Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Juraj Majzlan, Martin Števko, Jakub Plášil, Jiří Sejkora, Edgar Dachs

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

Journal of Geosciences, volume 68 (2023), issue 1, 67 - 80

DOI: http://doi.org/10.3190/jgeosci.367

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 [Cu_0.95Zn_1.05(AsO₄)(OH)], adamite [Zn₂(AsO₄)(OH)], ludjibaite [Cu₅(PO₄)₂(OH)₄], natural strashimirite [(Cu_7.75Zn_0.09)_7.84(AsO₄)_3.89(SO₄)_0.11(OH)_3.79·5H₂O], and a slavkovite sample dehydrated to the composition Cu₁₂(AsO₄)₄(AsO₃OH)₆·14H₂O 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·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·K^-1·mol^-1) and estimated for other phases. Gibbs free energies of formation (all in kJ·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 (P-1), with unit-cell parameters a = 6.4042(11) Å, b = 13.495 (2) Å, c = 13.574 (2) Å, α = 87.009(15)°, β = 85.564(14)°, γ = 79.678(15)°. 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·mol^-1. Ludjibaite is metastable with respect to its polymorph pseudomalachite. Slavkovite is probably restricted to local acidic environments, rich in Cu and As.