What is Kaolin?

aolin is a hydrated aluminum silicate. It is a naturally occurring clay that is prepared for pharmaceutical purposes by washing with water to remove sand and other impurities.

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. Rocks that are rich in kaolinite are known as china clay or kaolin.

The name is derived from Gaolin ("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 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, and the USA.

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.

Structural Transformation

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. 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.

2 Al₂Si₂O₅(OH)₄ --> 2 Al₂Si₂O₇ + 4 H₂O

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

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 also used in most paints and inks. 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 fruits, vegetables, and other vegetation to repel or deter insect damage. A traditional use is to soothe an upset stomach, similar to the way parrots (and later, humans) in South America originally used it. Until the early 1990s it was the active substance of anti-diarrhoea medicine Kaopectate.