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"Kaolin" redirects here. For the community in the United States, see Kaolin, Pennsylvania.
Category Phyllosilicates
Kaolinite-serpentine group
(repeating unit)
Strunz classification 09.ED.05
Crystal symmetry Triclinic pedial
H-M symbol: (1)
Space group: P1
Unit cell a = 5.13 Å, b = 8.89 Å, c = 7.25 Å; α = 90°, β = 104.5°, γ = 89.8°; Z = 2
Color White, sometimes red, blue or brown tints from impurities
Crystal habit Rarely as crystals, thin plates or stacked, More commonly as microscopic pseudohexagonal plates and clusters of plates, aggregated into compact, claylike masses
Crystal system Triclinic
Cleavage Perfect on {001}
Tenacity Flexible but inelastic
Mohs scale hardness 2–2.5
Luster Pearly to dull earthy
Streak White
Specific gravity 2.16–2.68
Optical properties Biaxial (–)
Refractive index nα = 1.553–1.565,
nβ = 1.559–1.569,
nγ = 1.569–1.570
2V angle Measured: 24° to 50°, Calculated: 44°
References [1][2][3]

Kaolinite (/ˈkəlɨˌnt/) is a clay mineral, part of the group of industrial minerals, 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.[4] Rocks that are rich in kaolinite are known as kaolin or china clay.[5]

The name is derived from Chinese Kao-Ling (高岭/高嶺, pinyin Gāolǐng), a village near Jingdezhen, Jiangxi province, China.[6] The name entered English in 1727 from the French version of the word: kaolin, following Francois Xavier d'Entrecolles's reports from Jingdezhen.[7] In Africa, kaolin is sometimes known as kalaba (in Gabon[8] and Cameroon[9]), calaba, and calabachop (in Equatorial Guinea).

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. Lighter 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, semi-dry noodle or as liquid slurry.



The chemical formula for kaolinite as used in mineralogy is Al2Si2O5(OH)4,[3] however, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is Al2O3·2SiO2·2H2O.[10] Cement chemist notation is even more terse: AS2H2, with the oxides represented as A = Al2O3, S = SiO2, H = H2O.[citation needed]

Structural transformations[edit]

Kaolinite structure

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


Below 100 °C, 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 550 °C, any remaining liquid water is expelled from kaolinite. The end state for this transformation is referred to as "bone dry". Through this state, 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.


Endothermic dehydration of kaolinite begins at 550–600 °C producing disordered metakaolin, but continuous hydroxyl loss is observed up to 900 °C.[11] 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 (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.[11]

Al2Si2O5(OH)4 → Al2Si2O7 + 2 H2O.


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[edit]

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[edit]

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.


Kaolinite is one of the most common minerals; it is mined, as kaolin, in Pakistan, Vietnam, Brazil, Bulgaria, France, United Kingdom, Iran, Germany, India, Australia, Korea, the People's Republic of China, the Czech Republic, Spain and the United States.[1]

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'Etude Agronomique au Congo Belge (INEAC) classification system, soils in which the clay fraction is predominantly kaolinite are called kaolisol (from kaolin and soil).[12]

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. The deposits were formed between the late Cretaceous and early Paleogene, about 100 million to 45 million years ago, in sediments derived from weathered igneous and metakaolin rocks.[13] Kaolin production in the US during 2011 was 5.5 million tonnes.[14]


The largest use is in the production of paper, including ensuring the gloss on some grades of paper.

In April 2008, the US Naval Medical Research Institute announced the successful use of a kaolinite-derived aluminosilicate nanoparticle infusion in traditional gauze, known commercially as QuikClot Combat Gauze.[15]

Kaolin is used (or was used in the past):

According to the American National Precast Concrete Association metakaolin is a supplementary cementitious material (SCM) acting 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).


Kaolin is eaten for health or to suppress hunger,[9] a practice known as geophagy. Consumption is greater among women, especially during pregnancy.[21] This practice has also been observed within a small population of African-American women in the Southern United States, especially Georgia.[22][23] There, the kaolin is called white dirt, chalk or white clay.[22]

See also[edit]


  1. ^ a b "Kaolinite mineral information and data". MinDat.org. Retrieved 2009-08-05. 
  2. ^ "Kaolinite Mineral Data". WebMineral.com. Retrieved 2009-08-05. 
  3. ^ a b Kaolinite in the Handbook of Mineralogy
  4. ^ Deer, W.A.; Howie, R.A.; Zussman, J. (1992). An introduction to the rock-forming minerals (2 ed.). Harlow: Longman. ISBN 0-582-30094-0. 
  5. ^ Pohl, Walter L. (2011). Economic geology: principles and practice : metals, minerals, coal and hydrocarbons – introduction to formation and sustainable exploitation of mineral deposits. Chichester, West Sussex: Wiley-Blackwell. p. 331. ISBN 978-1-4443-3662-7. 
  6. ^ Schroeder, Paul (2003-12-12). "Kaolin". New Georgia Encyclopedia. Retrieved 2008-08-01. 
  7. ^ Harper, Douglas. "kaolin". Online Etymology Dictionary. 
  8. ^ 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.
  9. ^ a b Franklin Kamtche. "Balengou : autour des mines." (Balengou: around the mines) Le Jour. 12 January 2010. (French)
  10. ^ Handbook of Inorganic Compounds, Dale L. Perry, Taylor & Francis, 2011, ISBN 978-1-4398-1461-1
  11. ^ 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 (4): 207–214. Bibcode:1995PCM....22..207B. doi:10.1007/BF00202253. 
  12. ^ Young, Anthony (1980). Tropical soils and soil survey. Cambridge Geographical Studies 9. CUP Archive. p. 132. ISBN 0-521-29768-0. 
  13. ^ Paul A. Schroeder (December 2003). "Kaolin". New Georgia Encyclopedia. 
  14. ^ Virta, Robert (2012). Mineral Commodity Summaries (Technical report). U.S. Geological Survey. pp. 44–45. 
  15. ^ Rowe, Aaron (24 April 2008). "Nanoparticles help gauze stop gushing wounds". Wired.com. Retrieved 2009-08-05. 
  16. ^ Ciullo, Peter A. (1996). Industrial minerals and their uses: a handbook and formulary. William Andrew. pp. 41–43. ISBN 978-0-8155-1408-4. 
  17. ^ Edison Diamond Disc information
  18. ^ Diamond, Jared M. (1999). "Evolutionary biology: Dirty eating for healthy living". Nature (Nature) 400 (6740): 120–121. Bibcode:1999Natur.400..120D. doi:10.1038/22014. PMID 10408435. 
  19. ^ "Secrets et rituels des femmes camerounaises." (Secrets and rituals of women in Cameroon) at Gennybeauté.com (French)
  20. ^ Leiviskä, Tiina; Gehör, Seppo; Eijärvi, Erkki; Sarpola, Arja; Tanskanen, Juha (10 April 2012). "Characteristics and potential applications of coarse clay fractions from Puolanka, Finland". Central European Journal of Engineering 2 (2): 239–247. Bibcode:2012CEJE....2..239L. doi:10.2478/s13531-011-0067-9. 
  21. ^ Gerald N. Callahan. "Eating Dirt." Emerging Infectious Diseases. 9.8 (August 2003).
  22. ^ a b R. Kevin Grigsby "Clay Eating." New Georgia Encyclopedia. 3 February 2004.
  23. ^ Chen, Linda (2014-04-02). "The Old And Mysterious Practice Of Eating Dirt, Revealed". NPR. Retrieved 2014-04-12. 

External links[edit]