Worldwide Deposits of the Sillimanite Minerals

Because of the abundance of O, Si, and Al in the earth’s crust, it is not surprising that there exist numerous deposits of the sillimanite minerals worldwide. However, the qualities, or grades, of most of these deposits are not amenable to large-scale commercial mining and beneficiation. Many are just small quarrying operations. For those reasons, only a few of the major worldwide deposits will be noted. The text by Varley [2] has an excellent summation of the deposits worldwide, while Chang [1] addresses the mineral processing and beneficiation quite well.

The major industrial sources of andalusite are in South Africa. The Transvaal andalusite often approaches 60 wt% alumina and so the quality is very high indeed. Sometimes, these ores contain other high alumina minerals, so that the total alumina content may actually exceed the 62.9 wt% level of pure anadalusite. Many other deposits exist, notably in the United States and the former USSR, but they are not of the significance of the South African ones. Of course, new findings are always possible in other localities.

The major kyanite deposits are in the United States and India. The US deposits are in the eastern states along the Atlantic seaboard. They are of exceptionally high quality. Massive kyanite deposits also occur in India, where the mining is extensive as they too are excellent. Other less productive deposits occur in Africa, Australia, South America, and the former USSR.

Sillimanite occurs to some extent almost everywhere that andalusite and kyanite are found. However, the major commercial sources are in India and in the sillimanite beach 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 2Al2O3SiO2 variety, although it is possible to produce different stoichi­ometries 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.