Anhydrite HMNH1.jpg
   HOME

TheInfoList



OR:

Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the
orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...
crystal system, with three directions of perfect cleavage parallel to the three planes of
symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...
. It is not isomorphous with the orthorhombic
barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...
(
baryte Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
) and
strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
( celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The Mohs hardness is 3.5, and the specific gravity is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the
lustre Lustre or Luster may refer to: Places * Luster, Norway, a municipality in Vestlandet, Norway ** Luster (village), a village in the municipality of Luster * Lustre, Montana, an unincorporated community in the United States Entertainment * '' ...
is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring gypsum, (CaSO4·2H2O) by the absorption of water. This transformation is reversible, with gypsum or
calcium sulfate hemihydrate Bassanite is a calcium sulfate mineral with formula calcium sulfate, CaSO4·water of crystallization, H2O or 2CaSO4·H2O. In other words it has half a water molecule per CaSO4 unit, hence its synonym ''calcium sulfate hemihydrate''. Bassanite was ...
forming anhydrite by heating to around under normal atmospheric conditions. Anhydrite is commonly associated with
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
, halite, and
sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s such as
galena Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver. Galena is one of the most abundant and widely distributed sulfide minerals. It cryst ...
, chalcopyrite, molybdenite, and pyrite in vein deposits.


Occurrence

Anhydrite is most frequently found in evaporite deposits with gypsum; it was, for instance, first discovered in 1794 in a salt mine near
Hall in Tirol Hall in Tyrol is a town in the Innsbruck-Land district of Tyrol, Austria. Located at an altitude of 574 m, about 5 km (3 mi) east of the state's capital Innsbruck in the Inn valley, it has a population of about 13,000 (Jan 2013). History ...
. In this occurrence, depth is critical since nearer the surface anhydrite has been altered to gypsum by absorption of circulating ground water. From an
aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
, calcium sulfate is deposited as crystals of gypsum, but when the solution contains an excess of sodium or
potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a salt ...
, anhydrite is deposited if the temperature is above . This is one method by which the mineral has been prepared artificially and is identical with its mode of origin in nature. The mineral is common in salt basins.


Tidal flat nodules

Anhydrite occurs in a tidal flat environment in the Persian Gulf sabkhas as massive diagenetic replacement nodules. Cross sections of these nodular masses have a netted appearance and have been referred to as ''chicken-wire anhydrite''. Nodular anhydrite occurs as replacement of gypsum in a variety of sedimentary depositional environments.


Salt dome cap rocks

Massive amounts of anhydrite occur when salt domes form a caprock. Anhydrite is 1–3% of the minerals in salt domes and is generally left as a cap at the top of the salt when the halite is removed by pore waters. The typical cap rock is a salt, topped by a layer of anhydrite, topped by patches of gypsum, topped by a layer of calcite. Interaction of anhydrite with hydrocarbons at high temperature in
oil field A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presence ...
s can reduce sulfate () into
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
(H2S) with a concomitant precipitation of
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
. The process is known as
thermochemical sulfate reduction The sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (CHNOPS), being a consti ...
(TSR).


Igneous rocks

Anhydrite has been found in some igneous rocks, for example in the intrusive dioritic
pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
of El Teniente, Chile and in trachyandesite pumice erupted by
El Chichón El Chichón, also known as Chichonal, is an active volcano in Francisco León, Chiapas, Francisco León, north-western Chiapas, Mexico. El Chichón is part of a geologic zone known as the Chiapanecan Volcanic Arc. El Chichón is a complex of dome ...
volcano, Mexico.


Naming history

The name anhydrite was given by
A. G. Werner Abraham Gottlob Werner (; 25 September 174930 June 1817) was a German geologist who set out an early theory about the stratification of the Earth's crust and propounded a history of the Earth that came to be known as Neptunism. While most tene ...
in 1804, because of the absence of water of crystallization, as contrasted with the presence of water in gypsum. Some obsolete names for the species are muriacite and karstenite; the former, an earlier name, being given under the impression that the substance was a chloride (muriate). A peculiar variety occurring as contorted concretionary masses is known as tripe-stone, and a scaly granular variety, from Volpino, near
Bergamo Bergamo (; lmo, Bèrghem ; from the proto- Germanic elements *''berg +*heim'', the "mountain home") is a city in the alpine Lombardy region of northern Italy, approximately northeast of Milan, and about from Switzerland, the alpine lakes Como ...
, in
Lombardy Lombardy ( it, Lombardia, Lombard language, Lombard: ''Lombardia'' or ''Lumbardia' '') is an administrative regions of Italy, region of Italy that covers ; it is located in the northern-central part of the country and has a population of about 10 ...
, as vulpinite; the latter is cut and polished for ornamental purposes. A semi-transparent light blue-grey variety from Peru is referred to by the trade name ''angelite''.


Other uses

The
Catalyst Science Discovery Centre The Catalyst Science Discovery Centre and Museum is a science and technology science centre, museum in Widnes, Halton, North-West England. The centre has interactive exhibits, reconstructed historical scenes, an observatory, a live-science theat ...
in Widnes, England, has a relief carving of an anhydrite kiln, made from a piece of anhydrite, for the
United Sulphuric Acid Corporation United may refer to: Places * United, Pennsylvania, an unincorporated community * United, West Virginia, an unincorporated community Arts and entertainment Films * ''United'' (2003 film), a Norwegian film * ''United'' (2011 film), a BBC Two fi ...
. Extensive structural damage in the German city of Staufen im Breisgau has occurred since a 2007 geothermal drilling project allowed subsurface water to invade a layer of anhydrite below the city, causing extensive but uneven ground swelling as pockets of the anhydrite converted to gypsum.


References


Further reading

* Spencer, Leonard James. Anhydrite. ''
1911 Encyclopædia Britannica A notable ongoing event was the Comparison of the Amundsen and Scott Expeditions, race for the South Pole. Events January * January 1 – A decade after federation, the Northern Territory and the Australian Capital Territory ...
''
Mineralgalleries.com


{{Authority control Calcium minerals Sulfate minerals Evaporite Salt domes Baryte group Luminescent minerals Orthorhombic minerals Minerals in space group 63