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.
Physical Properties of Gypsum |
| Color |
clear, colorless, white, gray, yellow, red, brown |
| Streak |
white |
| Luster |
vitreous |
| Diaphaneity |
transparent to translucent |
| Cleavage |
perfect |
| Mohs Hardness |
2 |
| Specific Gravity |
2.3 |
| Distinguishing Characteristics |
cleavage, specific gravity, low hardness |
| Chemical Composition |
hydrous calcium sulfate, CaSO4.2H2O |
| Crystal System |
monoclinic |
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.
| Gypsum from Grand Rapids, Michigan. Specimen is approximately 4 inches (10 centimeters) across. |
| Gypsum from North Holston, Virginia. Specimen is approximately 1-1/2 inches (3.8 centimeters) across. |
| Satin spar, a fibrous variety of gypsum from Derbyshire, England. Specimen is approximately 3 inches (7.6 centimeters) across. |
| Alabaster, a variety of gypsum, from Pomaia, Italy. Specimen is approximately 3 inches (7.6 centimeters) across. |
| The translucent characteristic of alabaster, a variety of gypsum, from Pomaia, Italy. Specimen is approximately 3 inches (7.6 centimeters) across. |
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| Satin spar, a fibrous variety of gypsum from Derbyshire, England. Specimen is approximately 4 inches (10 centimeters) across. |
| Wallboard and construction plaster are the primary industrial uses of gypsum in the United States.Photo © George Peters and iStockphoto. |
| Jar made of beautiful translucent alabaster gypsum by David MacFarlane, photo © David MacFarlane and iStockphoto. |
| Selenite, a variety of gypsum from Penfield, New York. Specimen is approximately 2-1/2 inches (6.4 centimeters) across. |
| Selenite, a variety of gypsum from Penfield, New York. Specimen is approximately 2-1/2 inches (6.4 centimeters) across. |
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