By Chisom Ibemere
Sillimanite is an aluminosilicate mineral. It is named after an American chemist, Benjamin Silliman. It was first described in 1824 for an occurrence in Chester, Connecticut. It belongs to the aluminosilicate polymorphs including andalusite and kyanite.
Fibrolite remains the common variety of sillimanite whose name was given because of its appearance like bunch of twisted fibres.
The fibrous and traditional forms of sillimanite are found in metamorphosed sedimentary rocks. It is an index mineral indicating high temperature but variable pressure. Example rocks include gneiss and granulite. It occurs with andalusite, kyanite, potassium feldspar, almandine, cordierite, biotite and quartz in schist, gneiss, hornfels and also rarely in pegmatites. Dumortierite and mullite have similarities but occurs in porcelain.
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Natural sillimanite is used in the production of high alumina refractories or 55-60% alumina bricks. It has been replaced by the other aluminosilicate polymorphs, andalusite and kyanite minerals for the same purpose. In the year 1998, sillimanite was 2% of all aluminosilicate mineral production in the western world.
Sillimanite may be colorless, white, yellow, brown, green, blue, gray with sub-Vitreous, Greasy lustre. It has a hardness of 6.5 – 7.5 and a specific gravity between 3.23 – 3.27 and an orthorhombic crystal system. It is transparent to translucent with an uneven fracture and brittle.
The rare transparent Sillimanite crystals which are gotten from Burma, Sri Lanka, and India are highly valued and are occasionally cut into exquisite gemstones. other minerals associates includes Quartz, Biotite, Almandine, Plagioclase.
Sillimanite has been found in Brandywine Springs, New Castle County, Delaware and named by the State Legislature as the state mineral of Delaware by the suggestion of the Delaware Mineralogical Society in the year 1977.
