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Kaolin
General
Category Mineral
Chemical formula Al2Si2O5(OH)4
Identification
Color White, sometimes red, blue or brown tints from impurities
Crystal habit Earthy
Crystal system triclinic
Cleavage perfect on {001}
Fracture Perfect
Mohs scale hardness 2 - 2.5
Luster dull and earthy
Streak white
Specific gravity 2.16 - 2.68
Refractive index α = 1.553 - 1.565, β = 1.559 - 1.569, γ = 1.569 - 1.570
References [1][2]

Kaolinite is a clay mineral with the chemical composition Al2Si2O5(OH)4. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra.[3] Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin.

The name is derived from Chinese: 高陵/高嶺pinyin: Gaoling or Kao-ling ("High Hill") in Jingdezhen, Jiangxi province, China.[4] The English name is derived from the French version of the word: "kaolin." [5] Kaolinite was first described as a mineral species in 1867 for an occurrence in the Jari River basin of Brazil.[6]

Kaolinite has a low shrink-swell capacity and a low cation exchange capacity (1-15 meq/100g.) 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. Lighter concentrations yield white, yellow or light orange colours. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, USA.

Contents

Structural transformations

Kaolin-type clays undergo a series of phase transformations upon thermal treatment in air at atmospheric pressure. Endothermic dehydroxylation (or alternatively, dehydration) begins at 550-600 °C to produce disordered metakaolin, Al2Si2O7, but continuous hydroxyl loss (-OH) is observed up to 900 °C and has been attributed to gradual oxolation of the metakaolin.[7] Because of historic disagreement concerning the nature of the metakaolin phase, extensive research has led to general consensus that metakaolin is not a simple mixture of amorphous silica (SiO2) and alumina (Al2O3), but rather a complex amorphous structure that retains some longer-range order (but not strictly crystalline) due to stacking of its hexagonal layers [7].

2 Al2Si2O5(OH)4 → 2 Al2Si2O7 + 4 H2O

Further heating to 925-950 °C converts metakaolin to a defect aluminium-silicon spinel, Si3Al4O12, which is sometimes also referred to as a gamma-alumina type structure:

2 Al2Si2O7 → Si3Al4O12 + SiO2

Upon calcination to ~1050 °C, the spinel phase (Si3Al4O12) nucleates and transforms to mullite, 3 Al2O3 · 2 SiO2, and highly crystalline cristobalite, SiO2:

3 Si3Al4O12 → 2 Si2Al6O13 + 5 SiO2

Occurrence

A kaolin mine in Ruse Province, Bulgaria

Kaolinite is one of the most common minerals; it is mined, as kaolin, in Brazil, Bulgaria, France, United Kingdom, Iran[8], Germany, India, Australia, Korea, the People's Republic of China, the Czech Republic, and the United States.

Predominance in tropical soils

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 kaolonite 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.

Uses

Kaolin is used in ceramics, medicine, coated paper, as a food additive, in toothpaste, as a light diffusing material in white incandescent light bulbs, and in cosmetics. It is generally the main component in porcelain.

It is also used in paint to extend titanium dioxide (TiO2) and modify gloss levels; in rubber for semi-reinforcing properties; and in adhesives to modify rheology.[9]

Kaolin was also used in the production of common pipes for centuries in Europe and Asia.

The largest use is in the production of paper, including ensuring the gloss on some grades of paper. Commercial grades of kaolin are supplied and transported as dry powder, semi-dry noodle or as liquid slurry.

Kaolinite can contain very small traces of uranium and thorium, and is therefore useful in radiological dating. While a single magazine made using kaolin does not contain enough radioactive material to be detected by a security-oriented monitor, this does result in truckloads of high end glossy paper occasionally tripping an overly-sensitive radiation monitor.[10] [11] [12]

Kaolinite has also seen some use in organic farming, as a spray applied to crops to deter insect damage, and in the case of apples, to prevent sun scald.

In April 2008, the US Naval Medical Research Center announced the successful use of a Kaolinite-derived aluminosilicate nanoparticle infusion in traditional gauze, known commercially as QuikClot Combat Gauze.[13]

When heated to between 650 and 900 °C kaolinite dehydroxylates to form metakaolin. According to the American National Precast Concrete Association this is a supplementary cementitious material (SCM). When added to a concrete mix, metakaolin affects the acceleration of Portland cement hydration when replacing Portland cement by 20 percent by weight.

In ceramics applications, the formula is typically written in terms of oxides, thus the formula for kaolinite is:

Al2O3 ▪ 2(SiO2) ▪ 2(H2O)

This format is also useful for describing the firing process of clay as the kaolinite loses the 2 water molecules, termed the chemical water, when fired to a high enough temperature. This is different from clay's physical water which will be lost simply due to evaporation and is not a part of the chemical formula.

Medicinal and culinary uses

A folk medicine use is to soothe an upset stomach, similar to the way parrots (and later, humans) in South America originally used it.[14]

Kaolin is, or has been, used as the active substance in liquid anti-diarrhea medicines such as Kaomagma and Kaopectate. Such medicines were changed away from aluminium substances due to a scare over Alzheimer's disease[citation needed], but have since changed back to compounds containing aluminium as they are more effective.

Kaolin is known in traditional Chinese medicine by the name chìshízhī (赤石脂),[citation needed] literally "crimson stone resin".

In Africa, kaolin is sometimes known as kalaba (in Gabon[15] and Cameroon[16]), calaba, and calabachop (in Equatorial Guinea). It is used for facial masks or soap[17] and is eaten for pleasure or to suppress hunger.[16] Consumption is greater among women, especially during pregnancy.[18]

This practice is also seen among African-American women in the Southern United States, especially Georgia.[19] There, the kaolin is called white dirt, chalk or white clay.[19] The product is commercially marketed by companies such as White Dirt of Georgia.

See also

Kaolin. (unknown scale)

References

Bibliography

  • Deer, W.A., Howie, R.A., and Zussman, J. (1992) An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman ISBN 0-582-30094-0.
  • Hurlbut, Cornelius S., Klein, Cornelis (1985) Manual of Mineralogy - after J. D. Dana, 20th ed., Wiley, pp. 428 - 429, ISBN 0-471-80580-7.
  • Breck, D.W. (1984)Zeolite Molecular Sieves, Robert E. Brieger Publishing Company: Malabar, FL, pp. 314–315, ISBN 0-89874-648-5.
  • The Mineral KAOLINITE - Mineral Galleries
  • MSDS: Incandescent Light Bulb - GE

Notes

  1. ^ "Kaolinite mineral information and data". MinDat.org. http://www.mindat.org/min-2156.html. Retrieved 2009-08-05. 
  2. ^ "Kaolinite Mineral Data". WebMineral.com. http://www.webmineral.com/data/Kaolinite.shtml. Retrieved 2009-08-05. 
  3. ^ Deer, W.A.; Howie, R.A.; Zussman, J. (1992), An introduction to the rock-forming minerals (2 ed.), Harlow: Longman, ISBN 0582300940 
  4. ^ Schroeder, Paul (2003-12-12). "Kaolin". New Georgia Encyclopedia. http://www.georgiaencyclopedia.org/nge/Article.jsp?id=h-1178. Retrieved 2008-08-01. 
  5. ^ http://www.etymonline.com/index.php?search=Kaolin&searchmode=none
  6. ^ "Morro do Felipe, Boca do Jari district, Mazagão, Amapá, North Region, Brazil". MinDat.org. http://www.mindat.org/loc-30969.html. Retrieved 2009-08-05. 
  7. ^ a b Bellotto, M., Gualtieri, A., Artioli, G., and Clark, S.M. (1995). "Kinetic study of the kaolinite-mullite reaction sequence. Part I: kaolinite dehydroxylation". Phys. Chem. Minerals 22: 207–214. 
  8. ^ http://www.bgs.ac.uk/mineralsuk/commodity/world/home.html
  9. ^ "Imerys Performance Minerals: Kaolin (China Clay)". http://www.imerys-perfmins.com/kaolin/eu/kaolin.htm. Retrieved 2008-08-01. 
  10. ^ http://www.orau.org/PTP/collection/consumer%20products/magazines.htm
  11. ^ http://www.realmilkpaint.com/article-kaolinclay.html
  12. ^ http://scienceray.com/chemistry/radioactive-glossy-white-paper/
  13. ^ Rowe, Aaron. "Nanoparticles Help Gauze Stop Gushing Wounds". Wired.com. http://www.wired.com/medtech/health/news/2008/04/blood_clotting. Retrieved 2009-08-05. 
  14. ^ Diamond, Jared M., Evolutionary Biology: Dirty eating for healthy living, 400, Nature, pp. 120–121, http://cogweb.ucla.edu/Abstracts/Diamond_99.html 
  15. ^ Karine Boucher, Suzanne Lafage. "Le lexique français du Gabon: K." Le Français en Afrique: Revue du Réseau des Observatoires du Français Contemporain en Afrique. 2000.
  16. ^ a b Franklin Kamtche. "Balengou : autour des mines." (Balengou : around the mines) Le Jour. 12 January 2010. (French)
  17. ^ "Secrets et rituels des femmes camerounaises." (Secrets and rituals of women in Cameroon) at Gennybeauté.com (French)
  18. ^ Gerald N. Callahan. "Eating Dirt." Emerging Infectious Diseases. 9.8 (August 2003).
  19. ^ a b R. Kevin Grigsby "Clay Eating." New Georgia Encyclopedia. 3 February 2004.

External links


Redirecting to Kaolinite


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

KAOLIN, a pure white clay, know also as china-clay, since it is an essential ingredient in the manufacture of china, or porcelain. The word kaolin, formerly written by some authors caulin, is said to be a corruption of the Chinese Kau-ling, meaning "High Ridge," the name of a hill east of King-te-chen, whence the earliest samples of the clay sent to Europe were obtained by the Pere d'Entrecolles, a French Jesuit missionary in China in the early part of the 18th century. His specimens, examined in Paris by R. A. Reaumur, showed that true porcelain, the composition of which had not previously been known in Europe, contained two essential ingredients, which came to be known - though it now appears incorrectly - as kaolin and petuntse, corresponding respectively to our china-clay and china-stone. The kaolin confers plasticity on the paste and secures retention of form for the ware when exposed to the heat of the kiln, whilst the petuntse gives the translucency so characteristic of porcelain. Some of the earliest discoveries of kaolin in Europe were at Aue, near Schneeberg in Saxony, and at St Yrieix, near Limoges in France. In England it was discovered in Cornwall about the year 1750 by William Cookworthy, of Plymouth; and in 1768 he took out his patent for making porcelain from moorstone or growan (china-stone) and growan clay (kaolin), the latter imparting "whiteness and infusibility" to the china. These raw materials were found first at Tregonning Hill, near Breage, and afterwards at St Stephen's in Brannel, near St Austell; and their discovery led to the manufacture of hard paste, or true porcelain, at Plymouth and subsequently at Bristol.

Kaolin is a hydrous aluminium silicate, having the formula H I A] 2 Si 2 0 9, or Al 2 Si 2 0 7.2H 2 0, but in common clay this silicate is largely mixed with impurities. Certain clays contain pearly white hexagonal scales, usually microscopic, referable to the monoclinic system, and having the chemical composition of kaolin. This crystalline substance was termed kaolinite by S. W. Johnson and J. M. Blake in 1867, and it is now regarded as the basis of pure clay. The kaolinite of Amlwch in Anglesey has been studied by Allan Dick. The origin of kaolin may be traced to the alteration of certain aluminous silicates like feldspar, scapolite, beryl and topaz; but all large deposits of china-clay are due to the decomposition of feldspar, generally in granite, but sometimes in gneiss, pitchstone, &c. The turbidity of many feldspars is the result of partial "kaolinization," or alteration to kaolin. The china-clay rocks of Cornwall and Devon are granites in which the orthoclase has become kaolinized. These rocks are sometimes known as carclazite, a name proposed by J. H. Collins from a typical locality, the Carclaze mine, near St Austell. It has often been supposed that the alteration of the granite has been effected mainly by meteoric agencies, the carbonic acid having decomposed the alkaline silicate of the feldspar, whilst the aluminous silicate assumes a hydrated condition and forms kaolin. In many cases, however, it seems likely that the change has been effected by subterranean agencies, probably by heated vapours carrying fluorine and boron, since minerals containing these elements, like tourmaline, often occur in association with the china-clay. According to F. H. Butler the kaolinization of the west of England granite may have been effected by a solution of carbonic acid at a high temperature, acting from below.

The china-stone, or petuntse, is a granitic rock which still retains much of the unaltered feldspar, on which its fusibility depends. In order to prepare kaolin for the market, the chinaclay rock is broken up, and the clay washed out by means of water. The liquid containing the clay in mechanical suspension is run into channels called "drags" where the coarser impurities subside, and whence it passes to another set of channels known as "micas," where the finer materials settle down. Thus purified, the clay-water is led into a series of pits or tanks, in which the finely divided clay is slowly deposited; and, after acquiring sufficient consistency, it is transferred to the dryinghouse, or "dry," heated by flues, where the moisture is expelled, and the kaolin obtained as a soft white earthy substance. The clay has extensive application in the arts, being used not only in ceramic manufacture but in paper-making, bleaching and various chemical industries.

Under the species "kaolinite" may be included several minerals which have received distinctive names, such as the Saxon mineral called from its pearly lustre nacrite, a name originally given by A. Brongniart to a nacreous mica; pholerite found chiefly in cracks of ironstone and named by J. Guillemin from the Greek coXls, a scale; and lithomarge, the old German Steinmark, a compact clay-like body of white, yellow or red colour. Dr C. Hintze has pointed out that the word pholerite should properly be written pholidite (cboXis, 40XLSos). Closely related to kaolinite is the mineral called halloysite, a name given to it by P. Berthier after his uncle Omalius d'Halloy, the Belgian geologist. (F. W. R.*)


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