Mineral Information and Data:

Acanthite
Ag2S [CNMNC approved formula]

Named after the Greek for thorn because of the shape of crystals.

IMA status : Valid species (Pre-IMA 1855) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
02.04.01.01  
  (02) Sulfides - Including Selenides and Tellurides
  (02.04) where Am Bn Xp, with (m+n):p=2:1
  (02.04.01) Acanthite Group
  
Nickel-Strunz
(10th edition) :
02.BA.30a
  (02) Sulfides
  (02.B) Metal Sulfides, M:S > 1:1 (mainly 2:1)
  (02.BA) With Cu, Ag, Au
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Space group: P 21/n
Unit cell: a = 4.23 Å, b = 6.93 Å, c = 9.53 Å, b = 125.5°
Crystal Habit: Crystals short to long prismatic, crystals rare
Twinning: On {111}, polysynthetic; on {101}, contact
  
Color: Iron-black
Diaphaneity: Opaque
Luster: Metallic
Hardness (Mohs): 2 to 2.5
Measured Density: 7.22 g/cm3
Tenacity: Sectile
Streak: Black
  
Polymorphism and Series: Psuedomorphous with argentite, which is stable only above 177°C. Below this temperature acanthite is the stable phase and forms directly.
Geologic Setting: A common silver species in moderately low-temperature hydrothermal sulfide veins, and in zones of secondary enrichment.
Mineral Association: Silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite, quartz
  
Synonyms/varieties: α-Argentite, Acantite, Akanthite, Argentite, Argentite-β
Other Properties: BioMineral.

 

Acanthite
Acanthite on Calcite
Origin: Uchucchacua Mine, Oyon Province, Lima Department, Peru
Sample size: 2.1 x 2. 0 x 1.3 cm
Photo : Dan Weinrich

 

Acanthite
(zoom in of above)

 

Acanthite
Acanthite on Pyrargyrite
Origin: Uchucchacua mine, Oyon Province, Lima Department, Peru
Sample size: 5 x 4.5 x 2.5 cm
Photo : Dan Weinrich

 

Acanthite
(zoom in of above)

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