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Sillimanite (Photo) [abbr. Sil, Fi ]

Year of discovery: 1824 (Carnatic Coast, India, and Chester, USA), 1899 (Japan)

Locality list: 257 localities of sillimanite

• Al₂SiO₅
• [Al³⁺][Al³⁺][O|SiO₄]^6-

• Class: Nesosilicate - Oxide
• Insoluble in: HCl, HNO₃, H₂SO₄

• Polymorph: Kyanite (High pressure, Triclinic)
• Polymorph: Andalusite (Low temperature, Orthorhombic)

• Al => Mn: Kanonaite

• Color: Colorless, Gray
• Streak: White
• Luster: Vitreous
• Diaphaneity: Transparent-Translucent

• Habit: Fibrous
• Aggregate: Columnar

• Cleavage: Perfect on {010} (1 plane)
• Hardness: 6.5-7.5
• Density: 3.2

• System: Orthorhombic
• Group: mmm, Pnma
• Lattice: a=7.5, b=7.7, c=5.8

Etymology: Professor Benjamin Silliman (1779-1864), Chemist and Geologist of Yale University, UK