| Autor: |
Diakonov, Igor, Khodakovsky, Igor, Schott, Jacques, Sergeeva, Elvira |
| Zdroj: |
European Journal of Mineralogy; November 1994, Vol. 6 Issue: 6 p967-984, 18p |
| Abstrakt: |
Abstract Critical analysis of experimental data available for the dehydration reaction of goethite to hematite, and of calorimetric measurements reported in the literature resulted in improved thermodynamic properties for goethite. Taking account of heats of solution reported by Ferrier (1966), the surface free energies of goethite and hematite were found to be 1400 ± 200 and 950 ± 200 mJ/m2, respectively. The heat capacity of goethite as a function of temperature (CP(T) in J/mol·K) was found to be consistent with CP(T) = 100.671-0.83486- 10-2·T-0.21199·107/T2 where T refers to temperature in kelvins (298 = T = 500 K). The standard enthalpy of formation of goethite at 298 K was derived from measurements of the heats of solution of goethite and hematite. Using the enthalpy of formation of hematite at 298 K from Hemingway (1990), the enthalpy of formation of goethite at 298 K has been calculated to be -562.9 ±1.5 kJ/mol. Good agreement was found between this value and those derived from experiments carried out at high temperature and pressure on the goethite-hematite equilibrium. Based on our value for the standard enthalpy of formation at 298 K and that of King & Weller (1970) for the standard entropy at 298 K, we have calculated the standard Gibbs free energy of formation of goethite at 298 K to be - 492.1 ±1.5 kJ/mol. Calculation of thermodynamic properties of goethite up to 500 K revealed that coarse-grained goethite is stable relative to coarse-grained hematite up to 373 K. |
| Databáze: |
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