•Class: cyclosilicates •Crystal system: hexagonal; 6/m2/m2/m •Crystal habit: prismatic crystals, elongate or flattened, are often vertically striated and grooved. Some crystals show varying degrees of etching. •Twinning: twinning is very rare and simple on pyramidal planes{3141} or{4041} •Specific gravity: 2.68 - 2.80 •Index of refraction: 1.563 - 1.583 •Birefringence: weak (0.005 - 0.007) •Pleochroism: distinct in strongly colored specimens, blue or green/colorless •Hardness: 7.5 - 8.0 •Color: blue to blue-green to sea-green •Luster: vitreous, some specimens have chatoyancy (cat's-eyes and stars) •Transparency: transparent to translucent •Cleavage: imperfect, parallel to the basal pinacoid {0001} •Fracture: conchoidal to uneven, brittle •Streak: white
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