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Mineral speciesFormula
 Acanthite(alpha)Ag2S 
Mineralogy
Family: Acanthite - Empressite Group 
Space group: Monoclinic - Prismatic H-M Symbol (2/m) Space Group: P 21/n 
Dana class: 2.4.1.1 (2) Sulfides - Including Selenides and Tellurides (2.4) where Am Bn Xp, with (m+n):p=2:1 (2.4.1) Dana Group 
Geological context
High temperature: Acanthite is stable under a normal atmospheric conditions to the weak hydrothermal conditions. 
Geological context: Low-temperature sulfide veins and in zones of secondary enrichment. 
Thermodynamics
Individual properties
Molar weight: 247.802 g/mol  ΔGºf-39,448 J/mol ref.  Sº: 143.51 J/mol/K ref. 
  ΔHºf-31,589 J/mol ref.  Cp: 76.12 J/mol/K ref. 
Reaction properties
Chemical reaction:  Acanthite(alpha) + H+ = 2Ag+ + HS- 
log10(Keq) = A + B.T + C.T-1 + D.log10(T) + E.T−2 (with T expressed in Kelvin)
A B C D E
 -8.86686825e+2   -1.32494314e-1   3.79562797e+4   3.21770221e+2   -2.96773773e+6 
Van t'Hoff
@ 25°C, 1 bar: log 10(Keq)-36.07
ΔH°r-641,936 J/mol ref.
T (ºC) : 0 25 60 100 150 200 250 300
log 10(Keq):  -39.73   -36.07   -31.92   -28.16   -24.50   -21.68   -19.50   -17.91 
Maier Kelley
Cp(T) = a + b*10-3T + C*105T-2 (with T expressed in Kelvin)  ref.
a:65.40 J/mol/K 
b:35.98 J/mol/K2 
c:0.00 J.K/mol 

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