Mineral Focus: Kaolinite
Kaolinite is a clay mineral, first described in 1867 for its occurrence in the Jari River basin, Brazil. It is very common and is extensively mined in the UK, France, Brazil, Australia, India, Korea, USA and China. The mineral is mined under the name of Kaolin, but is more commonly known as ‘China Clay’.
This soft, earthy white mineral is produced by chemical weathering of aluminium silicate minerals, such as feldspar. In Cornwall, in the UK, where there are the largest reserves of this mineral in the world, it is has been mined since the eighteenth century and is effectively weathered granite. Although white is its usual colour, it can also be rusty orange, as a result of the presence of iron oxide, or sometimes yellow or even light orange at lower concentrations of this chemical (that is ‘rust’). Kaolinite can form in bands of alternating colours, such as at Canyon State Park in Georgia, USA.
The mineral has a wide range of uses apart from being the main component in porcelain. These include being used by the ceramics industry, for coated paper, medical uses (such as anti-diarrhoea medicines), for cosmetics, as food additives, in toothpaste and as a light diffusing material used in white incandescent light bulbs. In spite of being famous for its use in porcelain, its largest use is in the production of paper. In particular, it is used to give a gloss effect. Recent discoveries about how it can be used include as a spray for organic fruits and vegetables to deter insects.
The chemical formula for Kaolinite is Al2Si2O5(OH)4. It occurs in abundance in natural soils that have formed from the chemical weathering of rocks in hot, moist climates. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra.