Thermodynamic properties, crystal structure and phase relations of pushcharovskite [Cu(AsO₃OH)(H₂O) ⋅ 0.5H₂O], geminite [Cu(AsO₃OH)(H₂O)] and liroconite [Cu₂Al(AsO₄)(OH)₄ ⋅ 4H₂O]

Abstract

The phases pushcharovskite, geminite and liroconite were synthesized or acquired and characterized by powder X-ray diffraction, infrared spectroscopy, electron microprobe analysis, thermogravimetric analysis and optical emission spectrometry, as needed. Their thermodynamic properties were determined by a combination of acid-solution calorimetry and relaxation calorimetry, resulting in Gibbs free energies of formation (ΔfGo, all values in kilojoules per mole) of -1036.4±3.8 (pushcharovskite, Cu(AsO3OH)(H2O)⋅0.5H2O) and -926.7±3.2 (geminite, Cu(AsO3OH)(H2O)). For the natural liroconite (Cu2Al[(AsO4)0.83(PO4)0.17](OH)4⋅4H2O), ΔfGo=-2996.3±9.2 kJ mol−1. The estimated ΔfGo for the endmember Cu2Al(AsO4)(OH)4⋅4H2O is −2931.6 kJ mol−1. The crystal structure of liroconite was refined (R1=1.96 % for 962 reflections with I>3σ(I)) by single-crystal X-ray diffraction and the positions of H atoms, not known previously, were determined. Liroconite is a rare mineral, except for several localities, notably Wheal Gorland in England. Thermodynamic modelling showed that liroconite will be preferred over olivenite if the Al(III) concentration in the fluid reaches levels needed for saturation with X-ray amorphous Al(OH)3. We assume that such fluids are responsible for the liroconite formation during contemporaneous oxidation of primary Cu–As ores and pervasive kaolinization of the host peraluminous granites. pH had to be kept in mildly acidic (5–6), and the activities of dissolved silica were too low to form dioptase. The main stage with abundant liroconite formation was preceded by an acidic episode with scorodite and pharmacosiderite and followed by a late neutral to mildly basic episode with copper carbonates. Geminite and pushcharovskite, on the other hand, are minerals typical for very acidic solutions. At the studied site in Jáchymov (Czech Republic), extremely acidic water precipitates arsenolite; sulfate is removed by formation of gypsum. Geminite associates with other acidic minerals, such as slavkovite, yvonite and minerals of the lindackerite group. Pushcharovskite is metastable with respect to geminite and probably converts quickly to geminite under field conditions.