Thermodynamic properties of feroxyhyte (δ'-FeOOH)

Feroxyhyte (δ'-FeOOH) is a relatively uncommon Fe oxide mineral and one of the few phases in the system Fe₂O₃-H₂O for which thermodynamic properties are not known. In natural occurrences, it is always fine-grained, although samples with larger particle sizes and better crystallinity (labeled as δ-FeOOH) can be prepared in the laboratory. This contribution presents a thermochemical study on a series of feroxyhyte samples. One is fine-grained and poorly crystalline, similar to natural materials, while the other three are of better crystallinity. The enthalpy of formation of feroxyhyte at 298.15 K is -547.4±1.3 kJ mol^-1 for the poorly crystalline sample (surface area 88 m²/g), -550.6±1.4, -1.550.9±1.3, and 552.6±1.2 kJ mol^-1 for the samples with better crystallinity. The entropy of feroxyhyte can be estimated only crudely, because it is influenced to a great extent by its magnetic properties, particle size, and structural disorder. The S°₂₉₈ of feroxyhyte is estimated here to be 65±5 J K^-1 mol^-1. The Gibbs free energy of the reaction feroxyhyte → hematite + liquid water -7.4 to -12.6 kJ mol^-1 at 298.15 K. The Gibbs free energy of formation (ΔG_f^o) of the fine-grained, poorly crystalline feroxyhyte is -478.1±2.0 kJ mol^-1 at 298.15 K. Since this sample is closest in its physical properties to natural feroxyhyte, this ΔG_f^o value should be used in thermodynamic modeling related to processes involving naturally occurring feroxyhyte. In terms of Gibbs free energy and enthalpy, feroxyhyte is very similar to lepidocrocite and maghemite, and, like these two phases, has no thermodynamic stability field in the system Fe₂O₃-H₂O, except possibly at the nanoscale.