Petalite
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Petalite, also known as castorite, is a lithium aluminum tektosilicate mineral LiAlSi₄O₁₀, crystallizing in the monoclinic system. Petalite occurs as colorless, pink, grey, yellow, yellow grey, to white tabular crystals and columnar masses. Wikipedia
2V angle: 82–84° measured
Cleavage: Perfect on, poor on, with 38.5° angle between the two
Color: Colorless, grey, yellow, pink, to white
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Tabular prismatic crystals and columnar masses
Crystal system: Monoclinic
Formula (repeating unit): LiAlSi4O10