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Gypsum

by Rick Moore last modified Apr 24, 2014 08:53 PM
A short overview of the calcium sulfate minerals Gypsum and Anhydrite.

Calcium Sulfate Minerals


Gypsum is the more common, hydrated form of calcium sulfate. In fact, it is the most common of all sulfate minerals. Gypsum's chemical formula is CaSO4.2H2O. The anhydrous (without water) mineral is called Anhydrite (CaSO4).

Gypsum and Anhydrite both occur as primary minerals in Marine Evaporites.Evaporite deposits occur when sea water evaporates under extreme conditions over long periods of time. Gypsum may also be produced by the action of sulfuric acid solutions on carbonate rocks.In this case it occurs in pockets or veins in the host rock. Gypsum occurrences in the Lockport Dolomite were formed in this way. Anhydrite may also forms as the result of the dewatering of Gypsum.

Selenite is a variety of Gypsum that forms perfectly clear crystals. It occurs locally in many places including Erie Blvd, Jamesville and Fayetteville. Satin Spar is a variety that consists of masses of long needle-like crystals which sometimes form spectacular curved specimens called Ram's Horn. Alabaster is a massive, very fine grained variety of Gypsum which often has wonderful color patterns. It is popular with stone carvers and can be easily stained to accentuate patterns in the rock.

Ram's Horn Gypsum

Ram's Horn

Alabaster Cup

Alabaster

The Ram's Horn photo is from the Gem & Mineral Collection of the University of Texas' Non-vertebrate Paleontology Laboratory. The Alabaster cup photo is from the Crystals, rocks and Gems Healing Stones web site. Their web site has many more examples of Alabaster carvings.

The Hyperphysics Group in the Dept. of Physics and Astronomy at Georgia State University has a couple of pages with excellent photos of all of the many varieties of Gypsum.

Uses for Gypsum


The main uses of Gypsum are in the construction industry. It is the primary ingredient in Drywall (also called sheetrock or wallbaord) and is added to cement to keep it from drying too quickly. Plaster of Paris is produced by heating Gypsum to about 300°F.

Gypsum has a multitude of uses in agriculture, especially in improving the quality of soil.

Gypsum is the primary coagulating agent used to make Tofu,

Gypsum was used as mortar for the Egyptian pyramid of Cheops in 3000 BC.

Gypsum CaSO4.2H2O

Selenite

Selenite crystals

Satin Spar

Satin Spar

The Selenite crystal photo is from the Bob Campbell Geology Museum at Clemson University. The Satin Spar photo is from the Green Cross Toad metaphysical crystal web site.

Gypsum is monoclinic, usually forming tabular prisms. The crystal structure is oriented such that CaSo4 molecules alternate with H2O in only one dimension. This results in the sheet-like nature of Selenite and the fibrous texture of Satin Spar.

tGypsum Crystal Lattice

This image is from the Warwick Electrochemistry and Interfaces Group of the Chemistry Department at the University of Warwick

Check out Mindat.org's interactive 3D model of this crystal form. You might also want to check out Webmineral's interactive 3D crystal lattice for Gypsum.

Interesting Facts

White Sands

Gypsum is rarely found as sand, yet the largest deposit of gypsum in the world is White Sands National Monument in New Mexico where pure white Gypsum sand dunes cover an area of over 275 square miles and can be seen from space.

Giant Crystal Cave

The largest mineral crystals in the world are composed of varieties of Gypsum and occur in the Cueva de los Cristales (aka Giant Crystal Cave) in the Naica Mine, Chihuahua, Mexico. To see these behemoths, watch the National Geographic video on YouTube.

In 2004, the European Space Agency's Mars Express Orbiter used it's OMEGA IR Mineralogical Mapping Spectrometer to confirm the presence of Gypsum sand dunes in the North Polar region. In 2001, Mars Rover "Opportunity" discovered veins of Gypsum exposed on the surface of Mars.

Mineral_Name Photo Galleries Mineral_Name Mineral Data
Mindat.org Gypsum photo gallery Mindat.org Gypsum data
Mineral Atlas Gypsum photo gallery Webmineral Gypsum data
Arkenstone Gypsum photo gallery Minerals.net Gypsum data
John Betts Gypsum photo gallery
Geology.com Gypsum data
Google search for Gypsum crystal images
Google search for Gypsum rock images Interested in Anhydrite ?
Interested in Anhydrite images ? Mindat.org Anhydrite page
Google search for Anydrite inages
Webmineral Anhydrite data

Literature References

  • Deer,W.A., R.A.Howie, J.Zussman, 1966, An Introduction to the Rock Forming Minerals, Longman Group Ltd., London, pp. 462-469
  • Ford,W.E., 1949, Dana's Textbook of Mineralogy, 4thEd., John Wiley & Sons, London, pp, 758-759
  • Hurlbut,C.S., 1971, Dana's Manual of Mineralogy, 18thEd., John Wiley & Sons, London, pp. 347-350