Acanthite

Acanthite
Acanthite
	Crystallized acanthite (4.0 × 2.5 × 1.5 cm) from Imiter mine, Jbel Saghro mountain range, Morocco
General
Category	Sulfide mineral
Formula; (repeating unit)	Ag2S
IMA symbol	Aca
Strunz classification	2.BA.30a
Crystal system	Monoclinic
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

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.

^ 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.
^ Bonewitz, Ronald Louis (2012). Rocks and Minerals. Dorling Kindersley Limited. ISBN 978-0-7566-9042-7.
^ ^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.
^ Mindat.org
^ Webmineral data
^ ^a ^b Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., 1985, pp. 271-2 ISBN 0-471-80580-7

[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.

[R._L._Bonewitz-2] Bonewitz, Ronald Louis (2012). Rocks and Minerals. Dorling Kindersley Limited. ISBN 978-0-7566-9042-7.

[Handbook-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.

[Mindat-4] Mindat.org

[Webmin-5] Webmineral data

[Klein-6] 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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