Mineral Information and Data:

Gypsum
CaSO4 • 2H2O [CNMNC approved formula]

Named from the Greek gypos for calcined gypsum used as plaster.

IMA status : Valid species (Pre-IMA Ancient) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
29.06.03.01  
  (29) Hydrated Acid and Sulfates
  (29.06) where A XO4 · x(H2O)
  (29.06.03)
  
Nickel-Strunz
(10th edition) :
07.CD.40
  (07) Sulfates (selenates, tellurates, chromates, molybdates, wolframates)
  (07.C) Sulfates (selenates, etc.) without Additional Anions, with H2O
  (07.CD) With only large cations
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Unit cell: a = 6.29 Ǻ, b = 15.21 Ǻ, c = 5.68 Ǻ, β = 114.1°
Crystal Habit: Crystals thin to thick tabular, diamond-shaped, short to long prismatic; acicular; massive, granular, concretionary
Twinning: Common on {100}, contact; on {-101}, butterfly or heart-shaped; uncommon on {-111}, elongated
  
Color: Colorless, white, gray, yellowish, greenish, reddish, brownish
Diaphaneity: Transparent, Translucent
Luster: Sub-vitreous, pearly on {010} cleavages, silky if fibrous
Hardness (Mohs): 2
Measured Density: 3.32 g/cm3
Cleavage: 3; {010} perfect, {100} and {011} distinct
Tenacity: Flexible, inelastic
Fracture: Splintery parallel [001], conchoidal on {100}
Streak: White
  
Geologic Setting: A common constituent of sedimentary rocks, particularly marine salt deposits, and soils formed directly by evaporation or later by hydration of anhydrite. Formed by the reaction between sulfuric acid and carbonate rock in oxidizing sulfide deposits, and by the action of sulfurous volcanic gases on surrounding Ca-bearing rock. As efflorescences in mines and speleothems in caves.
Mineral Association: Halite, celestine, calcite, aragonite, anhydrite, dolomite, sulfur
  
Synonyms/varieties: Alabaster (massive, fine-grained var.), gips, selenite (colorless, transparent var.), satin spar (columnar, fibrous var.)
Luminescent Properties: Sometimes fluoresces and phosphorescence greenish white.
Other Properties: Gemstone Extremely Rare. BioMineral.

 

Gypsum
Origin: Newton, Alberta, Canada
Sample size: about 2 cm
Photo and Owner: Milton Speckels

 

Gypsum
Origin: Las Salinas halite mine, near Otume Village, south of Pisco, Ica Department, Peru
Sample size: 6 x 3 x 4.5 cm
Photo: Dan Weinrich

 

Gypsum
Variety satin spar
Origin: Watchet Cliffs, Somerset, England
Sample size: 8.5 x 3.5 x 3 cm
Photo: Tom Loomis

 

Gypsum
(zoom-in of above)

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