Polarized electronic absorption spectra of colourless chalcocyanite, CuSO4, with a survey on crystal fields in Cu2+ minerals

Abstract

Polarized electronic absorption spectra of colourless chalcocyanite, CuSO4, have been measured using microscope-spectrometric techniques. The spectra are characterized by a structured and clearly polarized band system in the near-infrared spectral range with components centred at 11,720, 10,545, 9,100, and 7,320 cm−1, which have been assigned to crystal field d–d transitions of Cu2+ cations in pseudo-tetragonally elongated CuO6 polyhedra with point symmetry Ci (\(\bar{1}\)). The polarization behaviour is interpreted based on a D2(C2″) pseudo-symmetry. Crystal field calculations were performed for the actual triclinic point symmetry by applying the Superposition Model of crystal fields, as well as in terms of a ‘classic’ pseudo-tetragonal crystal field approach yielding the parameters Dq(eq) = 910, Dt = 395, and Ds = 1,336 cm−1, corresponding to a cubically averaged Dqcub = 679 cm−1. A comparative survey on crystal fields in Cu2+ minerals shows that the low overall crystal field strength in chalcocyanite, combined with a comparatively weak pseudo-tetragonal splitting of energy levels, is responsible for its unique colourless appearance among oxygen-based Cu2+ minerals. The weak crystal field in CuSO4 can be related to the lower position of the SO42− anion compared to, e.g. the H2O molecule in the spectrochemical series of ligands.