Spectroscopy of selected copper group minerals: ktenasite,orthoserpierite and kipushite

Near-infrared (NIR) and IR spectroscopy have been applied for the characterisation of three complex Cu–Zn sulphate/phosphate minerals, namely ktenasite, orthoserpierite and kipushite. The spectral signatures of the three minerals are quite distinct in relation to their composition and structure. The effect of structural cation substitution (Zn²⁺ and Cu²⁺) on band shifts is significant both in the electronic and in the vibrational spectra of these Cu–Zn minerals. The variable Cu:Zn ratio between Zn-rich and Cu-rich compositions shows a strong effect on Cu(II) bands in the electronic spectra. The Cu(II) spectrum is most significant in kipushite (Cu-rich) with bands displayed at high wavenumbers, 11,390 and 7,545 cm⁻¹. The isomorphic substitution of Cu²⁺ for Zn²⁺ is reflected in the NIR and IR spectroscopic signatures. The multiple bands for ν₃ and ν₄ (SO₄)²⁻ stretching vibrations in ktenasite and orthoserpierite are attributed to the reduction in symmetry of the sulphate ion from T_d to C_2V. The IR spectrum of kipushite is characterised by strong (PO₄)³⁻ vibrational modes at 1,090 and 990 cm⁻¹. The range of IR absorption is higher in ktenasite than in kipushite, while it is intermediate in orthoserpierite.