Gypsum | Wikipedia audio article
Gypsum is a soft sulfate mineral composed
of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and
is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk
chalk, and drywall. A massive fine-grained white or lightly tinted variety of gypsum,
called alabaster, has been used for sculpture by many cultures including Ancient Egypt,
Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval
England. Gypsum also crystallizes as translucent crystals of selenite. It also forms as an
evaporite mineral and as a hydration product of anhydrite.
The Mohs scale of mineral hardness defines hardness value 2 as gypsum based on scratch
hardness comparison.==Etymology and history==
The word gypsum is derived from the Greek word γύψος (gypsos), “plaster”. Because
the quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined
gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. Upon
addition of water, after a few tens of minutes plaster of Paris becomes regular gypsum (dihydrate)
again, causing the material to harden or “set” in ways that are useful for casting and construction.
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 mid-18th
century, the German clergyman and agriculturalist Johann Friderich Mayer investigated and publicized
gypsum’s use as a fertilizer. Gypsum may act as a source of sulfur for plant growth, and
in the early 19th 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 1820. In the 19th century, it was
also known as lime sulfate or sulfate of lime.==Physical properties==Gypsum is moderately water-soluble (~2.0–2.5
g/l at 25 °C) and, in contrast to most other salts, it exhibits retrograde solubility,
becoming less soluble at higher temperatures. When gypsum is heated in air it loses water
and converts first to calcium sulfate hemihydrate, (bassanite, often simply called “plaster”)
and, if heated further, to anhydrous calcium sulfate (anhydrite). As for anhydrite, its
solubility in saline solutions and in brines is also strongly dependent on NaCl (common
table salt) concentration.Gypsum crystals are found to contain anion water and hydrogen
bonding.==Crystal varieties==Gypsum occurs in nature as flattened and often
twinned crystals, and transparent, cleavable masses called selenite. Selenite contains
no significant selenium; rather, both substances were named for 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, called alabaster, 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. It also forms some of
the largest crystals found in nature, up to 12 m (39 ft) long, in the form of selenite.==Occurrence==
Gypsum 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 Archaean
eon. Gypsum is deposited from 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. Gypsum is the most common sulfate mineral. Pure gypsum is white, but other substances
found as impurities may give a wide range of colors to local deposits.
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 km2 (270 sq mi) expanse of white gypsum sand, enough to
supply the US construction industry with drywall for 1,000 years.
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 sulfide
oxidation, amongst others by pyrite oxidation, 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. Electric
power stations burning coal with flue gas desulfurization produce large quantities of
gypsum as a byproduct from the scrubbers. Orbital pictures from the Mars Reconnaissance
Orbiter (MRO) have indicated the existence of gypsum dunes in the northern polar region
of Mars, which were later confirmed at ground level by the Mars Exploration Rover (MER)
Commercial quantities of gypsum are found in the cities of Araripina and Grajaú in
Brazil; in Pakistan, Jamaica, Iran (world’s second largest producer), Thailand, Spain
(the main producer in Europe), Germany, Italy, England, Ireland and Canada and the United
States. Large open pit quarries are located in many places including Fort Dodge, Iowa,
which sits on one of the largest deposits of gypsum in the world, and Plaster City,
California, United States, and East Kutai, Kalimantan, Indonesia. Several small mines
also exist in places such as Kalannie in Western Australia, where gypsum is sold to private
buyers for additions of calcium and sulfur as well as reduction of aluminum toxicities
on soil for agricultural purposes. Crystals of gypsum up to 11 m (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 tonnes (61 short tons) and is around 500,000 years
Synthetic gypsum is recovered via flue-gas desulfurization at some coal-fired 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 calcite scaling, depending on the water source, can also be
important considerations in distillation, as well as in heat exchangers, where either
the salt solubility or concentration can change rapidly.
A new study has suggested that the formation of gypsum starts as tiny crystals of a mineral
called bassanite (CaSO4·1⁄2H2O). This process occurs via a three-stage pathway: homogeneous nucleation of nanocrystalline
bassanite; self-assembly of bassanite into aggregates,
and transformation of bassanite into gypsum.==Occupational safety==
People can be exposed to gypsum in the workplace by breathing it in, skin contact, and eye
The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible
exposure limit) for gypsum exposure in the workplace as TWA 15 mg/m3 for total exposure
and TWA 5 mg/m3 for respiratory exposure over an 8-hour workday. The National Institute
for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL)
of TWA 10 mg/m3 for total exposure and TWA 5 mg/m3 for respiratory exposure over an 8-hour
workday.==Uses==Gypsum is used in a wide variety of applications: Gypsum board is primarily used as a finish
for walls and ceilings, and is known in construction as drywall, wallboard, sheetrock or plasterboard.
Gypsum blocks are used like concrete blocks in building construction.
Gypsum mortar is an ancient mortar used in building construction.
Plaster ingredients are used in surgical splints, casting moulds and modeling.
Fertilizer and soil conditioner: In the late 18th and early 19th centuries, 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 high-sodium soils, such as in the Zuiderzee
Works. A binder in fast-dry tennis court clay
As alabaster, a material for sculpture, it was used especially in the ancient world before
steel was developed, when its relative softness made it much easier to carve.
A wood substitute in the ancient world: For example, when wood became scarce due to deforestation
on Bronze Age Crete, gypsum was employed in building construction at locations where wood
was previously used. 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 brewing
Used in baking as a dough conditioner, reducing stickiness, and as a baked-goods source of
dietary calcium. The primary component of mineral yeast food.
A component of Portland cement used to prevent flash setting of concrete
Soil/water potential monitoring (soil moisture) A common ingredient in making mead
In the medieval period, scribes and illuminators mixed it 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
Used in mushroom cultivation to stop grains from clumping together
Tests have shown that gypsum can be used to remove pollutants such as lead or arsenic
from contaminated waters.==Gallery==
Unusual gypsum specimens from around the world==See also==
Gypcrust Gypsum flora of Nova Scotia
Gypsum recycling Phosphogypsum