Silicates: Sillimanite

Composition: Al₂SiO₅

Crystal system: Orthorhombic; fibrous to prismatic aggregates

5.25.1
fibrous sillimanite
SA Australia

Properties:

• Vitreous to silky lustre
• White color; less commonly brownish or greenish
• White streak
• Knife does not scratch (Mohs Hardness 7)
• Above average density
• One perfect cleavage
• Cleaves rather than fractures
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Sillimanite occurs in high grade regional and contact metamorphosed shales .

5.25.2
fibrous sillimanite
SA Australia