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One outstanding feature of sillimanite is that the volume porosity
of grains hardly exceeds 6% after heating and that it tends to
decrease as a consequence of vitrification, if heating is prolonged.
East of the Metchin River, as the road rises over the Red Wine
Mountains, rock exposures appear more frequently than in the sandy
esker terrain to the west. The main rock type in this area is
sillimanite gneiss, formed during the Labradorian Orogeny about 1650
million years ago. Sillimanite gneiss occurs in flat, smooth outcrops
on the north side of the road between the Metchin River and Middle
The Present Scenario
Sillimanite refractories cut into various shapes and sizes or made
out of bonded particles are used in industries like cement, ceramics,
glass-making, metal smelting, refinery and treatment, tar
distillation, coal carbonisation, chemicals manufacture, and iron
is one of the three allotropic varieties of mineral corresponding to
the formula Al2O3. SiO2. The other two are anadalusite and kyanite.
They all contain 63.2% Al2
and 36.8% SiO2
They are distringuished from one another by their modes of occurrence
and physical and optical properties.
And Optical Properties Of Alumino-Silicate Minerals
||Pink, Red, Grey, Brown
||Blue, Green, White
||White, Grey, Brown, Yellow
||Stout, prismatic crystal, grain
||Bladed, enlongated, tabular
||Accicula, needle like
||Elongated, needle like
||1.629 - 1.647
||1.712 - 1.729
||1.657 - 1.684
||1.659 - 1.692
||1350º - 1380ºC
||1550º - 1650ºC
||Very slight increase
properties of sillimanite are similar to mullite. As a matter of fact
all the alumino-silicate minerals are used as refractory material in
the form of mullite (3Al2
It is stable in all temperatures up to 1550ºC. Between 1545º
to 1550ºC there is a fairly rapid permanent expansion of 2-3%.
The maximum expansion by volume observed in some sillimanite is 4%.
The linear expansion is less than 1%. This mineral thus shows
negligible expansion while changing over to mullite and hence it is
used as quarried without pre-calcination. Kyanite, on the other hand,
shows very great expansion in volume (of about 20%) and it is
necessary to precalcine it before use. Andalusite is the atablest of
all, scarcely undergoing any expansion. One of the largest sources of
andalusite is the metamorphosed rocks of Pretoria series of
Chuniespoort, northern Transvaal, Rep. Of South Africa. Other
producing countries are Russia, Sweden, Korea and the USA.
One outstanding feature of sillimanite is that the volume porosity of
grains hardly exceeds 6% after heating and that it tends to decrease
as a consequence of vitrification, if heating is prolonged. Such
compactness offers great advantage in all applications in which
problems of corrosion and abrasion arise. The porosity of the calcined
kyanite is 17 to 20%. Even in the denset variety it hardly decreases
below 10%. On the other hand low porosity offers great advantage in
sillimanite over kyanite. Sillimanite has many other valuable
properties which distinguish it as a superduty refractory material.
These include strength and toughness, resistance to mechanical shock,
high melting point, low co-efficient of expansion, low electrical
conductivity, neutral reaction and resistivity to corrosive slags and
to oxidising and reducing conditions.
|7 - Quartz
% SiO237.08 %
||Bluish Brownish Greenish
Colorless Gray Gray Green
||fibers are brittle
distinguishing them from asbestos.
||crystal habit, color,
brittlness and hardness if not fiberous
Appreciable quantity of sillimanite powder mixed with high-grade
china-clay is used in the manufacture of spark plugs and other
insulating materials required at high voltages. Sillimanite
refractories cut into various shapes and sizes or made out of bonded
particles are used in industries like cement, ceramics, glass-making,
metal smelting, refinery and treatment, tar distillation, coal
carbonisation, chemicals manufacture, and iron foundries. It is very
resistant to glass slag and hence widely used as lining material in
glass melting furnace. Iron corrodes it repidly hence it is not
suitable in iron smelting. It is also not highly resistant to molten
metallic oxides and, therefore, rather unsuitable for metallurgical
Countries reporting production of sillimanite are:
In USA, North and South Carolina and Georgia are the producing States.
Deposits of some importance have been discovered near the Orange
River, in the Namaqualand district of Cape Province and south-west of
Pell Mission farm about 172 km. from the nearest railway station at
Kakamas, Cape Province, Rep. of South Africa. The deposits occur on a
flat, low-lying and partly sand covered terraces. The present rate of
production is 1,500 long tons.
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