Editing Anhydrite (section)

==Occurrence==
[[Image:Anhydrite.png|left|thumb|Crystal structure of anhydrite]]
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]]. 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]], calcium sulfate is deposited as crystals of gypsum, but when the solution contains an excess of [[sodium]] or [[potassium chloride]], anhydrite is deposited if the temperature is above {{convert|40|°C}}. 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 [[Depression (geology)|basin]]s.

===Tidal flat nodules===
Anhydrite occurs in a [[tidal flat]] environment in the [[Persian Gulf]] [[sabkha]]s as massive [[Diagenesis|diagenetic]] replacement [[nodule (geology)|nodule]]s. 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.<ref>{{cite book|last=Michael A.|first=Church|title=Encyclopedia of Sediments & Sedimentary Rocks|publisher=Springer|date=2003|pages=17–18|isbn=978-1-4020-0872-6}}</ref>

===Salt dome cap rocks===
Massive amounts of anhydrite occur when [[salt dome]]s 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.<ref>{{cite journal|first=C. W.|last=Walker|title=Origin of Gulf Coast salt-dome cap rock|journal=AAPG Bulletin|date=December 1976|volume=60|issue=12|pages=2162–2166|doi=10.1306/c1ea3aa0-16c9-11d7-8645000102c1865d}}</ref> Interaction of anhydrite with [[hydrocarbon]]s at high temperature in [[oil field]]s can reduce [[sulfate]] ({{chem|SO|4|2–}}) into [[hydrogen sulfide]] (H<sub>2</sub>S) with a concomitant precipitation of [[calcite]].<ref>{{cite journal|title=Origin of 'exotic' minerals in Mississippi salt dome cap rocks: results of reaction-path modeling|journal=Applied Geochemistry|volume=11|issue=5|date=September 1996|pages=667–676|doi=10.1016/S0883-2927(96)00032-7|first1=James A.|last1=Saunders|first2=Robert C.|last2=Thomas|bibcode=1996ApGC...11..667S}}</ref> The process is known as [[thermochemical sulfate reduction]] (TSR).
[[Image:Chickenwire Gypsum anhydrite.jpg|left|thumb|Hand sample of gypsum and anhydrite from diapir caprock showing "chicken wire" texture.]]

===Igneous rocks===
Anhydrite has been found in some [[igneous rock]]s, for example in the [[Intrusive rock|intrusive]] [[Diorite|dioritic]] [[pluton]] of El Teniente, Chile and in [[trachyandesite]] [[pumice]] erupted by [[El Chichón]] volcano, Mexico.<ref>{{cite journal|title=Primary igneous anhydrite: Progress since its recognition in the 1982 El Chichón trachyandesite|last=Luhr|first=James F.|journal=Journal of Volcanology and Geothermal Research|year=2008|volume=175|issue=4|pages=394–407|doi=10.1016/j.jvolgeores.2008.02.016|bibcode=2008JVGR..175..394L}}</ref>