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Mineral speciesFormula
 ChalcedonySiO2 
Mineralogy
Family: Quartz Group 
Space group: Trigonal 
Geological context
High temperature: Very low temperature environments (amorphous) 
Geological context: Stable in very low temperature environments, even sedimentary mineral. Traditionally defined as a fibrous cryptocrystalline variety of Quartz, more recently, it has been shown that much Chalcedony is a mixture of Quartz and Moganite, another Silica mineral. 
Thermodynamics
Individual properties
Molar weight: 60.084 g/mol  ΔGºf-854,659 J/mol ref.  Sº: 41.34 J/mol/K ref. 
  ΔHºf-909,108 J/mol ref.  Cp: 44.47 J/mol/K ref. 
Reaction properties
Chemical reaction:  Chalcedony + 2H2O = H4SiO4 
log10(Keq) = A + B.T + C.T-1 + D.log10(T) + E.T−2 (with T expressed in Kelvin)
A B C D E
 -1.78233202e+1   -3.34992617e-3   -3.47399329e+2   6.74822309e+0   -1.47898431e+4 
Van t'Hoff
@ 25°C, 1 bar: log 10(Keq)-3.46
ΔH°r19,574 J/mol ref.
T (ºC) : 0 25 60 100 150 200 250 300
log 10(Keq):  -3.77   -3.46   -3.09   -2.75   -2.42   -2.16   -1.95   -1.78 
Maier Kelley
Cp(T) = a + b*10-3T + C*105T-2 (with T expressed in Kelvin)  ref.
a:46.94 J/mol/K 
b:34.31 J/mol/K2 
c:-11.30 J.K/mol 

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