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Kyanite and Related Minerals
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Kyanite, andalusite, and sillimanite are anhydrous aluminosilicate minerals that have the same chemical formula, Al2SiO5, but differ in crystal structure and physical properties.  When calcined at high temperatures (around 1,350° C to 1,380° C for kyanite and slightly higher for andalusite and sillimanite), these minerals are converted to mullite, 3Al2O3·2SiO2, and silica, SiO2, which are refractory materials.  In the conversion to mullite, 1 metric ton (t) of mineral concentrate yields about 0.88 t of mullite.  Synthetic mullite is made by heating mixtures of alumina and silica or bauxite and kaolin at around 1,550° C to 2,000° C.  Refractories are heat-resistant materials used in high-temperature applications such as furnaces, boilers, ladles, and kilns, in the metallurgical, glass, chemical, cement, and other industries.

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