kaoliin

Kaolin, also known as china clay, is a soft white clay mineral primarily composed of kaolinite, with the chemical formula Al2Si2O5(OH)4. The term kaolin is also used for the refined industrial product derived from this mineral. Kaolinite forms by chemical weathering of aluminosilicate minerals such as feldspar in hot, moist climates and is common in sedimentary deposits as well as residual weathering zones in granitic and other rocks.

Kaolinite is a 1:1 phyllosilicate with a layered crystal structure, giving it platy particles and a low shrink-swell tendency. It is white, has a Mohs hardness of about 2, and a refractive index around 1.54. Industrial kaolin is usually classified by brightness and particle size, with grades for paper, ceramics, and fillers. Primary deposits occur near the source rock; secondary deposits are transported and concentrated by water or wind, often with higher impurity content.

Mining and processing involve washing, separation, and sometimes flotation to remove impurities. Calcination (heating to 600–800 °C) produces calcined kaolin with increased whiteness, brightness, and thermal stability; metakaolin, produced at higher temperatures, is used as a pozzolanic additive in cement and as a catalyst support. Major industrial uses include paper production (coating and filler), ceramics (including porcelain and sanitaryware), paints, plastics, rubber, cosmetics, and pharmaceuticals.

China is the largest producer of kaolin, with significant deposits in the United States (notably Georgia), the United Kingdom (historic Cornwall deposits), Brazil, and India. The name derives from kaolin from Gaoling (High Ridge) in Jiangxi Province, China, where the clay was first described in connection with porcelain manufacture.

Exposure to respirable kaolin dust can irritate the eyes, skin, and lungs; it is generally less hazardous than crystalline silica, but dust-control measures are recommended in mining and processing.