Industrial Applications of the Sillimanite Minerals
It has already been noted that the primary use of the sillimanite minerals is for the production of mullite refractories. These refractories are used extensively in the steel and glass industries. Because of the association of the sillimanite minerals with higher alumina content minerals in their deposits, the range of alumina contents for the aluminosilicate refractories that they produce is approximately 50-80 wt% alumina.
There has also been a modest use of the sillimanite minerals as abrasives. This was originally a matter of convenience, perhaps even mistaking them for corundum. However, the recent globalization of trade has practically eliminated this application, given the modest hardness of the sillimanites, only about 7 on the mineralogical Mohs scratch hardness scale. This hardness level cannot compete with corundum and the recently developed and quite superior synthetic abrasives.
As with many other minerals, the sillimanites are sometimes found in large single crystal form at the quality level suitable for gemstones. This is particularly true for andalusite. Exquisite green andalusite is common in Brasil in the state of Minas Gerais. It can frequently be obtained at gem and mineral shows around the world.
The structures and properties of the three sillimanite minerals (andalusite, kyanite, and sillimanite) have been discussed. The uniqueness of the three, perhaps the result of their high-temperature, high-pressure genesis is described. Their decompositions at 1 atm pressure are also described. The availability and the properties of these minerals have served them well in industrial applications in the past. It appears that their worldwide source locations provide them with numerous opportunities for future uses in a growing market.