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Anhydrite, or anhydrous
calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that cove ...

calcium sulfate
, is a
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
with the
chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes ...
CaSO4. It is in the
orthorhombic In crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek language, Greek words ''crystallon'' "col ...

orthorhombic
crystal system, with three directions of perfect
cleavage Cleavage may refer to: Science * Cleavage (crystal), in mineralogy and materials science, a process of splitting a crystal * Cleavage (geology), the foliation perpendicular to stress as a result of ductile deformation * Cleavage (embryo), in embr ...
parallel to the three planes of
symmetry Symmetry (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

symmetry
. It is not
isomorphous In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse function, inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between the ...
with the orthorhombic
barium Barium is a chemical element with the Symbol (chemistry), 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 (chemistry), reactivity, ba ...

barium
(
baryte Baryte, barite or barytes (, ) is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs na ...

baryte
) and
strontium Strontium is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

strontium
( celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed
crystal A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformatio ...

crystal
s are somewhat rare, the mineral usually presenting the form of cleavage masses. The
Mohs hardness The Mohs scale of mineral hardness () is a qualitative ordinal scale Ordinal data is a categorical, statistical data type where the variables have natural, ordered categories and the distances between the categories are not known. These data e ...
is 3.5, and the
specific gravity Relative density, or specific gravity, is the ratio In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemon ...
is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the lustre is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring
gypsum Gypsum is a soft sulfate mineral The sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with ...

gypsum
, (CaSO4·2H2O) by the absorption of water. This transformation is reversible, with gypsum or calcium sulfate hemihydrate forming anhydrite by heating to around under normal atmospheric conditions. Anhydrite is commonly associated with
calcite Calcite is a carbonate mineral Carbonate minerals are those mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid E ...

calcite
,
halite Halite (), commonly known as rock salt, is a type of salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition a ...

halite
, and
sulfide Sulfide (British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar and usage a ...

sulfide
s such as
galena Galena, also called lead glance, is the natural mineral form of lead(II) sulfide Lead is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scienti ...

galena
,
chalcopyrite Chalcopyrite ( ) is a copper Copper is a chemical element with the Symbol (chemistry), symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductility, ductile metal with very high thermal conductivity, thermal and e ...

chalcopyrite
,
molybdenite Molybdenite is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure f ...

molybdenite
, and
pyrite The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. R ...

pyrite
in vein deposits.


Occurrence

Anhydrite is most frequently found in
evaporite An evaporite () is a water-solubility, soluble sedimentation, sedimentary mineral deposition (geology), deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposit ...
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 The Bezirk Innsbruck-Land is an administrative district (''Bezirk The German term ''Bezirk'' (plural ''Bezirke'', derived from la, circulus, "circle") translated as "district A district is a type ...

Hall in Tirol
. 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 Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the water the solvent. In chemistry ...
, calcium sulfate is deposited as crystals of gypsum, but when the solution contains an excess of
sodium Sodium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical eleme ...

sodium
or
potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. Covalently bonded metal halides may be discrete m ...

potassium chloride
, anhydrite is deposited if the temperature is above . This is one of the several methods 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 , intertidal and subtidal The neritic zone is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illumina ...

tidal flat
environment in the
Persian Gulf The Persian Gulf ( fa, خلیج فارس, translit=xalij-e fârs, lit=Gulf of , ) is a in . The body of water is an extension of the () through the and lies between to the northeast and the to the southwest.United Nations Group of Exper ...
sabkha A term typically used by Earth scientists, a sabkha ( ar, سبخة) is a coastal, supratidal mudflat or sandflat in which evaporite An evaporite () is a water-soluble Solubility is the property of a solid, liquid or gaseous chemical s ...
s as massive
diagenetic Diagenesis () is the process that describes physical and chemical changes in sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wi ...
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 dome A salt dome is a type of structural dome formed when salt (or other evaporite An evaporite () is a water-soluble Solubility is the property of a solid, liquid or gaseous chemical substance called ''solution, solute'' to dissolve in a s ...
s form a
caprockCaprock or cap rock is a harder or more resistant Rock (geology), rock type overlying a weaker or less resistant rock type.Kearey, Philip (2001). ''Dictionary of Geology'', 2nd ed., Penguin Reference, London, New York, etc., p. 41.. . Common types o ...
. 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 Halite (), commonly known as rock salt, is a type of salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition a ...

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 with oil can reduce SO4 creating
calcite Calcite is a carbonate mineral Carbonate minerals are those mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid E ...

calcite
, water, and
hydrogen sulfide Hydrogen sulfide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...

hydrogen sulfide
(H2S).


Igneous rocks

Anhydrite has been found in some
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock i ...
s, for example in the
intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magma leaving behind igneous rock * Saltwater intrusion, the movement of saline water into freshwa ...
dioritic
dioritic
pluton In , an igneous intrusion (or intrusive body or simply intrusion) is a body of that forms by crystallization of slowly cooling below the surface of the . Intrusions have a wide variety of forms and compositions, illustrated by examples like ...

pluton
of El Teniente, Chile and in
trachyandesite File:TAS-Diagramm-trachyandesite.png, upright=1.35, Trachyandesite is field S3 in the TAS diagram Trachyandesite is an extrusive igneous Rock (geology), rock with a composition between trachyte and andesite. It has little or no free quartz, but is d ...
pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a formed from erupted from a . In other words, it differs from other by being of origin. ...

pumice
erupted by
El Chichón
El Chichón
volcano, Mexico.


Naming history

The name anhydrite was given by A. G. Werner 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 (, also ; ; lmo, label=Eastern Lombard Eastern Lombard is a group of closely related dialects of Lombard language, Lombard, a Gallo-Italic languages, Gallo-Italic language spoken in Lombardy, mainly in the provinces of Bergamo, Bresc ...

Bergamo
, in
Lombardy (man), (woman) lmo, lombard, links=no (man), (woman) , population_note = , population_blank1_title = , population_blank1 = , demographics_type1 = , demographics1_footnotes = ...
, as vulpinite; the latter is cut and polished for ornamental purposes. A semi-transparent light blue-grey variety from
Peru , , image_flag = Flag_of_Peru.svg , image_coat = Escudo_nacional_del_Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = Seal (device), National seal , national_mott ...

Peru
is referred to by the trade name angelite.


Other uses

The
Catalyst Science Discovery Centre
Catalyst Science Discovery Centre
in Widnes, England, has a relief carving of an anhydrite kiln, made from a piece of anhydrite, for the United Sulphuric Acid Corporation.


References

{{Reflist


Further reading

*Spencer, Leonard James. Anhydrite. '' 1911 Encyclopædia Britannica''
Mineralgalleries.com
Calcium minerals Sulfate minerals Evaporite Salt domes Luminescent minerals Orthorhombic minerals Minerals in space group 63