Sillimanite
Image: Strekeisen (CC BY-SA 4.0)
Mineral NameSillimanite
Chemical FormulaAl₂SiO₅
Mohs Hardness6.5
Crystal Systemorthorhombic
Lustersilky
Streak Colorwhite
Cleavageperfect on {010}
Fractureuneven to splintery
Specific Gravity3.25
Colorswhite, gray, pale green, brown
Transparencytransparent to translucent
Type LocalityElba Island, Italy
Mineral GroupSilicates
Usesrefractory material, ceramic production, collector
Associated Mineralsbiotite, quartz, feldspar, garnet
Rarityuncommon
Description
This microscopic image displays fibrous, radiating aggregates of sillimanite crystals, appearing pale gray-blue, intergrown with larger, brown, platy crystals of biotite. The sillimanite forms a distinctive 'flow' texture within the rock.
Geological Context
Sillimanite is a high-temperature, high-pressure index mineral characteristic of high-grade metamorphic rocks, often forming in pelitic schists and gneisses. Its presence in a mafic microgranular enclave within granite suggests complex magmatic and metamorphic interactions.
