Energetics of stable and metastable low temperature iron oxides and oxyhydroxides

Iron oxides and oxyhydroxides are widespread in nature. Understanding the thermodynamic properties of the transformation in the iron oxide/hydroxide system permits the characterization of numerous geochemical processes including weathering, soil chemistry, and diagenesis in continental and oceanic environments. The aim of this study is to measure the energetics of a series of metastable oxides and oxyhydroxides in order to determine the effects of polymorphism, of oxidation state and of hydration. High temperature solution calorimetry using sodium molybdate as a solvent and transposed temperature drop calorimetry at 979 K were used. Enthalpies of formation from the elements of the metastable iron oxides and oxyhydroxides were calculated from the measured values of the enthalpies of drop solution. The experiments allow an estimate of the enthalpy of formation from the elements of γFe₂O₃ (-817.4+1.2 kJ/mol for sample with tetragonal symmetry and -816.9±1.2 kJ/mol for sample with cubic symmetry) and β-FeOOH (-561.3±3.1 kJ/mol) which are currently unavailable in the literature. The stability of the structure depends mostly on the degree of hydration and on the average oxidation state of iron. It depends less strongly on the topology of the structure itself and of the tunnel anion.