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Synthesis, characterization and thermochemistry of a Pb-jarosite |
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| Authors: | FL Forray AML Smith A Navrotsky K Wright WE Dubbin |
| Institution: | a Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA b Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, UK c CIRIMAT Carnot Institute, University of Toulouse, CNRS/INPT/UPS, ENSIACET, 118 route de Narbonne, 31077 Toulouse Cedex 04, France d Nanochemistry Research Institute, Department of Applied Chemistry, Curtin University of Technology, G.P.O. Box U1987, Perth, WA 6845, Australia e Research School of Earth Sciences at UCL-Birkbeck, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK f Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK |
| Abstract: | The enthalpy of formation from the elements of a well-characterized synthetic Pb-jarosite sample corresponding to the chemical formula (H3O)0.74Pb0.13Fe2.92(SO4)2(OH)5.76(H2O)0.24 was measured by high temperature oxide melt solution calorimetry. This value ( = −3695.9 ± 9.7 kJ/mol) is the first direct measurement of the heat of formation for a lead-containing jarosite. Comparison to the thermochemical properties of hydronium jarosite and plumbojarosite end-members strongly suggests the existence of a negative enthalpy of mixing possibly related to the nonrandom distribution of Pb2+ ions within the jarosite structure. Based on these considerations, the following thermodynamic data are proposed as the recommended values for the enthalpy of formation from the elements of the ideal stoichiometric plumbojarosite Pb0.5Fe3(SO4)2(OH)6:  = −3118.1 ± 4.6 kJ/mol,  = −3603.6 ± 4.6 kJ/mol and S° = 376.6 ± 4.5 J/(mol K). These data should prove helpful for the calculation of phase diagrams of the Pb-Fe-SO4-H2O system and for estimating the solubility product of pure plumbojarosite. For illustration, the evolution of the estimated solubility product of ideal plumbojarosite as a function of temperature in the range 5-45 °C was computed (Log(Ksp) ranging from −24.3 to −26.2). An Eh-pH diagram is also presented. |
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