Mineral Information and Data:

CaSO4 • 2H2O [CNMNC approved formula]

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

IMA status : Valid species (Pre-IMA Ancient) Grandfathered
(8th edition) :  
  (29) Hydrated Acid and Sulfates
  (29.06) where A XO4 · x(H2O)
(10th edition) :
  (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.


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


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


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


(zoom-in of above)