Kaolinite

"Kaolin" redirects here. For its use as a food additive, see Aluminium silicate.

Kaolinite
General
Category	Silicate minerals, phyllosilicate subclass
Chemical formula	Al2Si2O5(OH)4
Strunz classification	09.ED.05
Dana classification	71.1.1.2
Crystal symmetry	1
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	2–2½
Luster	dull and earthy
Streak	white
Specific gravity	2.6
Optical properties	biaxial
Refractive index	nα = 1.553–1.565 nβ = 1.559–1.569 nγ = 1.569–1.570
Pleochroism	colorless or pale yellow (x) buff or dark buff (y)
2V angle	24°–50° calculated: 44°
Solubility	insoluble
References

Kaolinite is a clay mineral with the chemical composition Al₂Si₂O₅(OH)₄. It is a layered silicate mineral (phyllosilicate), with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of AlO₆ octahedra. Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin.

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

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 aluminosilicate minerals like feldspar. In many parts of the world, it is colored pink-orange-red by iron oxide, giving it a distinct rusty hue. Lighter concentrations yield white, yellow or light orange colours. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, USA.

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. 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 .

2Al₂Si₂O₅(OH)₄ → 2Al₂Si₂O₇ + 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:

2Al₂Si₂O₇ → Si₃Al₄O₁₂ + SiO₂

Upon calcination to ~1050 °C, the spinel phase (Si₃Al₄O₁₂) nucleates and transforms to mullite, 3Al₂O₃·2SiO₂, and highly crystalline cristobalite, SiO₂:

3Si₃Al₄O₁₂ → 2Si₂Al₆O₁₃ + 5SiO₂

Occurrence

File:NGruev.kaolin.mine.bulgaria.jpg

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

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.

Uses

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.

Kaolin was also used in the production of common pipes for centuries in Europe and Asia. The practice of making and using kaolin pipes was brought to the colonies and reproduced once adequate sources of kaolin were discovered. (These distinctive pipes with unusually long stems when new are seen regularly in Renaissance paintings of après-hunt scenes or portraits of people relaxing with a kaolin pipe in one hand and the white stem stretching across the canvas.) The plain, long-stemmed, slender, small-bowled pipes were typically stored on the mantle. A member of the household would break off a 2 cm piece of the stem to provide a "fresh" mouthpiece before filling and lighting. (As a sign of hospitality, hosts would offer guests his/her own pipe from the mantle and break off the mouthpiece for them, ensuring a completely fresh pipe.) Though most pipes were undecorated, a few had monograms or coats of arms either embossed onto or pressed into the wet paste before firing. Some were even pressed into molds of human or animal heads. Archaeologists have been using kaolin pipes to date sites for decades now ever since the inverse relationship between time and bore diameter was confirmed. As ceramic technology improved through time, the diameter of the hole in the stem leading to the bowl decreased. The change has been found to be so constant and measurable as to allow dating of sites to specific decades. Kaolin pipes grew to be obsolete when paper began to be used to roll tobacco into cigarettes.

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 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.

A folk medicine use is to soothe an upset stomach, similar to the way parrots (and later, humans) in South America originally used it.

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. Its taste is sweet, astringent and warm. In traditional Chinese medicine, 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 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% by weight.

In ceramics or pottery applications, the formula is typically written in terms of oxides, thus the formula for kaolinite is Al₂O₃·2SiO₂·2H₂O.

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 from 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

Further reading

• Deer, W. A.; Howie, R. A.; Zussman, J. An introduction to the rock-forming minerals, 2nd ed.; Longman: Harlow, UK, 1992. ISBN 0-582-30094-0.
• Hurlbut, Cornelius S.; Klein, Cornelis Manual of Mineralogy, after J. D. Dana, 20th ed.; Wiley: New York, 1985; pp 428–29. ISBN 0-471-80580-7.
• Breck, D. W. Zeolite Molecular Sieves; Robert E. Brieger: Malabar, FL, 1984; pp 314–15. ISBN 0-89874-648-5.

External links

• China Clay Museum
• The Mineral KAOLINITE - Mineral Galleries
• MSDS: Incandescent Light Bulb - GE