Gypsum

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Other names:
  • Selenite is transparent and cleaves into thin sheets.
  • Satin spar is fibrous.
  • Massive or rock gypsum is granular.
  • Alabaster is fine-grained massive gypsum.
Mineral class:Sulfate
Composition:CaSO4 2H2O
Hardness:1 - 2.5
Luster:
  • A freshly broken fragment sparkles with pearly vitreous luster.
  • Satin spar, a fibrous variety, has a silky luster.
  • Selenite has a vitreous luster, pearly on cleavage surfaces.
Color:Mostly white to light gray; iron oxide can add red tints. Selenite is tranparent. With impurities it can be translusent, white, gray, yellow, red, etc.
Streak:White
Cleavage:Selenite has perfect cleavage in 1 plane; good cleavage in 2 other planes.
Crystal form:
  • Selenite is monoclinic (rhombic common, often flat crystals).
  • Massive gypsum has more randomly placed selenite crystals.
  • A fish-tail twin consists of two selenite crystals grown together.
  • Gypsum flowers are a crystal network that occurs in joint fillings.
Environment:Chemical formation by arid climate evaporation of saline lakes or sea water, in restricted or cut-off bays. For an interesting account, read chapter 1 of Kenneth Hsu, The Mediterranean was a Desert.
Uses:Plaster of Paris, fertilizer, Portland cement, wallboard, and other building materials
Comments:May be scratched by fingernail. When the water of hydration is absent, calcium sulfate occurs as the harder, heavier mineral Anhydrite. [gypsum.jpg]