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Sillimanite is a mineral species of the group of silicates under the group of the neosubsilicates of formula Al2O (SiO4) with traces of iron. The malformed crystals do not exceed 2 cm.

Described by the American mineralogist Bowen in 1824, his name comes from the American chemist and mineralogist Benjamin Silliman (1779-1864).

Sillimanite consists of parallel chains of octahedra AlO6 and tetrahedra alternately SiO4 and AlO4.

Morphology of crystals: prismatic crystals, acicular. This morphology results from the structure of sillimanite.

A relatively stable mineral, but can be altered into kaolinite, muscovite or sericite.

It is a frequent mineral in high-grade thermal metamorphism rocks of clay rocks, by transformation of biotite or andalusite.
It can also be formed by regional metamorphism of clay rocks (gneiss) from muscovite and biotite, reactions between staurotide and biotite or between staurotide and quartz, or by polymorphic transformation of disthene.

Sillimanite is one of three aluminosilicate 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.

Rocks cut into the required shape and size are used mainly in glass industries. It 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. Dumortierite and mullite are similar mineral species found in porcelain.

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


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