Image: <bdi><a href="https://www.wikidata.org/wiki/Q56247090" class="extiw" title="d:Q56247090"><span title="mineral collector and dealer">Robert M. Lavinsky</span></a></bdi> (CC BY-SA 3.0)
orthorhombiccommonSilicates
Sillimanite
Al2SiO5
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.
Sillimanite Physical Properties
Mohs Hardness
6.5Crystal System
orthorhombicLuster
vitreousStreak Color
whiteCleavage
perfect on {010}Fracture
unevenSpecific Gravity
3.25Colors
brown, gray, green, white, blueTransparency
translucentType Locality
Chester, Middlesex County, Connecticut, USAMineral Group
SilicatesUses
refractory materials, ceramics, gemstone (rarely)Associated Minerals
quartz, muscovite, biotite, garnet, cordierite, kyanite, andalusiteRarity
commonGeological 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.
