other names: Bolus alba, China earth
mineral class: Silicates and germanates
chemical formula: Al4(OH)8| Si4O10 (Kaolinite)
Chemical elements: Aluminum, oxygen, hydrogen, silicon
Similar minerals: Allophane, Halloysite, Imogolite
crystal structure: triklin or monoclinic
mass density: 2,6
Mohs hardness: 2,5
stroke color: White
transparency: translucent to opaque
use: Porcelain production, cosmetics
General information about the kaolin:
kaolin describes a clay mineral, which owes its name to the Chinese city Gaoling. There, the originally known as white earth and today under the common name of China earth or bolus alba known rock was first found. In the German-speaking world, the name kaolin prevailed only in the 18th century. In its pure form, kaolin is snow-white in color and is counted among the group of fine-grained silicate minerals. It is of dry consistency, easily grated with fingers, and then assumes the texture of fine flour. It also contains water of crystallization, which is released when heated. By chemical admixtures kaolin sometimes appears yellow, reddish, beige or slightly bluish. Most hexagonal crystals can form bulky or leafy aggregates. Kaolin has a matte, rarely pearlescent shine and is characterized by complete cleavage and uneven or mussel break. Depending on the nature of the industry, a distinction is made between Flint clay, Fireclay, Underclays or Ball clay. With a maximum Mohs hardness of 2.5, kaolin is a comparatively soft rock that can be completely transparent as well as slightly translucent. In addition to kaolinite, which is the main constituent of kaolin, the rock also contains feldspar.
Origin, occurrence and localities:
As a weathering product kaolin is formed from silicate-containing rocks such as granite or rhyolite, which have a high content of quartz and feldspar. These weather under the influence of groundwater, surface water or hydrothermal fluids. Therefore kaolin is often associated with feldspars, quartz or mica. Kaolin produced in this way is mined in primary deposits. By contrast, secondary deposits are formed when primary kaolin is eroded and subsequently deposited together with other minerals. Such kaolin sediment usually contains a Feldspatanteil of well over 25 percent and is referred to as so-called arkose. Kaolin is found worldwide, but there are only a few deposits that are significant in size and economically significant. In addition to China and Japan, the most important countries in which large deposits are located are the United States, Brazil, India, England, Austria and Germany, where above all the hill country in central Saxony and the Upper Palatinate are important deposits. Every year between forty and fifty tons of kaolin are mined. Global reserves are estimated to be only 14 billion tons today.
Use of kaolin:
Kaolin plays an economically important role primarily as raw material in the production of white porcelain, in particular for the production of the famous Meissen porcelain. In addition, china clay also serves as the basis of white tiles for floor coverings and wall coverings. Kaolin is used as a snow-white pigment for the production of paper in the form of brightener, as an additive in white paint for interior and exterior coatings and in food under the name E 559 as a release agent. In the cosmetics industry, kaolin is mainly used as the basis for body and face powders and various skin care products.