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China Clay

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China Clay

"Kaolin" redirects here. For the Sailor Moon character, see Death Busters#Kaolinite.
Kaolinite
General
Category Phyllosilicates
Kaolinite-serpentine group
Formula
(repeating unit)
Al2Si2O5(OH)4
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
Identification
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 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.

Chemistry

Notation

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.

Structural transformations

Kaolinite group 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.[11] 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.[11]

2 Al2Si2O5(OH)4 → 2 Al2Si2O7 + 2 H2O.

Further heating to 925–950 °C converts metakaolin to an 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

Kaolinite is one of the most common minerals; it is mined, as kaolin, in Pakistan, in 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 a geological 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]

Uses

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 or was also used:

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).

Geophagy

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] There, the kaolin is called white dirt, chalk or white clay.[22]

See also

References

Notes
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

External links

  • China Clay Museum

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