Spectroscopy of selected copper group minerals: Chalcophyllite and chenevixite-implications for hydrogen bonding

Abstract

NIR and IR spectroscopy has been applied for detection of chemical species and the nature of hydrogen bonding in arsenate complexes. The structure and spectral properties of copper(II) arsenate minerals: chalcophyllite and chenevixite are compared with copper(II) sulphate minerals: devilline, chalcoalumite and caledonite. Split NIR bands in the electronic spectrum of two ranges 11,700–8500 cm −1 and 8500–7200 m −1 confirm distortion of octahedral symmetry for Cu(II) in the arsenate complexes. The observed bands with maxima at 9860 and 7750 cm −1 are assigned to Cu(II) transitions 2B 1g → 2B 2g and 2B 1g → 2A 1g. Overlapping bands in the NIR region 4500–4000 cm −1 is the effect of multi-anions OH −, (AsO 4) 3− and (SO 4) 2−. The observation of broad and diffuse bands in the range 3700–2900 cm −1 confirms strong hydrogen bonding in chalcophyllite relative to chenevixite. The position of the water bending vibrations indicates the water is strongly hydrogen bonded in the mineral structure. The strong absorption feature centred at 1644 cm −1 in chalcophyllite indicates water is strongly hydrogen bonded in the mineral structure. The H 2O-bending vibrations shift to low wavenumbers in chenevixite and an additional band observed at 1390 cm −1 is related to carbonate impurity. The characterisation of IR spectra by ν 3 antisymmetric stretching vibrations of (SO 4) 2− and (AsO 4) 3 ions near 1100 and 800 cm −1 respectively is the result of isomorphic substitution for arsenate by sulphate in both the minerals of chalcophyllite and chenevixite.