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Al2SiO5 A brown, pale-green, or white neosilicate mineral with vitreous luster crystallizing in the orthorhombic system; commonly occurs in slender crystals, often in fibrous aggregates; hardness is 6-7 on Mohs scale, and specific gravity is 3.23. Also known as fibrolite.



a mineral of the silicate class. Sillimanite is a high-temperature, polymorphic modification of the composition Al[AlSiO5] and contains 1–1.5 percent Fe2O3 in admixture form. The sillimanite structure is made up of chains of alternating SiO4 and AlO4 tetrahedrons, connected by chains of Al octahedrons. The mineral crystallizes in the orthorhombic system, forming acicular crystals, compact radiated masses, or thin fibrous aggregates. It sometimes forms dispersed, filiform inclusions in other minerals (fibrolite variety). Sillimanite is gray, light brown, or pale green; it has a vitreous luster, a hardness of 6.5-7.5 on Mohs’ scale, and a density of 3,270 kg/m3. At high temperatures (approximately 1545°C), it decomposes into mullite and silica.

Sillimanite occurs in thermally and regionally metamorphosed clay rocks. It serves as a raw material in the production of refractories and acid-resistant products with a high alumina content. The largest known sillimanite deposits are in India (Khasi Hills and Pipri).


Kostov, 1. Mineralogiia. Moscow, 1971. (Translated from English.)
References in periodicals archive ?
The occurrence of andalusite and sillimanite in these rocks can be associated to a superimposed low-P/high-T thermal event, although the presence of these polymorphs implies pressure below the [Al.
The protrusion of fibrolitic sillimanite from biotite into quartz is alarming because it creates tiny microscopic fractures which can make the affected quartz grains extremely susceptible to ASR in the presence of alkali dissolved water.
Beach placer deposits of India essentially consist of ilmenite, rutile, garnet, zircon, monazite, sillimanite and quartz.
Rock-forming minerals such as kyanite, vanadian phlogopite, fuchsite, quartz, sillimanite, and tourmaline group species occur in the vein rock.
Its trend NW-SE is also indicated by the orientation of sillimanite nodules in sillimanite-biotite paragneisses and migmatites.
These metamorphic rocks are made of minerals such as micas, garnet, kyanite, sillimanite and cordierite.
Haematite, tourmaline, zircon, rutile, ilmenite, sillimanite, geothite, and chlorite were also observed in Fe-Mn concretions.
So far, more than 120 kyanite mineral districts have been found, of which 23 have proven reserves of 75M mt grading 10-20% kyanite group minerals (kyanite, sillimanite, and andalusite).
The muffle used in tube furnaces is usually dust-free and impervious; the sillimanite (aluminum silicate) muffle former used in chamber furnaces is not.
It is the third-richest country in bauxite, the largest exporter of cut and polished diamonds, and a key exporter of fabricated sheet mica, non-coking coal, sillimanite, granite, vermiculite, magnesite, steatite and bentonite.
Recoverable reserves of selected Indian resources (Mt unless otherwise stated) Fuel minerals Reserve Coal 192,000 Lignite 6,473 Metallic minerals Chromite 88 Iron ore (hematile) 9,602 (magnetite) 3,143 Manganese ore 176 Bauxite 2,525 Copper ore 416 Gold ore 18 Lead-zinc ore 186 Titanium 102 Tungsten 26 Vanadium 13 Molybdenum (t) 5,389 Non-metallic minerals Apatite 13 Asbestos 23 Barytes 70 China clay 986 Diamond ('000 ct) 1,084 Dolomite 4,967 Gypsum 239 Kyanite 3 Limestone 76,446 Magnesite 233 Mica (t) 109,000 Rock phosphate 147 Beach sand sillimanite 51 Soapstone/steatite 84 Wollastonite 4 Source: Indian Bureau of Mines