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Kaolinite
Chemical Formula	Al2Si2O5(OH)4
Species	Silicates
Crystal System	Triclinic
Mohs Scale	2
Specific Gravity	2.16-2.68
Color	White, sometimes red, blue or brown tints from impurities
Streak	White
Luster	Waxy, Pearly, Dull, Earthy
Refractive Index	n = 1.553 - 1.563 n = 1.559 - 1.569 n = 1.560 - 1.570
Diaphaneity	Translucent, Opaque
Cleavage	Perfecton .
Fracture	Sub-Conchoidal
Crystal Habit：Rarely as crystals, thin platy or stacked, More commonly as microscopic pseudohexagonal plates and clusters of plates, aggregated into compact, claylike masses
Geological Setting：A primary constiuent of clay beds formed by the decomposition of feldspar-bearing rocks.

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 kaolin or china clay.

The name is derived from Kao-ling (Chinese: 高岭/高嶺; pinyin: Gaoling), a village near 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. In Africa, kaolin is sometimes known as kalaba (in Gabon and Cameroon), calaba, and calabachop (in Equatorial Guinea).

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. Commercial grades of kaolin are supplied and transported as dry powder, semi-dry noodle or as liquid slurry.

Chemistry

Notation

While the most common chemical notation for kaolin is Al₂Si₂O₅(OH)₄, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is Al₂O₃·2SiO₂·2H₂O. Cement chemist notation 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. 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.

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.

Occurrence

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.

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 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).

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.

Kaolin is or was also used:

• in ceramics. It is generally the main component in porcelain.
• in toothpaste
• as a light diffusing material in white incandescent light bulbs
• in cosmetics
• as paint to extend titanium dioxide (TiO₂) and modify gloss levels;
• for its semi-reinforcing properties in rubber
• in adhesives to modify rheology.
• the production of common smoking pipes in Europe and Asia
• in organic farming, as a spray applied to crops to deter insect damage, and in the case of apples, to prevent sun scald.
• as whitewash 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.
• as a filler in Edison Diamond Discs.
• as an indicator in radiological dating since Kaolinite can contain very small traces of uranium and thorium.
• to soothe an upset stomach, similar to the way parrots (and later, humans) in South America originally used it. 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.
• for facial masks or soap

According to the American National Precast Concrete Association metakaolin is a supplementary cementitious material (SCM). When added to a concrete mix, metakaolin accelerates the hydration of Portland cement.

Geophagy

Kaolin is eaten for pleasure or to suppress hunger, a practice known as geophagy. Consumption is greater among women, especially during pregnancy. This practice has also been observed within a small population of African-American women in the Southern United States, especially Georgia. There, the kaolin is called white dirt, chalk or white clay.