Thermodynamic Properties and Crystal Structure Refinement of Ferricopiapite, Coquimbite, Rhomboclase, and Fe3(SO4)2(H2O)5
Enthalpies of formation of ferricopiapite [nominally Fe{sub 4.67}(SO{sub 4}){sub 6}(OH){sub 2}(H{sub 2}O){sub 20}], coquimbite [Fe{sub 2}(SO{sub 4}){sub 3}(H{sub 2}O){sub 9}], rhomboclase [(H{sub 3}O)Fe(SO{sub 4}){sub 2}(H{sub 2}O){sub 3}], and Fe{sub 2}(SO{sub 4}){sub 3}(H{sub 2}O){sub 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{sub 4.78}(SO{sub 4}){sub 6}(OH){sub 2.34}(H{sub 2}O){sub 20.71} (ferricopiapite), (Fe{sub 1.47}Al{sub 0.53})(SO{sub 4}){sub 3}(H{sub 2}O){sub 9.65} (coquimbite), (H{sub 3}O){sub 1.34}Fe(SO{sub 4}){sub 2.17}(H{sub 2}O){sub 3.06} (rhomboclase), and Fe{sub 2}(SO{sub 4}){sub 3}(H{sub 2}O){sub 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{sub 2} and H{sub 2}) at T = 298.15 K are -4115.8{+-}4.1 [Fe{sub 2}(SO{sub 4}){sub 3}(H{sub 2}O){sub 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{center_dot}{center_dot}K) values for the iron sulfates are 488.2 [Fe{sub 2}(SO{sub 4}){sub 3}(H{sub 2}O){sub 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{sub 2}(SO{sub 4}){sub 3}(H{sub 2}O){sub 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{sup III}{sub 2}(SO{sub 4}){sub 3}-Fe{sup II}SO{sub 4}-H{sub 2}O 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.
- Research Organization:
- Brookhaven National Laboratory (BNL) National Synchrotron Light Source
- Sponsoring Organization:
- Doe - Office Of Science
- DOE Contract Number:
- AC02-98CH10886
- OSTI ID:
- 914002
- Report Number(s):
- BNL--78570-2007-JA
- Journal Information:
- Eur. J. Mineral., Journal Name: Eur. J. Mineral. Vol. 18; ISSN 0935-1221; ISSN EJMIER
- Country of Publication:
- United States
- Language:
- English
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