Thermodynamic properties and crystal structure refinement of ferricopiapite, coquimbite, rhomboclase, and Fe (sub 2) (SO (sub 4) ) (sub 3) (H (sub 2) O) (sub 5)

The enthalpies of formation of ferricopiapite, coquimbite, rhomboclase 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 having been characterized by wet chemical analyses and synchrotron powder XRD. The refinement of the XRD patterns gave cell parameters, atomic positions, thermal factors and site occupancies. The calculated formulae differ slightly from the nominal compositions: ferricopiapite Fe (sub 4.78) (SO (sub 4) ) (sub 6) (OH) (sub 2.34) (H (sub 2) O) (sub 20.71) , coquimbite (Fe (sub 1.47) Al (sub 0.53) )(SO (sub 4) ) (sub 3) (H (sub 2) O) (sub 9.65) , rhomboclase (H (sub 3) O) (sub 1.34) Fe(SO (sub 4) ) (sub 2.17) (H (sub 2) O) (sub 3.06) 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 of formation (in kJ/mol) from the elements at 298.15 K are: ferricopiapite, -12045.1 + or - 9.2; coquimbite, -5738.4 + or - 3.3; rhomboclase, -3201.1 + or - 2.6; and Fe (sub 2) (SO (sub 4) ) (sub 3) (H (sub 2) O) (sub 5.03) , -4115.8 + or - 4.1. The estimated standard entropies (S (super o) ) for these same four iron sulphates are 1449.2, 638.3, 380.1 and 488.2, respectively, and the calculated Gibbs free energies of formation (kJ/mol) are -10089.8 + or - 9.3, -4845.6 + or - 3.3, -2688.0 + or - 2.7 and -3499.7 + or - 4.2, respectively. These results contribute to the construction of mineral stability diagrams; as an example, the stability of ferric sulphate minerals with decreasing pH in the range 1.5 to -0.5 is hydronium jarosite, ferricopiapite and rhomboclase.