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Gypsum


An important construction material that has been used for thousands of years


Gypsum

Gypsum: Satin spar, a fibrous variety of gypsum from Derbyshire, England. Specimen is approximately 4 inches (10 centimeters) across.

What is Gypsum?

Gypsum is an evaporite mineral most commonly found in layered sedimentary deposits in association with halite, anhydrite, sulfur, calcite, and dolomite. Gypsum (CaSO4.2H2O) is very similar to Anhydrite (CaSO4). The chemical difference is that gypsum contains two waters and anhydrite is without water. Gypsum is the most common sulfate mineral.



gypsum wallboard and plaster

Gypsum wallboard and plaster: Wallboard and construction plaster are the primary industrial uses of gypsum in the United States. Photo © iStockphoto / George Peters.

Uses of Gypsum

Gypsum uses include: manufacture of wallboard, cement, plaster of Paris, soil conditioning, a hardening retarder in portland cement. Varieties of gypsum known as "satin spar" and "alabaster" are used for a variety of ornamental purposes; however, their low hardness limits their durability.



Physical Properties of Gypsum

Chemical Classification Sulfate
Color Clear, colorless, white, gray, yellow, red, brown
Streak White
Luster Vitreous, silky, sugary
Diaphaneity Transparent to translucent
Cleavage Perfect
Mohs Hardness 2
Specific Gravity 2.3
Diagnostic Properties Cleavage, specific gravity, low hardness
Chemical Composition Hydrous calcium sulfate, CaSO4.2H2O
Crystal System Monoclinic
Uses Used to manufacture dry wall, plaster, joint compound. An agricultural soil treatment.

Gypsum from Michigan

Gypsum from Michigan: Gypsum from Grand Rapids, Michigan. Specimen is approximately 4 inches (10 centimeters) across.

Alabaster Gypsum

Alabaster Gypsum: Alabaster, a variety of gypsum, from Pomaia, Italy. Specimen is approximately 3 inches (7.6 centimeters) across.

Alabaster gypsum jar

Alabaster gypsum jar: Jar made of beautiful translucent alabaster gypsum by David MacFarlane, photo © iStockphoto / David MacFarlane.

Gypsum translucency

Gypsum translucency: The translucent characteristic of alabaster, a variety of gypsum, from Pomaia, Italy. Specimen is approximately 3 inches (7.6 centimeters) across.

Mineral collection

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Selenite Gypsum

Selenite Gypsum: Selenite, a variety of gypsum from Penfield, New York. Specimen is approximately 2-1/2 inches (6.4 centimeters) across.

Gypsum from Virginia

Gypsum from Virginia: Gypsum from North Holston, Virginia. Specimen is approximately 1-1/2 inches (3.8 centimeters) across.

Satin spar Gypsum

Satin spar Gypsum: Satin spar, a fibrous variety of gypsum from Derbyshire, England. Specimen is approximately 3 inches (7.6 centimeters) across.

Gypsum from New York

Gypsum from New York: Selenite, a variety of gypsum from Penfield, New York. Specimen is approximately 2-1/2 inches (6.4 centimeters) across.

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