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, 10 cm long.
Desert rose, 10 cm long.
Category Mineral
Chemical formula CaSO4·2H2O
Colour White to grey, pinkish-red
Crystal habit Massive, flat. Elongated and generally prismatic crystals.
Crystal system Monoclinic
Cleavage 2 good (66 and 114)
Fracture Conchoidal, sometimes fibrous
Mohs Scale hardness 1.5-2
Luster Vitreous to silky or pearly lustre
Refractive index 1.522
Pleochroism None
Streak White
Specific gravity 2.31 - 2.33
Fusibility  ?
Solubility No reaction to acid.
Major varieties
Satin Spa Pearly, fibrous masses
Selenite Transparent and bladed crystals
Alabaster Fine-grained, slightly coloured

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


Chemical structure

Heating gypsum above approximately 150C (302F) partially dehydrates the mineral, by driving off exactly 75% of the water contained in its chemical structure.

CaSO42H2O + heat → CaSO4½H2O + 1½H2O (steam)

The dehydration (specifically known as calcination) begins at approximately 80C (176F) and the heat energy delivered to the gypsum at this time (the heat of hydration) goes into driving off water (turning it into water vapor), not into increasing the temperature of the mineral. As water is lost, the temperature of the gypsum slowly increases until all the water has been removed, then begins rising normally at a quicker rate. The ability of hydrated gypsum to remain at relatively low temperatures, even if a flame is applied directly to it for a short period of time, is exploited by drywall to confer fire resistance to the wooden frames of houses and other buildings. Even if a fire is impinging directly on a sheet of drywall, the wood frame behind it will remain at a relatively low temperature (until too much water has been lost from the gypsum), preventing the destruction of the wood and the collapse of the structure.

The partially dehydrated mineral is called calcium sulfate hemihydrate or calcined gypsum (though more commonly known as plaster of Paris) and has the chemical formula CaSO4½H2O. Calcined gypsum has an unusual property: when mixed with water at normal (ambient) temperatures, it recombines with the water that was driven off during calcination, and sets to form a strong gypsum crystal lattice:

CaSO4½H2O + 1½H2O → CaSO42H2O This reaction is exothermic.

The anhydrous form, called anhydrous calcium sulfate (sometimes anhydrite), is produced by further heating to above approximately 180C (356F) and has the chemical formula CaSO4. Anhydrite reacts slowly with water to return to the dihydrated state.

Most minerals, when rehydrated, simply form liquid or semi-liquid pastes, or remain powdery. Gypsum, on the other hand, forms a strong crystal structure immediately upon receiving the water, and this phenomenon is responsible for gypsum's ease of being cast into sheets (for drywall), sticks (for blackboard chalk), molds (to set broken bones, or create molds for metal casting), and other forms. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth , an alternative to adobe (which loses its strength when wet).


It occurs as flattened and often twinned crystals in 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 often called desert rose.

Gypsum is a very common mineral, with thick and extensive beds 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. 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.

Commercial quantities of gypsum are found in England, Canada, and in New York, Michigan, Iowa, Kansas, New Mexico, Colorado, Utah in the United States.

A growing source of gypsum is from flue gas desulfurisation which scrubs the sulfur emissions from fossil fuel burning power stations. This is done by using finely ground limestone which reacts with the sulfur dioxide to produce high purity gypsum as a by-product.


See also: List of minerals

External link

Place names

Gypsum is also the name of several towns in the United States of America:

Last updated: 06-02-2005 13:37:42
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