Thermodynamic properties and crystal structure refinement of ferricopiapite, coquimbite, rhomboclase, and Fe2(SO4)3(H2O)5
Enthalpies of formation of ferricopiapite [nominally Fe4.67(SO4)6(OH)2(H2O)20], coquimbite [Fe2(SO4)3(H2O)9], rhomboclase [(H3O)Fe(SO4)2(H2O)3],
and Fe2(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: Fe4.78(SO4)6(OH)2.34(H2O)20.71 (ferricopiapite), (Fe1.47Al0.53)(SO4)3(H2O)9.65
(coquimbite), (H3O)1.34Fe(SO4)2.17(H2O)3.06 (rhomboclase), and Fe2(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 O2 and H2) at T = 298.15 K are −4115.8±4.1 [Fe2(SO4)3(H2O)5.03], −12045.1±9.2 (ferricopiapite),
−5738.4±3.3 (coquimbite), and −3201.1±2.6 (rhomboclase). Standard entropy (Sº) was estimated as a sum of entropies of oxide, hydroxide, and sulfate components. The estimated Sº (in J/mol·K) values for the iron sulfates are 488.2 [Fe2(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 [Fe2(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 FeIII
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.
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