Каолін

What it is

Kaolinite is a clay mineral with the chemical composition Al₂Si₂O₅(OH)₄. Rocks that are rich in kaolinite and halloysite are known as kaolin or china clay. Kaolinite is a soft, earthy, usually white mineral. In many parts of the world, iron oxide colors kaolin pink-orange-red, giving it a distinct rust hue. Lower concentrations of iron oxide give white, yellow or light orange kaolin.

Structure and properties

Kaolinite is a layered silicate mineral. Kaolinite is chemically and structurally simple compared with other clay minerals. It has a low shrink-swell capacity and a low cation-exchange capacity of 1-15 meq/100 g. Kaolinite has a Mohs hardness of 2-2.5 and a white streak. When the clay is dried, most of the water molecules are removed and the plates hydrogen bond directly to each other, so the dried clay is rigid but still fragile. If the clay is moistened again, it becomes plastic once more.

Names and notation

The ending -ite was added to the name later, so that it covers nearly identical minerals from other places. Lithomarge is an archaic name for kaolinite; in modern use it refers specifically to a compacted, massive form of kaolin. In mineralogy the formula is written Al₂Si₂O₅(OH)₄, while in ceramics it is usually written as oxides, Al₂O₃·2SiO₂·2H₂O.

Production and uses

Kaolin is an important raw material in many industries and applications. Global production of kaolin in 2021 was estimated to be 45 million tonnes. In 2009, up to 70% of kaolin was used in the production of paper. In 2016 the market share was reported to be 36% paper, 31% ceramics, 7% paint and 26% other. Kaolin is the primary material from which porcelain is made, even though clay minerals might account for only a small proportion of the whole. Porcelain is a ceramic material made by heating raw materials, generally including kaolinite, in a kiln to temperatures between 1,200 and 1,400 °C. In whiteware ceramic bodies, kaolin can constitute up to 50% of the raw materials. The Jingdezhen kilns first perfected the use of kaolin in combination with petuntse to create world-class hard-paste porcelain. In organic farming, kaolin is sprayed on crops to deter insect damage.

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