Enthalpy of formation of sulfate green rusts

As a contribution to the systematic study of iron oxide thermodynamics, this work reports enthalpies of formation of green rust, a double layered (FeII, FeIII) hydroxide with the ideal stoichiometry FeII1-xFeIIIx(OH)2[Am-]y/m·nH2O, with sulfate as the anion in the interlayer. Samples were characterized by X-ray powder diffraction, thermogravimetric analysis, infrared spectroscopy, and Mössbauer spectroscopy. Full chemical analysis was performed. Contents of FeII, FeIII, water, and sulfate were obtained. We report standard enthalpies of formation for green rust with different FeII/FeIII ratios. Enthalpies of formation from single cation compounds, namely, Fe(OH)2, Fe(OH)3, FeSO4 and H2O show reasonable agreement with Gibbs free energies of formation from single cation compounds recalculated from the reported literature values. These values show that green rust has little stabilization over a mechanical mixture of these single cation compounds and there is no thermodynamic preference for any particular FeII/FeIII ratio.