Acanthite

Acanthite
Crystallized acanthite (4.0 × 2.5 × 1.5 cm) from Imiter mine, Jbel Saghro mountain range, Morocco
General
CategorySulfide mineral
Formula
(repeating unit)
Ag2S
IMA symbolAca[1]
Strunz classification2.BA.30a
Crystal systemMonoclinic[2]
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
References[3][4][5][6]

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.[3][6] Acanthite is the only stable form in normal air temperature.

  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. ^ a b 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. ^ a b Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., 1985, pp. 271-2 ISBN 0-471-80580-7