Thermodynamic Data of Copiapite-Group Minerals

Abstract Copiapite-group minerals are common products of weathering of sulfides, especially pyrite and pyrrhotite. They have a general formula AFe3+4(SO4)6(OH)2·20H2O and all crystallize in the triclinic crystal system (space group P). In this work, we have determined the thermodynamic properties of copiapite (A = Fe2+), ferricopiapite (A = 2/3Fe3+), magnesiocopiapite (A = Mg), and aluminocopiapite (A = 2/3Al). Enthalpies of formation were calculated from enthalpies of dissolution in 5 mol/dm3 HCl (measured by acid-solution calorimetry) and entropies from low-temperature heat capacity data (measured by relaxation calorimetry). Differential scanning calorimetry was used in a restricted temperature range to verify the accuracy of the heat capacity measurements. We also present calculated molar volumes and densities for the copiapite-group phases. The calculated Gibbs free energies of formation are (all values in kJ/mol) −10,322.6 ± 11.8 (A = Mg), −9862.7 ± 12.4 (A = 2/3Fe3+), −10,177.4 ± 10.9 (A = 2/3Al), and −9949.0 ± 12.1 (A = Fe2+). They correspond to solubility products (log Ksp) of −21.24 (A = Mg), −18.55 (A = 2/3Fe3+), −18.17 (A = 2/3Al), and −19.66 (A = Fe2+). They all relate to the dissolution reaction AxFe3+4(SO4)6(OH)2·20H2O + 2H+ → xA + 4Fe3+ + 6SO42− + 22H2O. Using an extended Pitzer model for highly concentrated ferric iron-sulfate solutions, positions of solubility curves for several ferric sulfates were calculated. The shape of the solubility envelope was reproduced, but the position of the curves was not matched. It seems that there is a considerable way to go before a satisfactory thermodynamic model can be constructed for this particular system.