Table 2 Gibbs free energies of low temperature iron and silica oxyhydroxides in the system Fe–Si–O–H at 25°C and 1 bar
| Â | Sadiq and Lindsay, 1979 (ref. 39) kcal mole-1 | Ponamperuma et al., 1967 (ref 25) kcal mole -1 | Langmuir, 1969 (ref. 360) kcal mole-1 | Winters and Buckley, 1986 (ref. 43) kcal mole-1 | SUPCRT92, Johnson et al., 1991 (ref. 19) kcal mole -1 |
|---|---|---|---|---|---|
Fe3+ (aq) | -4.02 | -- | -4.02 | -- | -4.12 |
Fe2+ (aq) | -21.80 | -20.3 | -21.8 | -18.85 | -21.87 |
Fe(OH)2 (s) | -117.584 | -115.6 | -117.584 | -116.3 | Â |
Fe(OH)3 (s) soil amorphous ferric iron | -170.250 | -- | -- | -- | -- |
Fe(OH)3 (s) aged amorphous ferric iron | -170.163 | -- | -- | -- | -- |
Fe(OH)3 (s) 4 day old amorphous ferric iron | -169.250 | -- | -169.040 | -- | -- |
Fe(OH)3 (s) freshly precipitated amorphous ferric iron | Nature uncertain | -166.0 | -167.460 | -166.5 | -- |
Fe3(OH)8 (s) freshly precipitated amorphous mixed-valence iron | -459.220 | -451.2 (nature uncertain) | -- | -- | -- |
H2O | -56.687 | -56.7 | -56.688 | -56.69 | -56.688 |
OH- | -37.594 | -- | -37.594 | -- | -37.595 |
SiO2 amorphous | -203.02 | -- | -- | -202.89 | -202.89 (SiO2- nH20) |
Ferrosilite | -257.6 | -- | -- | -267.16 | -267.16 |
Si(OH)4 (aq) | -312.66 | -- | -- | -- | -312.6 (SiO2 [aq)] |