Formula: CaSO4 . 2H2O
Origin of name: From Greek gypos - plaster
Crystal system: Monoclinic
Crystal class: 2/m
Twinning: Common on {100}, contact; on {-101}, butterfly or heart-shaped; uncommon on {-111}, elongated
Unit cell: a = 6.29 Ǻ, b = 15.21 Ǻ, c = 5.68 Ǻ, β = 114.1°
Colour: Colourless, white, gray, yellowish, greenish, reddish, brownish
Diaphaneity:
Luster: Subvitreous, pearly on cleavage
Habit: Crystals thin to thick tabular, diamond-shaped, short to long prismatic; acicular; massive, granular, concretionary
Hardness: 2
Specific gravity: 3.32
Cleavage: 3; {010} perfect, {100} and {011} distinct
Tenacity: Splintery; flexible, not elastic
Streak: White
Synonyms/varieties: Alabaster (massive, fine-grained var.), gips, selenite (colourless, transparent var.), satin spar (columnar, fibrous var.)
Comments: Sometimes fluoresces and phosphoresces greenish white
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