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Download hereGENERAL DESCRIPTION
Sillimanite is an aluminosilicate, which is polymorphic to kyanite and andalusite (i.e. even chemical composition, but with a different atomic structure).
This mineral forms in the schists and the gneisses of high temperature metamorphism and more rarely in some granites (anatexites). It is associated to cordierite, andalusite, biotite or quartz.
The crystals form fine and lengthened prisms, often grooved. It is frequent to find them forming "bouquets". The fibrolite variety is when sillimanite is fibrous. Sillimanite is, in general, white porcelain to gray in color but can be colored by many impurities.
The most beautiful sillimanite crystals (purple clearly gemmeous) come from Ratrupana and Elahera in Sri Lanka (gemmeous and colourless prismatic crystals completion) and beautiful samples comes from Brazil (Saquarema close to Rio de Janiero), India (Orissa), New England, Italy (Valley di Fredda, Piedmont), Pakistan and Burma (Mogok).
In France, major outcrops are : Beaulieu in Cantal, Lavoute-Chilhac, Mont Denise and the alluvia of the Métou and Sénouire rivers in Haute-Loire, the island of Groix and the Gulf of Morbihan (in Morbihan, then), near Boën in the Loire...
The place of conservation of this type is not known.
Did you know? Sillimanite is the rarest species of the three polymorphic ones of aluminosilicate. It is sometimes cut for gem (samples from Orissa, India).
HISTORY : Species dedicated to professor Benjamin Silliman (1779-1864), professor of chemistry and geology at Yale University
Species described in 1824 by Bowen
Type-locality : Chester, Middlesex, Connecticut, USA and Vltava, Bohemia, Czech Republic
CHEMICAL FORMULA : Al2 SiO5
CRYSTAL SYSTEM : Orthorhombic
COLOR : Colorless or white to gray, brown, green-gray
DIAPHANIETY : Transparent, translucent
LUSTER : Vitreous, subadamantine
STREAK : White
MORPHOLOGIE : Fibrous, massive. Prismatic crystals
HARDNESS : 6,5-7,5
CHEMICAL CLASS: 3,24
DENSITY : VIII - Silicates
GROUP : Sillimanite-andalousite-kyanite
STRUNZ CLASS BEFORE 2001 : 8/B.02-10
STRUNZ CLASS AFTER 2001 : 9.AF.05
