7 Photos
Sillimanite
Chemical Formula	Al2SiO5
Species	Silicates
Crystal System	Orthorhombic
Mohs Scale	6-7
Specific Gravity	3.24
Color	Colorless or white to gray, also brown, yellow, yellow-green, gray-green, blue-green, blue; colorless in thin section
Streak	White
Luster	Sub-Vitreous, Greasy, Silky
Refractive Index	n = 1.653 - 1.661 n = 1.654 - 1.670 n = 1.669 - 1.684
Diaphaneity	Transparent, Translucent
Cleavage	Perfect perfect
Fracture	Irregular/Uneven
Crystal Habit：Prismatic crystals, fibrous, acicular
Geological Setting：High pressure and high temperature thermal aureoles around intrusive rocks. Similar conditions in regionally metamorphosed rocks.

Sillimanite is an alumino-silicate mineral with the chemical formula Al₂SiO₅. Sillimanite is named after the American chemist Benjamin Silliman (1779-1864). It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA.

Occurrence

Sillimanite is one of three alumino-silicate polymorphs, the other two being andalusite and kyanite. A common variety of sillimanite is known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye. Both the fibrous and traditional forms of sillimanite are common 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.

Sillimanite has been found in Brandywine Springs, New Castle County, Delaware, USA. It was named by the State Legislature in 1977 as the state mineral of Delaware by suggestion of the Delaware Mineralogical Society, Inc.

Natural sillimanite rocks cut into the required shape and size are used mainly in glass industries. Sillimanite is the best raw material for the manufacture of high alumina refractories or 55-60 % alumina bricks. But its use on large scale is not possible due to its fine grading and high cost.^{[citation needed]} Dumortierite and mullite are similar mineral species used in quality porcelain.