https://en.wikipedia.org/wiki/Gypsum Gypsum occurs in nature as flattened and often twinned crystals, and transparent, cleavable masses called selenite. Selenite contains no significant selenium; rather, both substances were named for the ancient Greek word for the Moon
Category Sulfate minerals
Formula
(repeating unit) CaSO4·2H2O
IMA symbol Gp[1]
Strunz classification 7.CD.40
Crystal system Monoclinic
Crystal class Prismatic (2/m)
H-M symbol: (2/m)
Space group Monoclinic
Space group: I2/a
Unit cell a = 5.679(5), b = 15.202(14)
c = 6.522(6) Å; β = 118.43°; Z = 4