46 R.C. Bradtare considered. Mullite, the major product, has a density of about 3.2 Mg m−3,
similar to that of andalusite. Thus, andalusite experiences the lowest of the
volume expansions during its decomposition. The crystalline silica phase that
results from the kyanite decomposition is cristobalite with a density of 2.2–2.3 Mg
m−3 and the amorphous silica would be expected to be even less; thus, it is easy to
explain the volume expansion and cracking of the sillimanite minerals when they
decompose, purely on the basis of the densities of the decomposition products
compared with that of the original sillimanite mineral.
When the sillimanite minerals are milled into the nanoparticle regime, the decom-
positions are strongly modified in several ways. The first effect is that the temperature
range of the decomposition to mullite and silica is reduced significantly, often by
several hundred degrees Celsius. This decrease in temperature has potential energy
savings for subsequent industrial firing processes. Second, the volume expansion,
∆V%, is reduced, eventually achieving a situation where the sintering of the nanofine
particles supercedes the volume expansion of the decomposition and shrinkage occurs
upon heating. Finally, the products of the decomposition are altered, as the amorphous
silica of the andalusite and sillimanite decompositions becomes cristobalite comparable
to the decomposition of kyanite. Although there have not been high-resolution transmission
electron microscopy studies of the effects of fine milling on the decomposition, it is
highly probable that the detailed mechanisms of the decomposition of very fine sillimanite
mineral particles may be altered from those of coarse particles usually produced
industrially.
4 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