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.[2][5] Acanthite is the only stable form in normal air temperature.

Acanthite
Acanthite on white calcite from Freiberg District, Erzgebirge. Scale is one inch with a rule at one cm
General
Category	Sulfide mineral
Formula (repeating unit)	Ag2S
Strunz classification	2.BA.30a
Crystal system	Monoclinic[1]
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Unit cell	a = 4.229 Å, b = 6.931 Å c = 7.862 Å; β = 99.61°; Z = 4
Identification
Color	Iron-black
Crystal 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 scale hardness	2.0 - 2.5
Luster	Metallic
Streak	Black
Diaphaneity	Opaque
Specific gravity	7.20 - 7.22
References	[2][3][4][5]

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

• 
  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 x 1.1 x 1.1 cm.

References

1. Bonewitz, Ronald Louis (2012). Rocks and Minerals. Dorling Kindersley Limited. ISBN 978-0-7566-9042-7.
2. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). "Acanthite" (PDF). Handbook of Mineralogy. Chantilly, VA: Mineralogical Society of America.
3. Mindat.org
4. Webmineral data
5. Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., 1985, pp. 271-2 ISBN 0-471-80580-7