Anhydrite
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Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO₄. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. Wikipedia
Color: Colorless to pale blue or violet if transparent; white, mauve, rose, pale brown or gray from included impurities
Chemical formula: CaSO4
2V angle: 56–84°
Birefringence: δ = 0.042–0.044
Cleavage: perfect, perfect, good, resulting in pseudocubic fragments
Crystal class: Dipyramidal (mmm); H–M symbol: (2/m 2/m 2/m)
Crystal system: Orthorhombic