Gypsum
Gypsum is a very soft
mineral composed of
calcium sulfate dihydrate, with the
chemical formula CaSO
4·2H
2O.
Heating gypsum to between 100°C and 150°C (302°F) partially
dehydrates the mineral by driving off exactly 75% of the water contained in its chemical structure. The temperature and time needed depend on ambient partial pressure of H
2O. Temperatures as high as 170°C are used in industrial calcination, but at these temperatures the anhydrite begins to be formed.The reaction for the partial dehydration is:
CaSO4·2H
2O + heat '
CaSO4·½H
2O + 1½H
2O (steam)
The partially dehydrated mineral is called
calcium sulfate hemihydrate or
calcined gypsum (commonly known as
plaster of Paris) (
CaSO4·½H
2O).
The dehydration (specifically known as
calcination) begins at approximately 80°C (176°F), although in dry air, some dehydration will take place already at 50°C. 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:
CaSO4·½H
2O + 1½H
2O '
CaSO4·2H
2O 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). Mixed with polymers, it has been used as a bone repair cement. 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 conditions of dehydration can be changed to adjust the porosity of the hemihydrate, resulting in the so-called alpha and beta hemihydrates (which are more or less chemically identical).
The completely water-free 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, a property exploited in the commercial
dessicant Drierite®.
|
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.
|
Fibrous Gypsum from Brazil |
Gypsum is a very 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.
Commercial quantities of gypsum are found in
Germany,
Italy,
England,
Ireland, in
British Columbia,
Manitoba,
Ontario,
Nova Scotia and
Newfoundland in
Canada[http://mmsd1.mms.nrcan.gc.ca/mmsd/producers/commodityCompany_e.asp?nId=51&mineType=nonMetal - Gypsum Mines in Canada], and in
New York,
Michigan,
Iowa,
Kansas,
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.
A growing source of gypsum is from
flue gas desulfurization 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.
Gypsum is also a by-product of the phosphate fertilizer refining process. Simply stated, phosphate ore is mixed with sulfuric acid to produce phosphoric acid. The phosphroic acid is then ammoniated to form monammonium phosphate (MAP) and diammonium phosphate (DAP). The resulting waste product is a slurry of calcium sulfate (gypsum), that eventually dries and hardens. In fact, the phosphate industry creates several times more gypsum as a by-product than is produced or required by the commercial gypsum industry. In the case of the phosphate industry in West Central Florida, this gypsum by-product is prohibited from commercial use because of low levels of naturally occurring radioactive material (NORM) existing in the phosphate ore that becomes technically enhanced and deposited into the gypsum. Because of the sheer volume of gypsum produced by this process, there is also a considerable lobbying effort by the gypsum industry not to allow the phosphate byproduct on the market. The result is huge piles of gypsum waste that are visible along the highways in the vicinity of the Florida phosphate refineries.
*Blackboard
chalk*
Cement*
Drywall*
Plaster, a construction material
*
Dental modes
*
Surgical casts
*
Paint filler*
Toothpaste*
Gesso*Molds for
Casting metals
*Agricultural
soil amendment*Solidifying earth (
cast earth construction)
*
Tofu coagulation
*Improving mineral content of
brewing water
*Dietary
calcium additives in
breads and
cereals*
Pharmaceuticals*
Dessicant - anhydrous
calcium sulfate (
anhydrite) is sold under the brand name
Drierite®
*
BurtonisationSee also:
List of minerals*
lime (mineral) - related minerals
*
United States Gypsum Corporation - largest producer of gypsum products in North America
*
WebMineral data*
Mineral Data
