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 sheet of alumina () octahedra. Rocks that are rich in kaolinite are known as kaolin () or china clay. Kaolin is occasionally referred to by the antiquated term lithomarge, from the Ancient Greek ''litho-'' and Latin ''marga'', meaning 'stone of marl'. 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. The name entered English in 1727 from the French version of the word: , following François Xavier d'Entrecolles's reports on the making of Jingdezhen porcelain.

Kaolinite has a low shrink–swell capacity and a low cation-exchange capacity (1–15 meq/100 g). It is a soft, earthy, usually white, mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminium silicate minerals like feldspar. In many parts of the world it is colored pink-orange-red by iron oxide, giving it a distinct rust hue. Lower concentrations yield white, yellow, or light orange colors. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, United States. Commercial grades of kaolin are supplied and transported as dry powder, chips, or liquid slurry.

Chemistry

Notation

The chemical formula for kaolinite as used in mineralogy is , however, in ceramics 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 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 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 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:

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 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, Pakistan, Vietnam, Brazil, Bulgaria, Bangladesh, France, the United Kingdom, Iran, Germany, India, Australia, South Korea, the People's Republic of China, the Czech Republic, Spain, South Africa, Tanzania and the United States.

Mantles of kaolinitic saprolite are common in Western and Northern Europe. The ages of these mantles are Mesozoic to Early Cenozoic.

Kaolinite clay occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates—for example in tropical rainforest 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, 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, and in what is present-day White Clay Creek Preserve. The product was brought by train to Newark, Delaware, on the Newark-Pomeroy line, along which can still be seen many open-pit clay mines. The deposits were formed between the late Cretaceous and early Paleogene, about 100 to 45 million years ago, in sediments derived from weathered igneous and metakaolin rocks. Kaolin production in the US during 2011 was 5.5 million tons.

During the Paleocene–Eocene Thermal Maximum sediments deposited in the Esplugafreda area of Spain were enriched with kaolinite from a detrital source due to denudation.

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, because the thermal energy suffices to overcome the energy barriers involved in the nucleation 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 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 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 (and pressure).

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 silica gels precipitating from supersaturated solutions without reacting with the aluminium or magnesium cations to form crystalline silicates, 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 cations must be hexacoordinated with respect to oxygen (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. 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 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 basaltic rock in Kivu (Zaïre), they noted how the occurrence of kaolinite depended on the of the area involved. A clear distinction was found between areas with good drainage (i.e., areas with a marked difference between wet and dry seasons) and those areas with poor drainage (i.e., perennially swampy 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 (in the form of ethyl silicate) during at least two months. In addition, adjustments of the pH took place every day by way of adding either hydrochloric acid or sodium hydroxide. Such daily additions of Si and Al to the solution in combination with the daily titrations with hydrochloric acid or sodium hydroxide 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. Its use provides the gloss on some grades of coated paper.

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, 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 ceramics (it is the main component of porcelain)
* in toothpaste
* as a light-diffusing material in white incandescent light bulbs
* in cosmetics
* in industrial insulation material called Kaowool (a form of mineral wool)
* in 'pre-work' skin protection and barrier creams
* in paint to extend the titanium dioxide () white pigment and modify gloss levels
* for modifying the properties of rubber upon vulcanization
* in adhesives to modify rheology
* in organic farming as a spray applied to crops to deter insect damage, and in the case of apples, to prevent sun scald
* as whitewash 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
* as an indicator in radiological dating since kaolinite can contain very small traces of uranium and thorium
* to soothe an upset stomach, similar to the way parrots (and later, humans) in South America originally used it (more recently, industrially-produced kaolinite preparations were common for treatment of diarrhea; 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 and takes part in the pozzolanic reaction with the portlandite formed in the hydration of the main cement minerals (e.g. alite)
* in its altered metakaolin form, as a base component for geopolymer compounds

Geophagy

Humans sometimes eat kaolin for pleasure or to suppress hunger, a practice known as geophagy. In Africa, kaolin used for such purposes is known as ''kalaba'' (in Gabon and Cameroon), ''calaba'', and ''calabachop'' (in Equatorial Guinea). 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, especially Georgia, likely brought with the traditions of the aforementioned Africans via slavery. 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.

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 (OSHA) has set the legal limit ( permissible exposure limit) 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 (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 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