Kaolinite
General
Category	Silicate mineral
Chemical formula	Al2Si2O5(OH)4
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 platy 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 Kao-ling (Chinese: 高岭/高嶺; pinyin: Gaoling), 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]

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 colors. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, USA.

Structural transformations[link]

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.^[8] 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.^[8]

2 Al₂Si₂O₅(OH)₄ → 2 Al₂Si₂O₇ + 4 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.

• 

  Kaolinite

• 

  Mullite

Occurrence[link]

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, Germany, India, Australia, Korea, the People's Republic of China, the Czech Republic and the United States.^[1]

Predominance in tropical soils[link]

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).^[9]

Uses[link]

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.^[10]

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 in the possibility of a truckload of glossy paper occasionally tripping an overly-sensitive radiation monitor.^[11]

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.

Kaolin is extensively used as a paint or white wash 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.

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.^[12]

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.^[13]

Medicinal and culinary uses[link]

Kaolin. (unknown scale)

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] More recently, industrially-produced kaolinite preparations were formerly 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.

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] a practice known as geophagy. Consumption is greater among women, especially during pregnancy.^[18]

This practice has also been observed within a small population of African-American women in the Southern United States, especially Georgia.^[19] There, the kaolin is called white dirt, chalk or white clay.^[19]

See also[link]

References[link]

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[link]

Wikimedia Commons has media related to: Kaolinite

• China Clay Museum

Jack Brett (June 17, 1917 – 29 December 1982) was a British Grand Prix motorcycle road racer. He competed from 1949 to 1960 in the Grand Prix world championship. He won his only world championship race in the 500cc class at the 1952 500cc Swiss Grand Prix, riding an AJS.^[1] He was a two-time winner at the North West 200 race in Northern Ireland and finished on the podium four times at the Isle of Man TT.^[2]

Motorcycle Grand Prix results[link]

(Races in italics indicate fastest lap)

References[link]

This English motorcycle racing biographical article is a stub. You can help Wikipedia by expanding it. v t e