Magnetite thermodynamic constants

Correction of the thermodynamic constants of hematite raises the need for agreement with the new constants of magnetite. A combination of the linear equation AG=/(T) of the reaction of formation of Fc304 with the experimental measurements (Charette and Flengas, 1968) used earlier (Mel nik,... 

All the input data of this local model are either literature data (diffusion coefficients of vacancies and interstitials, thermodynamic constants of the formation reaction of interstitials and vacancies in the magnetite layer), or determined by experimental conditions or calculated (spinel stoichiometry, oxygen activities at the T91—spinel interface and at the spinel—magnetite interface). No adjustment is performed on the experimental points. Results obtained [65,66] show that the simulation is in relatively good agreement with the experimental points whatever the temperature, the oxygen concentration, and the fluid velocity (for temperatures between 450°C and 620°C in... 

If the initial oxygen/iron ratio is below the stoichiometric value, then some elemental iron will remain, in association with hematite and gaseous oxygen. The /02 for this assemblage would be 10 - bars (as calculated from the equilibrium constant for reaction (19.69) at 25°C). This is an interesting situation, because hematite and iron are thermodynamically metastable under these conditions with respect to magnetite. 

These values are almost identical to those recommended by Robie and Hemingway (1995) but with lower uncertainties. The data of Lemire et al. (2013) are retained in this review. Lemire et al. then utilised the measured solubility constant of magnetite (via reaction (11.15)) to determine thermodynamic data for Fe " ". The same methodology has been adopted in the present review. 

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