Ratnapura, Ratnapura District, Sabaragamuwa Province, Sri Lanka

Al₂OSiO₄

Crystal System: orthorhombic
Colour: colorless, white, yellow, brown, green or gray
Lustre: sub-vitreous, greasy, silky
Habitus: prismatic to fibrous (var. fibrolite) crystals
Hardness: 6½ - 7½
Fracture: irregular/uneven
Cleavage: perfect {010} 
Density: 3.24 g/cm³ 
Origin and geological occurrence: in high-grade regional metamorphic schists, gneisses, and hornfels. It can rarely occur in pegmatites. Detrital component in sedimentary rocks

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Samples:

9A.8 Sillimanite Dark gray prismatic crystals in quartz #neso soro silicates