Kaolinite is a clay mineral, with the chemical composition Al2Si2O5(OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO
4) linked through oxygen atoms to one octahedral sheet of alumina (AlO
6) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.
The name kaolin is derived from Gaoling, a Chinese village near Jingdezhen in southeastern China’s Jiangxi Province. The name entered English in 1727 from the French version of the word: kaolin, following François Xavier d’Entrecolles’s reports on the making of Jingdezhen porcelain.
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 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, United States. Commercial grades of kaolin are supplied and transported as dry powder, semi-dry noodle, or liquid slurry.
Kaolin is used (or was used in the past):
- in ceramics (it is the main component of porcelain)
- in toothpaste
- as a light-diffusing material in white incandescent light bulbs
- in cosmetics
- in industrial insulation material called Kaowool (a form of mineral wool)
- in ‘pre-work’ skin protection and barrier creams
- in paint to extend the titanium dioxide (TiO
2) white pigment and modify gloss levels
- for modifying the properties of rubber upon vulcanization
- in adhesives to modify rheology
- 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 a filler to give bulk, or a coating to improve the surface in papermaking
- 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 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 (the active ingredient in Pepto-Bismol))
- for facial masks or soap (known as “White Clay”)
- for spa body treatments, such as body wraps, cocoons, or spot treatments such as just the feet, back, or hands. Essential oil can be added to add a pleasant aroma, or seaweed can be added to boost the nutrient values of the treatment.
- as adsorbents in water and wastewater treatment.
- to induce blood clotting in diagnostic procedures, e.g. Kaolin clotting time
- in its altered metakaolin form, as a pozzolan; when added to a concrete mix, metakaolin accelerates the hydration of Portland cement and takes part in the pozzolanic reaction with the portlandite formed in the hydration of the main cement minerals (e.g. alite)
- in its altered metakaolin form, as a base component for geopolymer compounds