Thermodynamics of the Cu, Zn, and Cu-Zn phases: zincolivenite, adamite, olivenite, ludjibaite, strashimirite, and slavkovite

Secondary minerals, especially phosphates and arsenates of copper and zinc, form a group of phases with astonishing variability in crystal structures and chemical composition. Some of these minerals are more common than others and one has to ask whether the abundance is linked to their thermodynamic stability or rather to geochemical constraints. In this work, we used calorimetric techniques to determine the thermodynamic properties of synthetic olivenite [Cu2(AsO4)(OH)], zincolivenite [Cu0.95Zn1.05(AsO4)(OH)], adamite [Zn2(AsO4)(OH)], ludjibaite [Cu5(PO4)2(OH)4], natural strashimirite [(Cu7.75Zn0.09)7.84(AsO4)3.89(SO4)0.11(OH)3.79 center dot 5H2O], and a slavkovite sample dehydrated to the composition Cu12(AsO4)4(AsO3OH)6 center dot 14H2O that is used as a proxy for slavkovite. All thermodynamic data presented are based upon the compositions given above. The enthalpies of formation (at 298.15 K and 1 bar, all in kJ center dot mol-1) are -1401.7 +/- 2.6 (adamite), -1211.6 +/- 3.2 (zincolivenite), -3214.3 +/- 10.7 (ludjibaite), -5374.9 +/- 18.1 (strashimirite), and -12004 +/- 34 (dehydrated slavkovite). Entropy was measured only for ludjibaite (389.0 +/- 2.7 J center dot K-1 center dot mol-1) and estimated for other phases. Gibbs free energies of formation (all in kJ center dot mol-1) were calculated for ludjibaite (-2811.4 +/- 10.7), strashimirite (-4477.0 +/- 18.3), and dehydrated slavkovite (-9987 +/- 35). The dehydrated slavkovite is the consequence of H2O loss from the slavkovite holotype specimens during storage of the samples in air at room temperature. It is triclinic (P1 over bar ), with unit-cell parameters a = 6.4042(11) angstrom, b = 13.495 (2) angstrom, c = 13.574 (2) angstrom, alpha = 87.009(15)degrees, beta = 85.564(14)degrees, gamma = 79.678(15)degrees. Dehydration of slavkovite results in a collapse of the sheet structure into a framework structure and into reorganization of bonding, including protonation/deprotonation of AsO4 groups. Constructed activity-activity phase diagrams show that the less stable phases are those which are less common in nature, such as euchroite, strashimirite, or slavkovite. Zincolivenite is stabilized with respect to the end-members olivenite and adamite by a small enthalpy difference of -1.95 kJ center dot mol-1. Ludjibaite is metastable with respect to its polymorph pseudomalachite. Slavkovite is probably restricted to local acidic environments, rich in Cu and As.