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)
orthorhombicuncommonSilicates
Sillimanite
Al2SiO5
This specimen shows a fibrous, brownish-gray mass of sillimanite, exhibiting a somewhat silky luster due to its acicular habit.
Sillimanite Physical Properties
Mohs Hardness
7Crystal 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
refractories, industrial, collectorAssociated Minerals
quartz, kyanite, andalusite, garnet, biotite, muscoviteRarity
uncommonGeological Context
Sillimanite is a high-temperature, high-pressure index mineral formed during regional metamorphism of argillaceous rocks, indicating high-grade metamorphic conditions.
