Gypsum is a very soft
sulfate mineral composed of
calcium sulfatedihydrate, with the
chemical formula CaSO
4·2H
2O.
[3] It is found in
alabaster, a decorative stone used in
Ancient Egypt. It is the second softest mineral on the
Mohs Hardness Scale. It forms as an
evaporite mineral and as a hydration product of
anhydrite.
Fibrous Gypsum Selenite showing its translucent propertyEtymology and historyThe word gypsum is derived from the
Greek word
γύψοςgypsos, "chalk" or "plaster".
[4] 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.
Gypsum was known in Old English as
spærstān,
spear stone, referring to its crystalline projections. (Thus, the word
spar in mineralogy is by way of comparison to gypsum, referring to any non-ore mineral or crystal that forms in spearlike projections.) In the early nineteenth century it was regarded as an almost miraculous fertilizer. American farmers were so anxious to acquire it that a lively smuggling trade with Nova Scotia evolved, resulting in the so-called "Plaster War" of 1812.
[5]Physical propertiesGypsum is moderately water-soluble (~2.0–2.5 g/L at 25 °C)
[6] and, in contrast to most other salts, it exhibits a retrograde solubility, becoming less soluble at higher temperatures. When the
crystal lattice is heated it loses liquid water molecules to evaporation and thus gains solidity. As for
anhydrite, its solubility in saline solutions and in
brines is also strongly dependent on
NaCl concentration.
[6]Gypsum crystals are found to contain
anion water and
hydrogen bonding.
[7]Crystal varietiesGypsum occurs in nature as flattened and often
twinnedcrystals and transparent cleavable masses called
selenite. Selenite contains no significant
selenium; rather both substances were named from the Ancient Greek word for the
Moon.
Selenite may also occur in a silky, fibrous form, 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, and 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 forms some of the largest crystals found in nature, up to 11 metres long, in the form of selenite.
[8]OccurrenceGypsum is a common mineral, with thick and extensive
evaporite beds in association with
sedimentary rocks. Deposits are known to occur in
strata from as far back as the
Archaeaneon.
[9] Gypsum is deposited from lake and sea water, as well as in
hot springs, from
volcanic vapors, and sulfate solutions in
veins.
Hydrothermalanhydrite in veins is commonly hydrated to gypsum by groundwater in near surface exposures. It is often associated with the minerals
halite and
sulfur.
Veins of gypsum in the silts/marls of the Tea Green and Grey Marls, Blue Anchor, Somerset, UK.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 km
2 (270 sq mi) expanse of white gypsum sand, enough to supply the construction industry with
drywall for 1,000 years.
[10] 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.
Gypsum is also formed as a by-product of
sulfideoxidation, amongst others by
pyriteoxidation, when the
sulfuric acid generated reacts with
calcium carbonate. Its presence indicates oxidizing conditions. Under reducing conditions, the sulfates it contains can be reduced back to sulfide by
sulfate reducing bacteria.
Orbital pictures from the
Mars Reconnaissance Orbiter (MRO) indicate the existence of gypsum dunes in the northern polar region of Mars.
[11]MiningCommercial quantities of gypsum are found in the cities of
Araripina and
Grajaú,
Brazil,
Pakistan,
Jamaica,
Iran (world's second largest producer),
Thailand,
Spain (the main producer in
Europe),
Germany,
Italy,
England,
Ireland, in
British Columbia,
Manitoba,
Ontario,
Nova Scotia[12] and
Newfoundland in
Canada,
[13] and in
New York,
Michigan,
Indiana,
[13]Texas (in the
Palo Duro Canyon),
Iowa,
Kansas,
Oklahoma,
Arizona,
New Mexico,
Colorado,
Utah,
Arkansas and
Nevada in the
United States. There is also a large open pit quarry located at
Plaster City, California, in Imperial County, and in East
Kutai,
Kalimantan. Several small mines also exist in places like
Kalannie in
Western Australia where gypsum is sold to private buyers for changing the
pH levels of soil for agricultural purposes.
Veins of gypsum in the Chugwater Group, WyomingCrystals of gypsum up to 11 meters (36 ft) long have been found in the caves of the
Naica Mine of
Chihuahua,
Mexico. The crystals thrived in the cave's extremely rare and stable natural environment. Temperatures stayed at 58 °C (136 °F), and the cave was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 short tons (50,000 kg) and is around 500,000 years old.
[14][15]Synthesis
Synthetic gypsum is recovered via
flue gas desulfurization at some coal-fired electric power plants. It can be used interchangeably with natural gypsum in some applications.
Gypsum also precipitates onto brackish water
membranes, a phenomenon known as mineral salt
scaling, such as during
brackish water
desalination of water with high concentrations of
calcium and
sulfate.
Scaling decreases membrane life and productivity. This is one of the main obstacles in brackish water membrane desalination processes, such as
reverse osmosis or
nanofiltration. Other forms of
scaling such as
calcitescaling, depending on the water source, can also be important considerations in
distillation as well as in
heat exchangers where either the salt
solubility or salt
concentration can change rapidly.
Golden Gypsum crystals from Winnipeg.Uses of gypsumGypsum is used in a wide variety of applications:
- Gypsum Board[16] primarily used as a finish for walls and ceilings; known in construction as drywall.
- Plaster ingredient.
- Fertilizer and soil conditioner. In the late 18th and early 19th century, Nova Scotia gypsum, often referred to as plaster, was a highly sought fertilizer for wheat fields in the United States. It is also used in ameliorating sodic soils.[17]
- A binder in fast-dry tennis court clay.
- Plaster of Paris (surgical splints; casting moulds; modeling).
- A wood substitute in the ancient world; for example, when wood became scarce due to deforestation on Bronze AgeCrete, gypsum was employed in building construction at locations where wood was previously used.[18]
- A tofu (soy bean curd) coagulant, making it ultimately a major source of dietary calcium, especially in Asian cultures which traditionally use few dairy products.
- Adding hardness to water used for homebrewing.[19]
- A component of Portland cement used to prevent flash setting of concrete.
- Soil/water potential monitoring (soil moisture tension).
- A common ingredient in making mead.
- In the medieval period it was mixed, by scribes and illuminators, with lead carbonate (powdered white lead) to make gesso which was applied to illuminated letters and gilded with gold in illuminated manuscripts.
- In foot creams, shampoos and many other hair products.
- A medicinal agent in traditional Chinese medicine called Shi Gao.
- Impression plasters in dentistry
Gypsum sand from White Sands National Monument, New Mexico.