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

[摘要] Secondary minerals, especially phosphates and arsenates of copper and zinc, form a group of phases with astonishingvariability in crystal structures and chemical composition. Some of these minerals are more common than others andone 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], naturalstrashimirite [(Cu7.75Zn0.09)7.84(AsO4)3.89(SO4)0.11(OH)3.79·5H2O], and a slavkovite sample dehydrated to the compositionCu12(AsO4)4(AsO3OH)6·14H2O that is used as a proxy for slavkovite. All thermodynamic data presented are based uponthe 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 otherphases. 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 lossfrom 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 andinto reorganization of bonding, including protonation/deprotonation of AsO4 groups. Constructed activity–activity phasediagrams 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 enthalpydifference of –1.95 kJ·mol–1. Ludjibaite is metastable with respect to its polymorph pseudomalachite. Slavkovite isprobably restricted to local acidic environments, rich in Cu and As.

[学科分类] 自动化工程

[关键词] thermodynamics;crystal structure;zincolivenite;strashimirite;slavkovite;ludjibaite