Dictionary:

gypsum

  (jĭp'səm)

A widespread colorless, white, or yellowish mineral, CaSO₄·2H₂O, used in the manufacture of plaster of Paris, various plaster products, and fertilizers.

[Middle English gipsum, from Latin gypsum, from Greek gupsos, probably of Semitic origin; akin to Arabic jibs, jiṣṣ, jaṣṣ, from Akkadian gaṣṣu.]

The most common sulfate mineral, characterized by the chemical formula CaSO₄ · 2H₂O; it shows little variation from this composition.

Gypsum is one of the several evaporite minerals. This mineral group includes chlorides, carbonates, borates, nitrates, and sulfates. These minerals precipitate in seas, lakes, caves, and salt flats due to concentration of ions by evaporation. When heated or subjected to solutions with very large salinities, gypsum converts to bassanite (CaSO₄·H₂O) or anhydrite (CaSO₄). Under equilibrium conditions, this conversion to anhydrite is direct. The conversion occurs above 108°F (42°C) in pure water. The presence of halite (NaCl) or other sulfates in the solution lowers this temperature, although metastable gypsum exists at higher temperatures. See also Anhydrite; Halite.

Crystals of gypsum are commonly tabular, diamond-shaped, or lenticular; swallow-tailed twinsare also common. The mineral is monoclinic with symmetry 2/m. The common colors displayed are white, gray, brown, yellow, and clear. Cleavage surfaces show a pearly to vitreous luster. Gypsum is the index mineral chosen for hardness 2 on Mohs scale with a specific gravity of 2.32. In addition to free crystals, the common forms of gypsum are satin spar (fibrous), alabaster (finely crystalline), and selenite (massive crystalline).

Gypsum is used for a variety of purposes, but chiefly in the manufacture of plaster of paris, in the production of wallboard, in agriculture to loosen clay-rich soils, and in the manufacture of fertilizer. Plaster of paris is made by heating gypsum to 392°F (200°C) in air. A hemihydrate is formed as part of the water of crystallization is driven off. Later, when water is added, rehydration occurs. The interlocking, finely crystalline texture that results forms a uniform hardened mass. The slightly increased volume of the set plaster serves to fill the mold into which it has been poured. See also Plaster of paris.

Gypsum deposits are mined throughout the world, with the United States being a world leader in gypsum production. The majority of United States gypsum is mined in Michigan, Iowa, Texas, California, and Oklahoma. Canada is the world's second largest producer. Most Canadian production is in the province of Nova Scotia. Among the other leading producers are France, Japan, Iran, Russia, Italy, Spain, and the United Kingdom.

The dihydrate of calcium sulfate (CaSO₄-2H₂O). α Hemihydrate and β hemihydrate are derived from gypsum. See also plaster of paris.

For more information on gypsum, visit Britannica.com.

A soft mineral consisting of a hydrated calcium sulfate from which gypsum plaster is made (by heating); colorless when pure; used as a retarder in portland cement.

Environment

Sedimentary rocks as massive beds, in free crystals in clay beds, alkaline lake muds, and crystallized in cavities in limestone. Often in opaque, sand-filled crystal clusters. Crystals constantly grow and clog various contrivances of human making: water pipes, mines, and dumps.

Crystals are common, often assuming a tabular habit: model-like, backward-slanting, monoclinic plates, with the horizontal axis the shortest. "Fishtail" twins are frequent. The most common crystals--which may be large ill-formed sheets--are found loose and free-growing in clay beds and cover outcrops in mica-like sheets; glassy gypsum known as selenite. Stony bands of massive Italian gypsum, known as alabaster, are carved and dyed in Florence; fibrous warm-hued chatoyant veins known as satin spar are Russian sculptors' grist.

Colorless, white, and pale tints. Luster glassy, pearly (on cleavage face), and silky (avoid washing very lustrous surfaces, for even water has been found to dull them); hardness 2; specific gravity 2.3; fracture conchoidal and splintery; cleavage 2, perfect and micaceous. Sectile; often fluoresces yellow in an hourglass pattern within crystal; also phosphorescent.

Hydrous calcium sulfate (32.6% CaO, 46.5% SO ₃ , 20.9% H ₂ O).

Soluble in hot dilute hydrochloric acid; the addition of barium chloride solution makes a white precipitate. After firing, fluorescent and phosphorescent in longwave ultraviolet light.

With its low hardness, and flakes that are easily scratched by a fingernail, no other test is needed. The clear plates bend but lack the elastic rebound of mica; they are softer than uncommon brucite. Massive alabaster is softer than anhydrite or marble, and gypsum will not bubble in acid like the latter.

A widespread, commercially important mineral. The massive material is quarried, or mined, for the manufacture of plaster of Paris and various plaster products. The most abundant deposits are the sedimentary beds, some of which have formed from the alteration of the water-free variety, anhydrite. It is such beds that are mined for economic applications in New York, Michigan, Texas, Iowa, and California. Nova Scotia has great beds of altered anhydrite that show interesting crumpling of the layers as they swelled with the addition of water--metamorphic structures on a very small scale.

Good crystals are found in clay beds of Ohio and Maryland, and interesting cave rosettes of spreading fibers (gypsum flowers) come from Kentucky. The most beautiful gypsum (selenite) crystals are foreign in origin, though the largest probably came from a cave in Utah. The large water-clear crystals from the Sicilian sulfur mines, often with inclusions of sulfur, are classics of all collections. In Naica, Chihuahua, Mexico, a cavern in the mine (Cave of the Swords) contains meter-long, slender, slightly milky needles with tubular water-filled cavities and movable bubbles.

The name plaster of Paris comes from its early source in the Montmartre quarries of Paris. The name gypsum comes from the Greek word for the calcined (or "burned") material. Selenite comes from a Greek comparison of the pearly luster of a cleavage plate to moonlight. Decomposed gypsum is considered a source of sulfur in the Sicilian mines.

Native calcium sulfate, which, when calcined, becomes plaster of Paris; used in making plaster casts for fractures.

A mineral, calcium sulfate, that is used to add calcium to the soil or to improve the structure of clay soils without affecting the pH. See also flocculate.

It has been suggested that Selenite be merged into this article or section. (Discuss)

Gypsum
Desert rose, 10 cm long
General
Category	Mineral
Chemical formula	CaSO4·2H2O
Identification
Color	White to grey, pinkish-red
Crystal habit	Massive, flat. Elongated and generally prismatic crystals
Crystal system	Monoclinic 2/m
Twinning	common {110}
Cleavage	2 good (66° and 114°)
Fracture	Conchoidal, sometimes fibrous
Mohs Scale hardness	1.5-2
Luster	Vitreous to silky or pearly
Refractive index	α=1.520, β=1.523, γ=1.530
Optical Properties	2V = 58° +
Pleochroism	None
Streak	White
Specific gravity	2.31 - 2.33
Fusibility	3
Solubility	hot, dilute HCl
Diaphaneity	transparent to translucent
Major varieties
Satin Spar	Pearly, fibrous masses
Selenite	Transparent and bladed crystals
Alabaster	Fine-grained, slightly colored

Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO₄·2H₂O.

Crystal varieties

Gypsum from New South Wales, Australia

Gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite. It may also occur silky and fibrous, in which case it is commonly called satin spar. Finally it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or lightly-tinted variety of gypsum is called alabaster, which is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form typically opaque with embedded sand grains called desert rose. The most visually striking variety, however, is the giant crystals from Naica Mine. Up to the size of 11m long, these megacrystals are among the largest crystals found in nature. A recent publication shows that these crystals are grown under very constant temperature such that large crystals can grow slowly but steadily without excessive nucleation.^[1]

Occurrence

Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. The largest deposits known occur in strata from the Permian age. Gypsum is deposited in lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near surface exposures. It is often associated with the minerals halite and sulfur.

Fibrous Gypsum from Brazil

The word gypsum is derived from the aorist form of the Greek verb μαγειρεύω, "to cook", referring to the burnt or calcined mineral. Because the gypsum from the quarries of the Montmartre district of Paris has long furnished burnt gypsum used for various purposes, this material has been called plaster of Paris. It is also used in foot creams, shampoos and many other hair products.

Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions of the White Sands National Monument in the US state of New Mexico have created a 710 km² (275 sq mile) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.^[2] Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in 1933 when president Herbert Hoover declared the gypsum dunes a protected national monument.

Commercial quantities of gypsum are found in Thailand, Germany, Italy, England, Ireland, in British Columbia, Manitoba, Ontario, Nova Scotia and Newfoundland in Canada,^[3] and in New York, Michigan, Iowa, Kansas, Oklahoma, Arizona, New Mexico, Colorado, Utah and Nevada in the United States. There is also a large mine located at Plaster City, California in Imperial County. There are commercial quantities in East Kutai, Kalimantan.

Vast crystals of gypsum, up to 10metres in length have been found in the "Cueva de los Crystales" in Naica, Chihuahua, Mexico.^[4]

Uses of Gypsum

1. Drywall

2. Plaster ingredient.

3. Fertilizer and soil conditioner. In the late eighteenth and early nineteenth century, Nova Scotia gypsum, often referred to as plaister, was a highly sought fertilizer for wheat fields in the United States.

4. Plaster of Paris (surgical splints; casting moulds; modeling).

5. A tofu (soy bean curd) coagulant, making it ultimately a major source of dietary calcium, especially in Asian cultures which traditionally use very few dairy products.

6. Adding hardness to water used for brewing.

7. An ingredient in the popular snack food Twinkie

8. Blackboard chalk.

9. A component of Portland cement used to prevent flash setting of concrete.

References

1. ^ Juan Manuel García-Ruiz, Roberto Villasuso, Carlos Ayora, Angels Canals, and Fermín Otálora (2007). "Formation of natural gypsum megacrystals in Naica, Mexico". Geology 35 (4): 327-330. DOI:10.1130/G23393A.1. 
2. ^ Abarr, James. "Sea of Sand", The Albuquerque Journal, 1999-02-07. Retrieved on 2007-01-27. 
3. ^ Mines, Mills and Concentrators in Canada. Natural Resources Canada (2005-10-24). Retrieved on 2007-01-27.
4. ^ http://naica.laventa.it/naica-crystal-cave.en.html

• Klein, Cornelis and Cornelius S. Hurlbut, Jr., 1985, Manual of Mineralogy, John Wiley, 20th ed., pp. 352-353, ISBN 0-471-80580-7
• WebMineral data
• Mineral Data from Mindat
• Mineral Galleries- Gypsumzh-yue:石膏

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - gips, gipssten

Nederlands (Dutch)
gips

Français (French)
n. - (Minér, Géol) gypse

Deutsch (German)
n. - Gips

Ελληνική (Greek)
n. - (ορυκτολ.) γύψος, θειικό ασβέστιο

Italiano (Italian)
ingessatura, pietra da gesso

Português (Portuguese)
n. - gesso (m) (Quím.)

Русский (Russian)
гипс

Español (Spanish)
n. - yeso

Svenska (Swedish)
n. - gips

中文（简体） (Chinese (Simplified))
石膏

中文（繁體） (Chinese (Traditional))
n. - 石膏

한국어 (Korean)
n. - 기브스, 석고

日本語 (Japanese)
n. - 石膏, ギプス
v. - 石膏で処理する

العربيه (Arabic)
‏(الاسم) جص, جبص‏

עברית (Hebrew)
n. - ‮גבס‬

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