Sillimanite

Aluminum Silicate
Al₂SiO₅

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.

The History Says

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 Brook.

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 foundries.

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SILLIMANITE 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% Al₂O₃ and 36.8% SiO₂. They are distringuished from one another by their modes of occurrence and physical and optical properties.

Physical And Optical Properties Of Alumino-Silicate Minerals

Properties	Alumino-silicate minerals
	Andalusite	Kyanite	Sillimanite	Mullite
Crystal system	Orthorhombic	Triclinic	Orthorhombic	Orthorhombic
Colour	Pink, Red, Grey, Brown	Blue, Green, White	White, Grey, Brown, Yellow	-
Form	Stout, prismatic crystal, grain size	Bladed, enlongated, tabular	Accicula, needle like	Elongated, needle like
Hardness	7.5	4-7	6-7	7
Sp. Gravity	3.15	3.6	3.23	3.16
Refractive Index	1.629 - 1.647	1.712 - 1.729	1.657 - 1.684	1.659 - 1.692
Decomposition	1380º -1400ºC	1350º - 1380ºC	1550º - 1650ºC	1810ºC
Volume change	Very slight increase	Notable increase	Slight increase	Stable

The 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 (3Al₂O₃. 2SiO₂). 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.

Hardness	Associated Minerals	Chemical/Typical composition	Colour	characteristics	Luster	Field Indicators
7 - Quartz	garnets biotite feldspars quartz kyanite andalusite	Al2O362.92 % SiO237.08 %	Bluish Brownish Greenish Colorless Gray Gray Green	fibers are brittle distinguishing them from asbestos.	Vitreous (Glassy)	crystal habit, color, brittlness and hardness if not fiberous

Industrial Applications

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 operations.

World Resources

Countries reporting production of sillimanite are:

• USA
• Brazil
• South Africa

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.