Coordinates | 28°36′36″N77°13′48″N |
---|---|
name | Kaolin |
category | Silicate mineral |
formula | Al2Si2O5(OH)4 |
symmetry | Triclinic pedialH-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 |
habit | Rarely as crystals, thin platy or stacked, More commonly as microscopic pseudohexagonal plates and clusters of plates, aggregated into compact, claylike masses |
system | Triclinic |
cleavage | Perfect on {001} |
tenacity | Flexible but inelastic |
mohs | 2–2.5 |
luster | Pearly to dull earthy |
refractive | nα 1.553–1.565, nβ 1.559–1.569, nγ 1.569–1.570 |
opticalprop | Biaxial (–) |
2v | Measured: 24° to 50°, Calculated: 44° |
streak | White |
gravity | 2.16–2.68 |
references | }} |
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. Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin.
The name is derived from ("High Hill") in Jingdezhen, Jiangxi province, China. The name entered English in 1727 from the French version of the word: "kaolin", following Francois Xavier d'Entrecolles's reports from Jingdezhen.
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.
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.
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).
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.
Kaolin was long used in the production of common smoking pipes 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 could result the possibility of a truckload of glossy paper occasionally tripping an overly-sensitive radiation monitor.
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 Institute announced the successful use of a kaolinite-derived aluminosilicate nanoparticle infusion in traditional gauze, known commercially as QuikClot Combat Gauze.
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 Al2O3·2SiO2·2H2O
Using the cement chemist notation this can be even written as AS2H2, with the oxides represented as A = Al2O3, S = SiO2, H = H2O.
Kaolinite was also used as filler in Edison Diamond Discs.
In Africa, kaolin is sometimes known as kalaba (in Gabon and Cameroon), calaba, and calabachop (in Equatorial Guinea). It is used for facial masks or soap and is eaten for pleasure or to suppress hunger, a practice known as geophagy. Consumption is greater among women, especially during pregnancy.
This practice is also seen among black women in the Southern United States, especially Georgia. There, the kaolin is called white dirt, chalk or white clay.
;Bibliography
Category:Aluminium minerals Category:Aluminosilicates Category:Clay minerals group Category:Medicinal clay Category:Phyllosilicates
ar:كاولين bg:Каолин ca:Caolinita cs:Kaolinit da:Kaolinit de:Kaolinit et:Kaoliniit es:Caolín eo:Kaolinito eu:Kaolinita fa:کائولینیت fr:Kaolinite ko:고령석 io:Kaolino it:Caolinite he:קאוליניט kw:Pri gwynn hu:Kaolinit nl:Kaolien ja:カオリナイト no:Kaolinitt pl:Kaolinit pt:Caulinita ro:Caolin ru:Каолинит sq:Kaolini sk:Kaolinit sr:Каолинит fi:Kaoliniitti sv:Kaolinit tr:Kaolin uk:Каолініт vi:Kaolinit zh:高嶺石This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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