Article · Wikipedia archive · Last revised Jun 6, 2026

Acanthite

Acanthite 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. Acanthite is the only stable form in normal air temperature.

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Acanthite
Crystallized acanthite (4.0 × 2.5 × 1.5 cm) from Imiter mine, Jbel Saghro mountain range, Morocco
General
CategorySulfide mineral
FormulaAg2S
IMA symbolAca1
Strunz classification2.BA.30a
Crystal systemMonoclinic2
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
LusterMetallic
StreakBlack
DiaphaneityOpaque
Specific gravity7.20–7.22
References3456

Acanthite 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.36 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.3

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.456

  • Acanthite. Locality: Chispas Mine, Arizpe, Sonora, Mexico. Scale is one inch with a ruled line at one cm.
    Acanthite. Locality: Chispas Mine, Arizpe, Sonora, Mexico. Scale is one inch with a ruled line at one cm.
  • Classic acanthite specimen from the Rayas Mine at Guanajuato, Mexico. Size: 2.4 × 1.1 × 1.1 cm.
    Classic acanthite specimen from the Rayas Mine at Guanajuato, Mexico. Size: 2.4 × 1.1 × 1.1 cm.
See also

See also

References

References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Bonewitz, Ronald Louis (2012). Rocks and Minerals. Dorling Kindersley Limited. ISBN 978-0-7566-9042-7.
  3. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). "Acanthite" (PDF). Handbook of Mineralogy. Chantilly, VA: Mineralogical Society of America.
  4. Mindat.org
  5. Webmineral data
  6. Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., 1985, pp. 271-2 ISBN 0-471-80580-7