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Chemical Formula | CaSO4 · 2H2O | ||||||||||||||
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Composition | Hydrous calcium sulfate | ||||||||||||||
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Color | Colorless, white, gray, brown, beige, orange, pink, yellow, light red, green | ||||||||||||||
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Streak | White | ||||||||||||||
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Hardness | 1½ - 2 | ||||||||||||||
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Crystal Forms and Aggregates |
(Monoclinic) Commonly occurs as tabular crystals, often with no imperfections. Also
occurs prismatic, acicular, bladed, and as dense bundles of fragile acicular crystals. Other forms are fibrous, scaly,
grainy, lenticular, rosette, massive, earthy, and as parallel, cactus-like growths.
Crystals may also be curved, some severely. Crystals frequently twin, forming perfect fishtail or swallowtail twins. |
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Transparency | Transparent to opaque | ||||||||||||||
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Specific Gravity | 2.3 - 2.4 | ||||||||||||||
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Luster | Vitreous to pearly | ||||||||||||||
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Cleavage | 1,1 - micaceous ; 2,2 | ||||||||||||||
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Fracture | Uneven | ||||||||||||||
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Tenacity | Sectile and slightly flexible | ||||||||||||||
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Other ID Marks | May fluoresce light yellow in shortwave ultraviolet light, and may also phosphoresce. | ||||||||||||||
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Other Names |  Gypsite |
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Varieties |  Selenite - Transparent and
colorless (or very lightly colored) Gypsum Satin
Spar - Fibrous variety of Gypsum Alabaster
- Massive, fine-grained variety of Gypsum Desert
Rose - Rosette shaped Gypsum with brown sand inclusions Gypsum
Flower - Rosette shaped Gypsum found in caverns with
spreading fibers Gypsum
Rock - Rock
composed mostly of the mineral Gypsum, but also contain
impurities such as Calcite, Anhydrite, Rock Salt, Dolomite, Limonite, and clay |
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In Group | Sulfates ; Hydrous sulfates | ||||||||||||||
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All About | Gypsum has many interesting properties; the
most notable is its crystal habits. Many Gypsum crystals
are found perfectly intact without distortions or parts
broken off. Such crystals are found in a clay beds as floating crystals, where they fully form without
being attached to a matrix. Gypsum crystals are known for their flexibility, and slim crystals can be slightly bent. It is not recommended to bend good crystals, since they are only slightly flexible, and if flexed just a little too much will break. Gypsum has the same composition as Anhydrite, but contains water in its structure whereas Anhydrite does not. Many Anhydrite specimens absorb water, transforming into the much-more-common Gypsum. Some Gypsum specimens show evidence of this, containing growths of crumpling layers that testify that they expanded from the addition of water. In a small number of Gypsum specimens, water gets trapped inside a crystal in a hollow channel while the crystal forms. When such a crystal is rotated, a water "bubble" moves around inside it toward the lowest point in the channel. Such specimens are considered a mineralogical oddity, and are very much sought by collectors. Gypsum sometimes forms in sandy areas, and crystals may trap sand inside when forming, causing a specimen to become brown and opaque. These sand inclusions sometimes form hourglass formations in a crystal. They are also present in the well-known "Desert Rose", which is rosette shaped Gypsum with sand inclusions. The term "Desert Rose" also applies to rosette shaped Barite with sand inclusions, and the two should not be confused. The variety Selenite usually refers to the transparent, colorless variety of Gypsum, but some people use it as a synonym for Gypsum. Gypsum specimens should only be cleaned with water. Soaps and detergents should be avoided, for they enter cracks and crevices of a crystal, destroying its true luster. |
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Uses | Gypsum is an industrially important mineral.
It is the primary ingredient of plaster-of-Paris (finely
ground Gypsum) and is also used in the production of
cement. It is also the main component of sheet-rock. It
is used as a flux
for creating earthenware, and can be used as a
fertilizer. The variety Alabaster is is carved
for ornamental use, such as artistic sculptures and
pottery. It is porous and is therefore easily dyed. The fibrous Satin Spar variety is sometimes cut into cabochons for collectors because of its strong cat's eye effect. Fine Gypsum specimens are very popular among mineral collectors, especially the varieties Selenite and Desert Rose. |
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Striking Features | Crystal habits, low hardness, flexibility, and sectility | ||||||||||||||
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Popularity (1-4) | 2 | ||||||||||||||
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Prevalence (1-3) | 1 | ||||||||||||||
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Demand (1-3) | 1 | ||||||||||||||
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Distinguishing Similar Minerals |
Mica - much better cleavage, greater flexibility, and elastic Brucite - harder Calcite - harder (3), not flexible and not sectile. Barite Desert Rose can be distinguished from Gypsum Desert Rose, since it is harder (3 - 3½) and much heavier (4.3 - 4.6) |
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Commonly Occurs With |
Halite, Dolomite, Barite, Anhydrite, Sulfur | ||||||||||||||
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Noteworthy Localities |
Gypsum is a very common mineral, and occurs
in numerous localities. Only the finest are mentioned
here: Beautiful crystals occur in Tyrol, Austria, and huge examples come from Bologna and Pavia, Italy. The Sicilian Sulfur mines in Italy have produced crystal clear specimens with bright-yellow Sulfur inclusions. Gypsum "Desert Roses" occur in a number of places in the Sahara Desert in Africa. Fragile, acicular bunches occur in Whyalla, South Australia, and green, grass-like "mats" in Pernatty Lagoon, South Australia. Many fine localities also exist in North America. In Niaca, Chihuahua, Mexico, a cavern in the Maravilla Mine (dubbed "cave of the swords" because of the gigantic crystals found there) has produced magnificent, enormous, perfect crystals. In Nova Scotia, Canada, a large Gypsum deposit formed by altered Anhydrite is noted for individual crystal masses where expansion from the hydration is apparent. Gypsum "Desert Roses" occur in abundance in Alfalfa Co., Oklahoma. Stand-alone, perfect "floating" crystals are found in clay at Ellsworth, Mahoning Co., Ohio. Huge crystals were obtained from a cave in South Wash, Wayne Co., Utah. Severely curved Gypsum crystals and "Gypsum Flowers" come from the famous Mammoth Caves of Kentucky, and lustrous crystals have come from Lockport, Niagara Co., New York. Many states contain large deposits that stretch over large regions. Such deposits exist in New York, Ohio, Kentucky, Maryland, Tennessee, Iowa, Michigan, Texas, New Mexico, Arizona, and California. |
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Picture Icon Links |
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Picture Links | 1. Selenite
crystal 2. Brown crystal cluster 3. Gypsum "Desert Rose" 4. Group of fragile acicular crystals 5. Platy rosette crystals 6. Long, slender, bent crystal 7. Satin Spar |
