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]

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 (Δ1f Go, all values in kilojoules per mole) of −1036.4 ± 3.8 (pushcharovskite, Cu (AsO3 OH)(H2 O) · 0.5H2 O) and −926.7 ± 3.2 (geminite, Cu (AsO3 OH)(H2 O)). For the natural liroconite (Cu 2 Al[(AsO4 )0.83 (PO4 )0.17 ](OH)4 · 4H2 O), Δ1f Go = − 2996.3 ± 9.2 kJ mol −1. The estimated Δ1f Go for the endmember Cu 2 Al(AsO4 )(OH)4 · 4H2 O 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.