Description: Is the most common sulphate mineral. Gypsum is found in enormous deposits and thick and extensive evaporite beds in association with sedimentary rocks, specifically limestone and in the caprock of salt domes; also, in clay sedimentary deposits and in dry caves. Occasionally in igneous traprock and in the oxidation zone of sulfide deposits. Occurs in wide variety of forms, as both massive material, including the alabaster variety; and clear crystals, the selenite variety; and, parallel fibrous, the satin spar variety. Rarely found as sand; one exception is at White Sands National Park in New Mexico.
Gypsum is an important industrial mineral, and it touches and impacts your daily life. It is used as an ingredient and to manufacture wallboard (or "dry wall"), plaster of Paris, soil conditioners, and as a hardening retarder in Portland cement.
Chemical Formula: CaSO4 · 2H2O (hydrous calcium sulfate)
Crystal Form: Massive, flat. Elongated and generally prismatic crystals
Crystal System: Monoclinic
Color: Typically colourless to white, transparent crystals, thick tabular to lenticular, sometimes prismatic.
Streak: White
Luster: Vitreous to pearly
Fracture: Uneven
Cleavage: Perfect in one direction, only fair in two others to produce thin rhombic
Hardness: 2 on Moh’s scale (very soft)
Density/Specific Gravity: ranges from 2.31-2.33 g/cm3
Magnetism: N/A
Taste: N/A
Hydrochloric acid: N/A
Radioactivity: N/A
Fluorescence: Most common colors of fluorescence are baby-blue and shades of golden yellow to yellow. Selenite crystals often exhibit zoned "hourglass" fluorescence in zones that may, or may not, be evident in ordinary light.
Distinguishing Physical Properties:
Hardness: 2, it can be scratched with a fingernail
Cleavage
Solubility in water
Photograph Attribution: Gypsum specimen photograph by Sean C. Murphy, 2020.