Kaolin Overview

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 china clay, white clay, or kaolin.
The name is derived from Chinese: 高陵/高嶺; pinyin: Gaoling or Kao-ling (“High Hill”) in 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.
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 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.
2 Al2Si2O5(OH)4 → 2 Al2Si2O7 + 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:
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