Utilisation of sillimanite minerals, their geology, and potential occurrences in Norway - an overwiev
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- Artikler 
The aluminium-silicate polymorphs sillimanite, andalusite and kyanite decompose to a mixture of mullite and silica glass during calcination. Mullite is an essential component of high-alumina refractories forming the inner lining of furnaces and high-temperature vessels widely used in the production of metals, ceramics, glass and cement. sillimanite, andalusite and kyanite constituted 2%,74% and 24%, respectively, of the total western world's production of sillimanite minerals in 1998 of a total of about 401 ooo tonnes, of which about 95% was consumed by the refractory industry. The sillimanite minerals are mined mainly in South Africa (62% andalusite), USA (23%, kyanite) and France (11% andalusite), whereas the major consumers are found in the major iron- and steel producing areas of the world. Production has declined over the last decade, due mainly to cutbacks in iron and steel production. Most economic deposits of sillimanite minerals can be classified as metamorphogenic and include contact-metamophic andalusite deposits, stratabound podiform deposits of massive (corundum)-sillimanite and (corundum)-kyanite rocks and stratiform deposits of kyanite quartzites. The former occur within metapelitic sequences in the contact aureoles of plutonic massifs, whereas the two latter in most cases are interpreted as representing metamophosed high-alumina sediments, derived from hydothermal alteration centres in subaerial felsic to intermediate volcanites. All of these deposit types have also been recognised in Norway. The most promising occurrences are the andalusite schist within the contact aureoles of the Fongen-Hyllingen and Øyungen Gabbro Complexes in the Caldonides of Central Norway.