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. |
Origin: Newton, Alberta, CanadaSample size: about 2 cm
Photo and Owner: Milton Speckels
Origin: Las Salinas halite mine, near Otume Village, south of Pisco, Ica Department, PeruSample size: 6 x 3 x 4.5 cm
Photo: Dan Weinrich
Variety satin sparOrigin: Watchet Cliffs, Somerset, England
Sample size: 8.5 x 3.5 x 3 cm
Photo: Tom Loomis
(zoom-in of above)