3 The Sillimanite Minerals: Andalusite, Kyanite, and Sillimanite 47beach sands of several locales. The Assam deposits in India are perhaps the most
famous, as large pure sillimanite boulders are found there. Beach sands in Australia,
the United States, and Africa are also sources, often as a byproduct of the beneficiation
of the heavy metals in those sands.
Although the sillimanite minerals are the major source of mullite for ceramics
and refractories, there was an increase in the production of synthetic mullites by
electrofusion starting in the 1950s and shortly thereafter by sol–gel chemical methods.
These industries were the outgrowth of the inability of the commercial sillimanite
producers to provide sufficiently high quality amounts of the sillimanite minerals to
meet the worldwide demand for mullite. Synthetic mullite was the result. However,
the use of synthetic mullite in many applications requires extensive manufacturing
process alterations when substituted for the sillimanites. For example, fused mullite is
often of the 2Al 2 O 3 ·SiO 2 variety, although it is possible to produce different stoichi-
ometries with different alumina contents. Chemical mullites prepared by sol–gel
methods are highly pure, but they are quite expensive relative to the varieties naturally
produced from the sillimanites.
5 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.6 Conclusions
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 world-
wide source locations provide them with numerous opportunities for future uses in a
growing market.