystallization rates of kaolinite from solution at room temperature Fripiat and Herbillon (1971) postulated the existence of high activation energies in the low-temperature nucleation of kaolinite. At high temperatures, equilibrium thermodynamic models appear to be satisfactory for the description of kaolinite dissolution and
nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
, because the thermal energy suffices to overcome the
energy barrier In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...
s involved in the
nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
process. The importance of syntheses at ambient temperature and atmospheric pressure towards the understanding of the mechanism involved in the nucleation of clay minerals lies in overcoming these energy barriers. As indicated by Caillère and Hénin (1960) the processes involved will have to be studied in well-defined experiments, because it is virtually impossible to isolate the factors involved by mere deduction from complex natural physico-chemical systems such as the
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
environment. Fripiat and Herbillon (1971), in a review on the formation of kaolinite, raised the fundamental question how a disordered material (i.e., the
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language, Gr ...
fraction of tropical soils) could ever be transformed into a corresponding ordered structure. This transformation seems to take place in soils without major changes in the environment, in a relatively short period of time, and at ambient
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
(and
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
). Low-temperature synthesis of clay minerals (with kaolinite as an example) has several aspects. In the first place the silicic acid to be supplied to the growing crystal must be in a monomeric form, i.e., silica should be present in very dilute solution (Caillère et al., 1957; Caillère and Hénin, 1960; Wey and Siffert, 1962; Millot, 1970). In order to prevent the formation of
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language, Gr ...
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
gel A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still di ...
s precipitating from supersaturated solutions without reacting with the
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 ha ...
or
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
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 to form crystalline
silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is a ...
s, the silicic acid must be present in concentrations below the maximum solubility of amorphous silica. The principle behind this prerequisite can be found in structural chemistry: "Since the polysilicate ions are not of uniform size, they cannot arrange themselves along with the metal ions into a regular crystal lattice." (Iler, 1955, p. 182) The second aspect of the low-temperature synthesis of kaolinite is that the
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 ha ...
cations must be hexacoordinated with respect to
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
(Caillère and Hénin, 1947; Caillère et al., 1953; Hénin and Robichet, 1955). Gastuche et al. (1962) and Caillère and Hénin (1962) have concluded that kaolinite can only ever be formed when the aluminium hydroxide is in the form of
gibbsite Gibbsite, Al(OH)3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al(OH)3 (but sometimes as α-Al(OH)3.). It is also sometimes called hydrargillite (or hydrargyllite). Gibbsite is an important ore of aluminiu ...
. Otherwise, the precipitate formed will be a "mixed alumino-silicic gel" (as Millot, 1970, p. 343 put it). If it were the only requirement, large amounts of kaolinite could be harvested simply by adding gibbsite powder to a silica solution. Undoubtedly a marked degree of adsorption of the silica in solution by the gibbsite surfaces will take place, but, as stated before, mere adsorption does not create the layer lattice typical of kaolinite crystals. The third aspect is that these two initial components must be incorporated into one mixed crystal with a layer structure. From the following equation (as given by Gastuche and DeKimpe, 1962) for kaolinite formation
2Al(OH)3 + 2H4SiO4 -> Si2O5 . Al2(OH)4 + 5H2O
it can be seen that five molecules of water must be removed from the reaction for every
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
of kaolinite formed. Field evidence illustrating the importance of the removal of water from the kaolinite reaction has been supplied by Gastuche and DeKimpe (1962). While studying soil formation on a
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
ic rock in
Kivu Kivu was the name for a large "region" in the Democratic Republic of the Congo under the rule of Mobutu Sese Seko that bordered Lake Kivu. It included three "Sub-Regions" ("Sous-Régions" in French): Nord-Kivu, Sud-Kivu and Maniema, correspon ...
(
Zaïre Zaire (, ), officially the Republic of Zaire (french: République du Zaïre, link=no, ), was a Congolese state from 1971 to 1997 in Central Africa that was previously and is now again known as the Democratic Republic of the Congo. Zaire was, ...
), they noted how the occurrence of kaolinite depended on the of the area involved. A clear distinction was found between areas with good
drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
(i.e., areas with a marked difference between wet and dry seasons) and those areas with poor
drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
(i.e.,
perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also widel ...
ly
swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...
y areas). Kaolinite was only found in the areas with distinct seasonal alternations between wet and dry. The possible significance of alternating wet and dry conditions on the transition of allophane into kaolinite has been stressed by Tamura and Jackson (1953). The role of alternations between wetting and drying on the formation of kaolinite has also been noted by Moore (1964).


Laboratory syntheses

Syntheses of kaolinite at high temperatures (more than ) are relatively well known. There are for example the syntheses of Van Nieuwenberg and Pieters (1929); Noll (1934); Noll (1936); Norton (1939); Roy and Osborn (1954); Roy (1961); Hawkins and Roy (1962); Tomura et al. (1985); Satokawa et al. (1994) and Huertas et al. (1999). Relatively few low-temperature syntheses have become known (cf. Brindley and DeKimpe (1961); DeKimpe (1969); Bogatyrev et al. (1997)). Laboratory syntheses of kaolinite at room temperature and atmospheric pressure have been described by DeKimpe et al. (1961). From those tests the role of periodicity becomes convincingly clear. DeKimpe et al. (1961) had used daily additions of alumina (as ) and
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
(in the form of
ethyl silicate Tetraethyl orthosilicate, formally named tetraethoxysilane (TEOS), ethyl silicate is the organic chemical compound with the formula Si(OC2H5)4. TEOS is a colorless liquid. It degrades in water. TEOS is the of orthosilicic acid, Si(OH)4. It i ...
) during at least two months. In addition, adjustments of the pH took place every day by way of adding either
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...
or
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and al ...
. Such daily additions of Si and Al to the solution in combination with the daily titrations with
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...
or
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and al ...
during at least 60 days will have introduced the necessary element of periodicity. Only now the actual role of what has been described as the "aging" (''Alterung'') of amorphous alumino-silicates (as for example Harder, 1978 had noted) can be fully understood. As such, time is not bringing about any change in a closed system at equilibrium; but a series of alternations of periodically changing conditions (by definition, taking place in an open system) will bring about the low-temperature formation of more and more of the stable phase kaolinite instead of (ill-defined) amorphous alumino-silicates.


Uses

, up to 70% of kaolinite was used in the production of
paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distribu ...
. Its use provides the gloss on some grades of
coated paper Coated paper (also known as enamel paper, gloss paper, and thin paper) is paper that has been coated by a mixture of materials or a polymer to impart certain qualities to the paper, including weight, surface gloss, smoothness, or reduced ink absor ...
. Kaolin is also known for its capabilities to induce and accelerate blood clotting. In April 2008 the US Naval Medical Research Institute announced the successful use of a kaolinite-derived aluminosilicate infusion in traditional
gauze Gauze is a thin, translucent fabric with a loose open weave. In technical terms "gauze" is a weave structure in which the weft yarns are arranged in pairs and are crossed before and after each warp yarn keeping the weft firmly in place. ...
, known commercially as QuikClot Combat Gauze, which is still the hemostat of choice for all branches of the US military. Kaolin is used (or was used in the past): * in
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...
s (it is the main component of
porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises main ...
) * in
toothpaste Toothpaste is a paste or gel dentifrice used with a toothbrush to clean and maintain the aesthetics and health of teeth. Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from the teeth, ...
* as a light-diffusing material in white
incandescent light bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxi ...
s * in
cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protec ...
* in industrial insulation material called Kaowool (a form of mineral wool) * in 'pre-work' skin protection and barrier creams * in
paint 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 ...
to extend the
titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolu ...
() white pigment and modify gloss levels * for modifying the properties of
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
upon
vulcanization Vulcanization (British: Vulcanisation) is a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to includ ...
* in adhesives to modify
rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid ( liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an ap ...
* in
organic farming Organic farming, also known as ecological farming or biological farming,Labelling, article 30 o''Regulation (EU) 2018/848 of the European Parliament and of the Council of 30 May 2018 on organic production and labelling of organic products and re ...
as a
spray Spray or spraying commonly refer to: * Spray (liquid drop) ** Aerosol spray ** Blood spray ** Hair spray ** Nasal spray ** Pepper spray ** PAVA spray ** Road spray or tire spray, road debris kicked up from a vehicle tire ** Sea spray, refers to ...
applied to crops to deter
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
damage, and in the case of apples, to prevent sun scald * as
whitewash Whitewash, or calcimine, kalsomine, calsomine, or lime paint is a type of paint made from slaked lime (calcium hydroxide, Ca(OH)2) or chalk calcium carbonate, (CaCO3), sometimes known as "whiting". Various other additives are sometimes used. ...
in traditional stone masonry homes in Nepal (the most common method is to paint the upper part with white kaolin clay and the middle with red clay; the red clay may extend to the bottom, or the bottom may be painted black) * as a filler in Edison Diamond Discs * as a filler to give bulk, or a coating to improve the surface in
papermaking Papermaking is the manufacture of paper and cardboard, which are used widely for printing, writing, and packaging, among many other purposes. Today almost all paper is made using industrial machinery, while handmade paper survives as a speciali ...
* as an indicator in
radiological dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...
since kaolinite can contain very small traces of
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
and
thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...
* to soothe an upset
stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
, similar to the way
parrot Parrots, also known as psittacines (), are birds of the roughly 398 species in 92 genera comprising the order Psittaciformes (), found mostly in tropical and subtropical regions. The order is subdivided into three superfamilies: the Psittacoide ...
s (and later, humans) in
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sou ...
originally used it (more recently, industrially-produced kaolinite preparations were common for treatment of
diarrhea Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin w ...
; the most common of these was Kaopectate, which abandoned the use of kaolin in favor of attapulgite and then (in the United States) bismuth subsalicylate (the active ingredient in Pepto-Bismol)) * for facial masks or soap (known as "White Clay") * for spa body treatments, such as body wraps, cocoons, or spot treatments such as just the feet, back, or hands. Essential oil can be added to add a pleasant aroma, or seaweed can be added to boost the nutrient values of the treatment. * as adsorbents in water and wastewater treatment * to induce blood clotting in diagnostic procedures, e.g. Kaolin clotting time * in its altered metakaolin form, as a pozzolan; when added to a concrete mix, metakaolin accelerates the hydration of
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19t ...
and takes part in the pozzolanic reaction with the
portlandite Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)2) and the calcium analogue of brucite (Mg(OH)2). Occurrence Portlandite occurs in a variety ...
formed in the hydration of the main cement minerals (e.g. alite) * in its altered metakaolin form, as a base component for
geopolymer Geopolymers are inorganic, typically ceramic, alumino-silicate forming long-range, covalently bonded, non-crystalline (amorphous) networks. Obsidian (volcanic glass) fragments are a component of some geopolymer blends. Commercially produced geopo ...
compounds


Geophagy

Humans sometimes eat kaolin for pleasure or to suppress hunger, a practice known as
geophagy Geophagia (), also known as geophagy (), is the intentional practice of eating earth or soil-like substances such as clay, chalk, or termite mounds. It is a behavioural adaptation that occurs in many non-human animals and has been documented in ...
. In Africa, kaolin used for such purposes is known as ''kalaba'' (in
Gabon Gabon (; ; snq, Ngabu), officially the Gabonese Republic (french: République gabonaise), is a country on the west coast of Central Africa. Located on the equator, it is bordered by Equatorial Guinea to the northwest, Cameroon to the nort ...
and
Cameroon Cameroon (; french: Cameroun, ff, Kamerun), officially the Republic of Cameroon (french: République du Cameroun, links=no), is a country in west-central Africa. It is bordered by Nigeria to the west and north; Chad to the northeast; the ...
), ''calaba'', and ''calabachop'' (in
Equatorial Guinea Equatorial Guinea ( es, Guinea Ecuatorial; french: Guinée équatoriale; pt, Guiné Equatorial), officially the Republic of Equatorial Guinea ( es, link=no, República de Guinea Ecuatorial, french: link=no, République de Guinée équatoria ...
). Consumption is greater among women, especially during pregnancy, and its use is sometimes said by women of the region to be a habit analogous to cigarette smoking among men. This practice has also been observed within a small population of African-American women in the
Southern United States The Southern United States (sometimes Dixie, also referred to as the Southern States, the American South, the Southland, or simply the South) is a geographic and cultural region of the United States of America. It is between the Atlantic Ocean ...
, especially
Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to the ...
, likely brought with the traditions of the aforementioned Africans via
slavery Slavery and enslavement are both the state and the condition of being a slave—someone forbidden to quit one's service for an enslaver, and who is treated by the enslaver as property. Slavery typically involves slaves being made to perf ...
. There, the kaolin is called ''white dirt'', ''chalk'', or ''white clay''.


Safety

Kaolin is generally recognized as safe, but may cause mild irritation of the skin or mucous membranes. Kaolin products may also contain traces of crystalline silica, a known
carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...
. People can be exposed to kaolin in the workplace by breathing in the powder or from skin or eye contact.


United States

The
Occupational Safety and Health Administration The Occupational Safety and Health Administration'' (OSHA ) is a large regulatory agency of the United States Department of Labor that originally had federal visitorial powers to inspect and examine workplaces. Congress established the agen ...
(OSHA) has set the legal limit (
permissible exposure limit The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agent such as high level noise. Permissible exposure limits are established by the Occupationa ...
) for kaolin exposure in the workplace as 15 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. The
National Institute for Occupational Safety and Health The National Institute for Occupational Safety and Health (NIOSH, ) is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. NIOSH is part of the C ...
(NIOSH) has set a recommended exposure limit (REL) of 10 mg/m3 total exposure TWA 5 mg/m3 respiratory exposure over an 8-hour workday.


Geotechnical engineering

Research results show that the utilization of kaolinite in
geotechnical engineering 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 ...
can be alternatively replaced by safer illite, especially if its presence is less than 10.8% of the total rock mass.


See also

* * * * * * * *


References


Citations


General references

* * * *


External links


CDC – NIOSH Pocket Guide to Chemical Hazards
{{Authority control Aluminium minerals Aluminosilicates Clay minerals group Industrial minerals Medicinal clay Phyllosilicates Triclinic minerals Minerals in space group 1