@article {Majzlan:2006-03-01T00:00:00:0935-1221:175,
author = "Majzlan, Juraj and Navrotsky, Alexandra and McCleskey, R. Blaine and Alpers, Charles N.",
title = "Thermodynamic properties and crystal structure refinement of ferricopiapite, coquimbite, rhomboclase, and Fe_{2}(SO_{4})_{3}(H_{2}O)_{5}
",
journal = "European Journal of Mineralogy",
volume = "18",
number = "2",
year = "2006-03-01T00:00:00",
abstract = "Enthalpies of formation of ferricopiapite [nominally Fe_{4.67}(SO_{4})_{6}(OH)_{2}(H_{2}O)_{20}], coquimbite [Fe_{2}(SO_{4})_{3}(H_{2}O)_{9}], rhomboclase [(H_{3}O)Fe(SO_{4})_{2}(H_{2}O)_{3}],
and Fe_{2}(SO_{4})_{3}(H_{2}O)_{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}(SO_{4})_{6}(OH)_{2.34}(H_{2}O)_{20.71} (ferricopiapite), (Fe_{1.47}Al_{0.53})(SO_{4})_{3}(H_{2}O)_{9.65}
(coquimbite), (H_{3}O)_{1.34}Fe(SO_{4})_{2.17}(H_{2}O)_{3.06} (rhomboclase), and Fe_{2}(SO_{4})_{3}(H_{2}O)_{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}(SO_{4})_{3}(H_{2}O)_{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 [Fe_{2}(SO_{4})_{3}(H_{2}O)_{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}(SO_{4})_{3}(H_{2}O)_{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^{III}
_{2}(SO_{4})_{3}–Fe^{II}SO_{4}–H_{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.",
pages = "175-186",
url = "http://www.ingentaconnect.com/content/schweiz/ejm/2006/00000018/00000002/art00004",
doi = "doi:10.1127/0935-1221/2006/0018-0175",
keyword = "CRYSTAL STRUCTURE, THERMODYNAMICS, SULFATE MINERALS, COPIAPITE, RHOMBOCLASE, COQUIMBITE"
}