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

Chemical structure

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

CaSO4·2H2O + heat → CaSO4·½H2O + 1½H2O (steam)

The partially dehydrated mineral is called calcium sulfate hemihydrate or calcined gypsum (commonly known as plaster of Paris) (CaSO4·½H2O).

The dehydration (specifically known as calcination) begins at approximately 80°C (176°F) and the heat energy delivered to the gypsum at this time (the heat of hydration) tends to go into driving off water (as water vapor) rather than increasing the temperature of the mineral, which rises slowly until the water is gone, then increases more rapidly.

The endothermic property of this reaction is exploited by drywall to confer fire resistance on residential and other structures. In a fire the structure behind a sheet of drywall will remain relatively cool as water is lost from the gypsum, thus preventing (or substantially retarding) damage to the framing (through combustion of wood members or loss of strength of steel at high temperatures) and consequent structural collapse.

In contrast to most minerals, which when rehydrated simply form liquid or semi-liquid pastes, or remain powdery, calcined gypsum has an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically “setting” to form a rigid and relatively strong gypsum crystal lattice:

CaSO4·½H2O + 1½H2O → CaSO4·2H2O This reaction is exothermic.

This phenomenon is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). 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).

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


It occurs in nature 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.


– Blackboard chalk
– Cement
– Plaster, a construction material
– Dental modes
– Surgical casts
– Paint filler
– Toothpaste
– Gesso
– Molds for Casting metals
– Agricultural soil additive
– Solidifying earth (cast earth construction)
– Tofu