Amorphous_silica

Formula : SiO2

High temperature: 
The stability of amorphous silica (opal) is limited to the low temperature springs <150C.
Geological context: 
low temperature hydrothermal systems, epithermal mineralizations, supergen alteration of epithermal systems

Dernière mise à jour le 25.01.2021

THERMODYNAMICS

Individual properties (298.15 K)
M
g·mol-1
 V°
cm3·mol-1
 ΔGºf
J·mol-1
 ΔHºf
J·mol-1
 Sº
J·mol-1·k-1
 CP
J·mol-1·K-1
60.084 29.000 (Ref.: 78hel/del) Helgeson, H.C., Delany, J.M, Nesbitt, H.W., and Bird, D.K., 1978. Summary and Critique of the Thermodynamic Properties of Rock Forming Minerals: Amer. J. Sci., 278A, 229 pp. -850359 (Ref.: Internal calculation) -903150 (Ref.: 00gun/arn) Gunnarson I., and Arnorsson S., 2000. Amorphous silica solubility and the thermodynamic properties of H4SiO4 in the range of 0 to 350C at Psat. Geochimica et Cosmochimica Acta, 64, p. 2295-2307. 46.900 (Ref.: 00gun/arn) Gunnarson I., and Arnorsson S., 2000. Amorphous silica solubility and the thermodynamic properties of H4SiO4 in the range of 0 to 350C at Psat. Geochimica et Cosmochimica Acta, 64, p. 2295-2307. 59.89 (Ref.: 00gun/arn) Gunnarson I., and Arnorsson S., 2000. Amorphous silica solubility and the thermodynamic properties of H4SiO4 in the range of 0 to 350C at Psat. Geochimica et Cosmochimica Acta, 64, p. 2295-2307.
CP(T) = a + b·T + ·c·T-2 (T in Kelvin)
a
J·mol-1·K-1
 b * 103
J·mol-1·K-2
 c * 10-5
J·K·mol-1
 Maier Kelley Ref.
34.51 13.09 19.09 (Ref.: 00gun/arn) Gunnarson I., and Arnorsson S., 2000. Amorphous silica solubility and the thermodynamic properties of H4SiO4 in the range of 0 to 350C at Psat. Geochimica et Cosmochimica Acta, 64, p. 2295-2307.
Reaction properties

Amorphous_silica + 2H2O ⇌ H4SiO4

298.15 K, 1 bar log10K: -2.70 ΔH°r:15949 J·mol-1 (Ref.: 00gun/arn) Gunnarson I., and Arnorsson S., 2000. Amorphous silica solubility and the thermodynamic properties of H4SiO4 in the range of 0 to 350C at Psat. Geochimica et Cosmochimica Acta, 64, p. 2295-2307.
T(°C) 0 25 60 100 150 200 250 300
log10K -3.00 -2.70 -2.44 -2.23 -2.03 -1.87 -1.74 -1.63
log10K(T) = A + B·T + C·T-1 + D·log10(T) + E·T−2 (T in Kelvin)
A B C D E
-353.396043 -0.040434 22631.079271 123.444526 -1653953.388315