Sillimanite
Image: Robert M. Lavinsky (CC BY-SA 3.0)
Mineral NameSillimanite
Chemical FormulaAl2SiO5
Mohs Hardness6.5
Crystal Systemorthorhombic
Lustervitreous
Streak Colorwhite
Cleavageperfect on {010}
Fractureuneven
Specific Gravity3.25
Colorsbrown, gray, green, white, blue
Transparencytranslucent
Type LocalityChester, Middlesex County, Connecticut, USA
Mineral GroupSilicates
Usesrefractory materials, ceramics, gemstone (rarely)
Associated Mineralsquartz, muscovite, biotite, garnet, cordierite, kyanite, andalusite
Raritycommon
Description
This specimen displays a fibrous to columnar mass of brownish-gray sillimanite, characteristic of its common habit. The fibers are tightly intergrown, giving it a somewhat silky appearance.
Geological Context
Sillimanite is a high-temperature, high-pressure index mineral formed during regional metamorphism of aluminous rocks, often indicating the highest grades of metamorphism. It is a polymorph of kyanite and andalusite, forming under different pressure-temperature conditions.
