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(mineralogy) A fibrous neosilicate mineral, polymorphic with andalusite and kyanite, with the chemical formula Al₂SiO₅.

Colourless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green, blue; colourless in thin section – Prismatic crystals, fibrous, acicular – {010} perfect – Splintery – Tough – 7 – Vitreous to subadamantine, silky – White – Transparent to translucent – 3.24 – Biaxial (+) – Nα = 1.653 – 1.661 nβ = 1.654 – 1.670 nγ = 1.669 – 1.684 – Δ = 0.020 – 0.022 – Colourless to pale brown to yellow – 21–30°.

(mineralogy) A fibrous neosilicate mineral, polymorphic with andalusite and kyanite, with the chemical formula Al₂SiO₅.

Phase diagram of Al₂SiO₅ (aluminosilicates).