Information about Plaster Of Paris

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 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.

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

• 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- Gypsum