Thermodynamics and crystal chemistry of copper arsenates

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Oxidation zones of ore deposits and ore occurrences can originate naturally or by a maninduced action. In either case, they constitute a window in the natural processes which form and shape minerals near the Earth¿s surface and determine the fate of minerals, elements, and biota around the weathering primary minerals. In this proposal, I am planning to focus on copper arsenate minerals, an astonishingly variable group of minerals encountered in many oxidation zones. They are capable of immobilizing or releasing copper, arsenic, and other elements, such as Pb, Zn, or Se. Within the proposed work, we will determine thermodynamic properties of endmembers and solid solutions of copper arsenate minerals (chalcophyllite, liroconite, strashimirite, tyrolite, parnauite, geminite, and euchroite) by a combination of acid-solution (enthalpy of formation) and relaxation (entropy) calorimetry. The mixing parameters for selected solid solutions will be derived by calorimetry or solubility studies and used to construct Lippmann diagrams that show the nature of the interaction between a solid solution and an aqueous phase. The calculated miscibility gaps will be critically compared to the composition of natural solid solutions (determined by electron microprobe). We will carry out field work, collect copper arsenate minerals and co-existing aqueous solutions in the field, and use the composition of these solutions to arrive at independent values of thermodynamic properties of the phases studied. In the final phases of the proposed work, the thermodynamic results andthe known crystallographic models for the copper arsenates will be combined into a mathematical model that predicts the thermodynamic properties of all copper arsenate minerals, based on the assignment of energies to the individual structural blocks (polyhedra) in the minerals.