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

摘要

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 (Zn~(2+) and Cu~(2+)) 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 Cu~(2+) for Zn~(2+) is reflected in the NIR and IR spectroscopic signatures. The multiple bands for v3 and v4 (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.