CaSO4 – 2(H2O), Hydrated calcium Sulfate
Plaster, wall board, some cements, fertilizer, paint filler, ornamental stone, etc..
Gypsum is one of the more common minerals in sedimentary environments. It is a major rock forming mineral that produces massive beds, usually from precipitation out of highly saline waters. Since it forms easily from saline water, gypsum can have many inclusions of other minerals and even trapped bubbles of air and water.
Gypsum has several variety names that are widely used in the mineral trade.
“Selenite” is the colourless and transparent variety that shows a pearl like luster and has been described as having a moon like glow. The word selenite comes from the greek for Moon and means moon rock.
Another variety is a compact fiberous aggregate called “satin spar” . This variety has a very satin like look that gives a play of light up and down the fiberous crystals.
A fine grained massive material is called “alabaster” and is an ornamental stone used in fine carvings for centuries, even eons.
Colour : usually white, colourless or gray, but can also be shades of red, brown and yellow
Luster : vitreous to pearly especially on cleavage surfaces
Transparency : crystals are transparent to translucent
Crystal System : monoclinic; 2/m
Crystal Habits : include the tabular, bladed or blocky crystals with a slanted parallelogram outline. The pinacoid faces dominate with jutting prism faces on the edges of the tabular crystals. Long thin crystals show bends and some specimens bend into spirals called “Ram’s Horn Selenite” Two types of twinning are common and one produces a “spear head twin” or “swallowtail twin” while the other type produces a “fishtail twin”. Also massive, crusty, granular, earthy and fiberous.
Cleavage : good in one direction and distinct in two others
Fracture : uneven but rarely seen
Hardness : 2 and can be scratched by a fingernail
Specific Gravity : approx. 2.3+ (light)
Streak : white
Other : thin crystals are flexible but not elastic, meaning they can be bent but will not bend back on their own. Also some samples are fluorescent. Gypsum has a very low thermal conductivity (hence it’s use in drywall as an insulating filler). A crystal of Gypsum will feel noticeably warmer than a like crystal of quartz.
Associated Minerals : halite, calcite, sulfur, pyrite, borax and many others
Major Occurrences : include Naica, Mexico; Sicily; Utah and Colorado, USA; and many other locallities throughout the world
Best Indicators : crystal habit, flexible crystals, cleavage and hardness