Gypsum
| Gypsum | |
|---|---|
 Desert rose, 10 cm long |
|
| General | |
| Category | Mineral |
| Chemical formula | CaSO4·2H2O |
| Identification | |
| 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 Spar | 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 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 ambiant partial pressure of H2O. 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:
CaSO4·2H2O + 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), 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:
CaSO4·½H2O + 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). 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 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.
Occurrence
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.
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, 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.
Uses
- 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)
- Tofucoagulation
- Improving mineral content of brewing water
- Dietary calcium additives in breads and cereals
- Pharmaceuticals
See also: List of minerals
Place names
Gypsum is also the name of several towns in the United States of America:
- Gypsum, Kansas
- Gypsum, Ohio
- Gypsum, Colorado