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 (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 [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 FeIII2(SO4)3-FeIISO4-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.

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