Thermodynamic and crystallographic properties of kornelite [Fe2(SO4)3 ~7.75H2O] and paracoquimbite [Fe2(SO4)3 9H2O]

Enthalpies of formation of kornelite [Fe-2(SO4)(3)center dot similar to 7.75H(2)O] and paracoquimbite [Fe-2(SO4)(3)center dot 9H(2)O] were measured by acid (5 N HCl) solution calorimetry at T = 298.15 K. The samples were characterized chemically by an electron microprobe, and structurally by the means of single-crystal, in-house powder, and synchrotron powder X-ray diffraction. The refined structures for the two phases are provided, including estimates of the positions and concentration of non-stoichiometric water in structural channels of kornelite, location of the hydrogen atoms and the hydrogen bonding system in this phase. The measured enthalpies of formation from the elements (crystalline Fe, orthorhombic S, ideal gases O-2 and H-2) at T = 298.15 K are -4916.2 +/- 4.2 kJ/mol for kornelite and -5295.4 +/- 4.2 kJ/mol for paracoquimbite. We have used several algorithms to estimate the standard entropy of the two phases. Afterward, we calculated their Gibbs free energy of formation and constructed a phase diagram for kornelite, paracoquimbite, Fe-2(SO4)(3)center dot 5H(2)O, and Fe-2(SO4)(3) as a function of temperature and relative humidity of air. The topology of the phase diagram is very sensitive to the entropy estimates and the construction of a reliable phase diagram must await better constraints on entropy or Gibbs free energy of formation. Possible remedies of these problems are also discussed.