Chemistry:Acanthite

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Acanthite [Aca]
Acanthite - Imiter mine, Jbel Saghro, Tinghir, Drâa-Tafilalet, Morocco.jpg
Crystallized acanthite (4.0 × 2.5 × 1.5 cm) from Imiter mine, Jbel Saghro mountain range, Morocco
General
CategorySulfide mineral
Formula
(repeating unit)
Ag2S
Strunz classification2.BA.30a
Crystal systemMonoclinic[1]
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/n
Unit cella = 4.229 Å, b = 6.931 Å
c = 7.862 Å; β = 99.61°; Z = 4
Identification
ColorIron-black
Crystal habitPrimary 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
TwinningPolysynthetic on {111}, may be very complex due to inversion; contact on {101}
CleavageIndistinct
FractureUneven
TenacitySectile
Mohs scale hardness2.0 - 2.5
|re|er}}Metallic
StreakBlack
DiaphaneityOpaque
Specific gravity7.20 - 7.22
References[2][3][4][5]

Acanthite (IMA symbol: Aca[6]) is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below 173 °C (343 °F). 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.[2][5] 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.[2]

Acanthite was first described in 1855 for an occurrence in the Jáchymov (St Joachimsthal) District, Krušné Hory Mts (Erzgebirge), Karlovy Vary Region, Bohemia, Czech Republic . The name is from the Greek "akantha" meaning thorn or arrow, in reference to its crystal shape.[3][4][5]

Gallery

References

  1. Bonewitz, Ronald Louis (2012). Rocks and Minerals. Dorling Kindersley Limited. ISBN 978-0-7566-9042-7. 
  2. 2.0 2.1 2.2 Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. et al., eds. "Acanthite". Handbook of Mineralogy. Chantilly, VA: Mineralogical Society of America. http://rruff.geo.arizona.edu/doclib/hom/acanthite.pdf. 
  3. 3.0 3.1 Mindat.org
  4. 4.0 4.1 Webmineral data
  5. 5.0 5.1 5.2 Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., 1985, pp. 271-2 ISBN:0-471-80580-7
  6. Warr, L.N. (2021). "IMA-CNMNC approved mineral symbols". Mineralogical Magazine 85: 291-320. https://www.cambridge.org/core/journals/mineralogical-magazine/article/imacnmnc-approved-mineral-symbols/62311F45ED37831D78603C6E6B25EE0A. 

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