The breakdown of hercynite at low fO2

Experimental data on the stability of hercynite yield very large discrepancies in the derived values of its free energy. Attempts were made to resolve these discrepancies through an electrochemical study of the equilibrium 2 FeAl₂O₄ = 2 Fe + 2 Al₂O₃ + O₂. These measurements proved, however, also to give a wide range of apparent free energies, so a detailed phase equilibrium study was made of the Fe-Fe₂O₃-Al₂O₃ system at 1 atm pressure. Our results demonstrate that difficulties in measuring the position of Equilibrium 1 in T-f_O2 space are due to the appearance of a new phase, a defect spinel of approximate composition Fe_0.5Al_2.33O₄ and a cell edge a₀ = 8.00 Å. The data show that previous experiments at 1 atm measured either the hercynite-corundum-Fe equilibrium or various equilibria involving the defect spinel phase, and hence the apparent discrepancies in the hercynite free energy. The results also indicate that hercynite is more stable at 1 atm than would be predicted from high pressure phase-equilibrium experiments. This appears to be due to the solution of excess Al₂O₃ in hercynite at low pressure. -from Authors