{{Short description|Mineral, silver sulfide}} {{infobox mineral | name = Acanthite | category = Sulfide mineral | boxwidth = | boxbgcolor = | image = Acanthite - Imiter mine, Jbel Saghro, Tinghir, Drâa-Tafilalet, Morocco.jpg | imagesize = 280px | alt = | caption = Crystallized acanthite (4.0 × 2.5 × 1.5 cm) from Imiter mine, Jbel Saghro mountain range, Morocco | formula = Ag<sub>2</sub>S |IMAsymbol=Aca<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 2.BA.30a | dana = | system = Monoclinic<ref name="R. L. Bonewitz">{{cite book|last1=Bonewitz|first1=Ronald Louis|title=Rocks and Minerals|date=2012|publisher=Dorling Kindersley Limited|isbn=978-0-7566-9042-7}}</ref> | class = Prismatic (2/m) <br/><small>(same H-M symbol)</small> | symmetry = ''P''2<sub>1</sub>/n | unit cell = a = 4.229 Å, b = 6.931 Å <br/>c = 7.862 Å; β = 99.61°; Z = 4 | color = Iron-black | colour = | habit = Primary crystals rare, prismatic to long prismatic, elongated along [001], may be tubular; massive. Commonly paramorphic after the cubic high-temperature phase ("argentite"), of original cubic or octahedral habit | twinning = Polysynthetic on {111}, may be very complex due to inversion; contact on {101} | cleavage = Indistinct | fracture = Uneven | tenacity = Sectile | mohs = 2.0–2.5 | luster = Metallic | streak = Black | diaphaneity = Opaque | gravity = 7.20–7.22 | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = <ref name=Handbook>{{cite encyclopedia|url=https://rruff.geo.arizona.edu/doclib/hom/acanthite.pdf |title=Acanthite |encyclopedia=Handbook of Mineralogy |editor1-first=John W. |editor1-last=Anthony|editor2-first=Richard A. |editor2-last=Bideaux|editor3-first=Kenneth W. |editor3-last=Bladh|editor4-first= Monte C. |editor4-last=Nichols |publisher=Mineralogical Society of America|location=Chantilly, VA}}</ref><ref name=Mindat>[http://www.mindat.org/min-10.html Mindat.org]</ref><ref name=Webmin>[https://www.webmineral.com/data/Acanthite.shtml Webmineral data]</ref><ref name=Klein>Klein, Cornelis and Cornelius S. Hurlbut, ''Manual of Mineralogy,'' Wiley, 20th ed., 1985, pp. 271-2 {{ISBN|0-471-80580-7}}</ref> }} '''Acanthite''' is a form of silver sulfide with the chemical formula Ag<sub>2</sub>S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below {{convert|173|C}}. Argentite is the stable form above that temperature. As argentite cools below that temperature its cubic form is distorted to the monoclinic form of acanthite. Below 173 °C acanthite forms directly.<ref name=Handbook/><ref name=Klein/> Acanthite is the only stable form in normal air temperature.
==Occurrence== Acanthite is a common silver mineral in moderately low-temperature hydrothermal veins and in zones of supergene enrichment. It occurs in association with native silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite and quartz.<ref name=Handbook/>
Acanthite was first described in 1855 for an occurrence in the Jáchymov (Joachimsthal) district, Ore Mountains, Bohemia (today Karlovy Vary Region, Czech Republic). The name is from the Greek "akantha" meaning thorn or arrow, in reference to its crystal shape.<ref name=Mindat/><ref name=Webmin/><ref name=Klein/>
==Gallery== <gallery widths="220px" heights="300px" perrow="2"> File:Acanthite - Chispas Mine, Arizpe, Sonora, Mexico.jpg|Acanthite. Locality: Chispas Mine, Arizpe, Sonora, Mexico. Scale is one inch with a ruled line at one cm. File:Acanthite-180757.jpg| Classic acanthite specimen from the Rayas Mine at Guanajuato, Mexico. Size: 2.4 × 1.1 × 1.1 cm.
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==See also== * List of minerals
==References== {{Commons category}} {{Wiktionary}} {{Reflist}} {{Ores}}
Category:Sulfide minerals Category:Silver minerals Category:Monoclinic minerals Category:Glances Category:Minerals in space group 14 Category:Minerals described in 1855