Acanthite

Acanthite

Acanthite on calcite - Locality: Freiberg District, Erzgebirge- Scale is one inch with a rule at one cm
General
Category Sulfide mineral
Formula
(repeating unit)		 Ag2S
Strunz classification 02.BA.30a
Crystal symmetry Monoclinic 2/m
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
Crystal system Monoclinic prismatic
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 [1][2][3][4]

Acanthite, Ag2S, crystallizes in the monoclinic system and is the stable form of silver sulfide below 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.[1][4] 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.[1]

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.[2][3][4]

Gallery

References

  1. 1 2 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.
  2. 1 2 Mindat.org
  3. 1 2 Webmineral data
  4. 1 2 3 Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., 1985, pp. 271-2 ISBN 0-471-80580-7
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