THERMODYNAMIC DATA FROM REDOX REACTIONS AT HIGH-TEMPERATURES .2. THE MNO-MN3O4 OXYGEN BUFFER, AND IMPLICATIONS FOR THE THERMODYNAMIC PROPERTIES OF MNO AND MN3O4

The mu(O2) defined by the reaction 6 MnO + O2 = 2 Mn3O4 has been determined from 917 to 1,433 K using electrochemical cells (with calcia-stabilized zirconia, CSZ) of the type: Pt, LaCrO3, ''MnO'' + Mn3O4\CSZ\Ni + NiO, Pt Steady emfs were achieved rapidly at all temperatures on both increasing and decreasing temperature, indicating that the MnO-Mn3O4 oxygen buffer equilibrates relatively easily. It therefore makes a useful alternative choice in experimental petrology to Fe2O3-Fe3O4 for buffering oxygen potentials at oxidized values. The results are (in J/mol, temperature in K, reference pressure 1 bar); mu(O2) (+/- 200) = - 563,241 + 1,761.758 T - 220.490 T ln T + 0.101819 T2 with an uncertainty of +/-200 J/mol. Third law analysis of these data, including a correction for the diviations in stoichiometry of MnO, implies S298.15 for Mn3O4 of 166.6 J/K.mol, which is 2.5 J/K.mol mol higher than the calorimetric determination of Robie and Hemingway (1985). The low value of the calorimetric entropy may be due to incomplete ordering of the magnetic spins. The third law value of DELTA(r)H298.15(0) is -450.09 kJ/mol, which is significantly different from the calorimetric value of -457.5 +/- 3.4kJ/mol, calculated from DELTA(f)H298.15(0) of MnO and Mn3O4, implying a small error in one or both of these latter.