Sillimanite
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Sillimanite or fibrolite is an aluminosilicate mineral with the chemical formula Al₂SiO₅. Sillimanite is named after the American chemist Benjamin Silliman. It was first described in 1824 for an occurrence in Chester, Connecticut. Wikipedia
Chemical formula: Al2SiO5
Optical properties: Biaxial (+)
2V angle: 21–30°
Birefringence: δ = 0.020 – 0.022
Cleavage: perfect
Color: Colourless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green, blue; colourless in thin section
Crystal class: Dipyramidal (mmm); H-M symbol: (2/m 2/m 2/m)