Kaolinite

Kaolinite is a clay mineral of the kaolin group with the formula Al2Si2O5(OH)4. It is a layered silicate consisting of alternating silica tetrahedral sheets and alumina octahedral sheets (a 1:1 structure). The layers are held together by hydrogen bonds, giving a relatively fine, platy habit and a low shrink-swell potential.

Kaolinite forms by the chemical weathering of feldspars and other aluminum silicates under acidic, hydrated conditions, commonly in tropical and subtropical soils. It develops from the alteration of feldspars in granitoid rocks and is part of the broader kaolin group, which also includes halloysite, dickite, and nacrite. These minerals differ mainly in interlayer water content and crystallinity.

Occurrence and deposits are worldwide, with major reserves in Georgia (USA), China, Brazil, and the United Kingdom. The name kaolin derives from Kaoling, a hill in Jiangxi Province, China, where large deposits were historically exploited for porcelain.

Properties and varieties: halloysite is a hydrated form that forms nanotubes; dickite and nacrite are more highly crystalline polymorphs. In general kaolinite is white to cream, with low cation exchange capacity, low plasticity, and low swelling. It has a Mohs hardness around 2–2.5 and a specific gravity near 2.6.

Uses: the principal application is as a white filler and coating pigment in ceramics, paper, paints, plastics, and cosmetics. It is also used as an adsorbent, catalyst support, and in pharmaceuticals and agriculture.

Processing and thermal behavior: upon heating, kaolinite dehydroxylates around 490–600 °C to form metakaolinite, an amorphous aluminosilicate; further heating can yield spinel and eventually mullite and silica.