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Andalusite (aluminum
silicate)
Color: brown, green, pink, violet, red.
Moh’s hardness: 7-7½
Refractive index: 1.641-1.648
Critical angle: 37.5°
Specific gravity: 3.12-3.18
Cleavage: Imperfect, but distinct in one
direction.
Fracture: Uneven, brittle.
Dispersion: Medium (.016)
Heat sensitivity: Low
Pleochroism: Trichroic – Strong; reddish
to yellow to olive green.
Birefringence (double refraction): Medium
(.007)
Crystal structure: Orthorhombic
Cutting: Cut on a medium lap.
Orient for
desired color. Sharp corners may chip because of brittleness. Stay clear of the
cleavage plane.
Polishing: Alumina or tin oxide on tin or
tin/lead lap at normal speed.
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