kaolinite

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 or kaolin.

The name is derived from Gaoling or Kao-Ling ("High Hill") in Jingdezhen, Jiangxi province, China.^[4] Kaolinite was first described as a mineral species in 1867 for an occurrence in the Jari River basin of Brazil.^[5]

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

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.

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.

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 (Bellotto et al., 1995). Due to 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 (Bellotto et al., 1995).

2 Al₂Si₂O₅(OH)₄ —> 2 Al₂Si₂O₇ + 4 H₂O

Further heating to 925-950 °C converts metakaolin to a defect aluminuim-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

Uses

A kaolin mine in Ruse Province, Bulgaria

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.^[6]

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.

A more recent, and more limited, use is as a specially formulated spray applied to especially organic fruits, vegetables, and other vegetation to repel or deter insect damage, and at least in the case of apples, to prevent sun scald. A traditional use is to soothe an upset stomach, similar to the way parrots (and later, humans) in South America originally used it.^[7]

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, but have since changed back to compounds containing aluminium as they are more effective. Kaolin is known in Traditional Chinese Medicine as an herb under the name 赤石脂 (chì shí zhī), "crimson stone resin" in a direct translation. It's taste is sweet, astringent and warm. In TCM it's used for restricting leakage from intestines and stopping diarrhea, blood containment and stopping bleeding, wounds healing.

In April 2008, the Naval Medical Research Center announced the successful use of a Kaolinite-derived aluminosilicate nanoparticles infusion in traditional gauze known commercially as QuikClot Combat Gauze.^{[8] [9]}

Miscellany

The Eden Project, constructed in a used kaolin pit

• The crystallography of kaolinite played a role in Linus Pauling's work on the nature of the chemical bond.

• The Eden Project, a large environmental complex near St Austell, Cornwall, United Kingdom, is constructed in a disused kaolin pit.

• Sandersville, a small town in the state of Georgia in the United States holds an annual kaolin festival. Sandersville has huge kaolin deposits throughout the town and the surrounding areas, and the kaolin industry is the economic backbone of the region.

• When heated to between 650°C 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.

• Ceramists, or more commonly potters, typically think of materials in terms of oxides, thus they write the formula for kaolinite as:

  Al₂O₃ ▪ 2(SiO₂) ▪ 2(H₂O)

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

See also

Kaolin. (unknown scale)

• Silicate minerals
• Dickite
• Geology of Cornwall
• Halloysite
• Nacrite
• Medicinal clay
• Porcelain
• Clay pit
• Kaolin Deposits of Charentes Basin, France

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.
• 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, Vol 22, 207-214.
• The Mineral KAOLINITE - Mineral Galleries
• MSDS: Incandescent Light Bulb - GE

Notes

External links

• China Clay Museum