Thermodynamic properties and crystal structure refinement of ferricopiapite, coquimbite, rhomboclase, and Fe2(SO4)3(H2O)5

Enthalpies of formation of ferricopiapite [nominally Fe-4.67(SO4)(6)(OH)(2)(H2O)(20)] coquimbite [Fe-2(SO4)(3)(H2O)(9)], rhomboclase [(H3O)Fe(SO4)(2)(H2O)(3)], and Fe-2(SO4)3(H2O)(5) were measured by acid (5 N HCl) solution calorimetry. The samples were characterized by wet chemical analyses and synchrotron powder X-ray diffraction (XRD). The refinement of XRD patterns gave lattice parameters, atomic positions, thermal factors, and occupancies of the sites. The calculated formulae differ slightly from the nominal compositions: Fe-4.78(SO4)(6) (OH)(2.34)(H2O)(20.71) (ferricopiapite), (Fe1.47Al0.53)(SO4)(3)(H2O)(9.65) (coquimbite), (H3O)(1.34)Fe(SO4)(2.17) (H2O)(3.06) (rhomboclase), and Fe-2(SO4)(3)(H2O)(5.03). All thermodynamic data are given per mole of these formulae. The measured standard enthalpies (in kJ/mol) of formation from the elements (crystalline Fe, Al, S, and ideal gases O-2 and H-2) at T = 298.15 K are -4115.8 +/- 4.1 [Fe-2(SO4)(3)(H2O)(5.03)], -12045.1 +/- 9.2 (ferricopiapite), -5738.4 +/- 3.3 (coquimbite), and -3201.1 +/- 2.6 (rhomboclase). Standard entropy (S') was estimated as a sum of entropies of oxide, hydroxide, and sulfate components. The estimated S' (in J/mol.K) values for the iron sulfates are 488.2 [Fe-2(SO4)(3)(H2O)(5.03)], 1449.2 (ferricopiapite), 638.3 (coquimbite), and 380.1 (rhomboclase). The calculated Gibbs free energies of formation (in kJ/mol) are -3499.7 +/- 4.2 [Fe-2(SO4)(3)(H2O)(5.03)], -10089.8 +/- 9.3 (ferricopiapite), -4845.6 +/- 3.3 (coquimbite), and -2688.0 +/- 2.7 (rhomboclase). These results combined with other available thermodynamic data allow construction of mineral stability diagrams in the Fe-2(III)(SO4)(3)-(FeSO4)-S-II-H2O system. One such diagram is provided, indicating that the order of stability of ferric sulfate minerals with decreasing pH in the range of 1.5 to -0.5 is: hydronium jarosite, ferricopiapite, and rhomboclase.