Spectroscopic characterisation of rosasite and aurichalcite

Abstract

Chemical and textural studies show fibrous rosasite crystals, with a spread Cu : Zn ratio, occuring in a calcite matrix together with goethite, psilomelane, quartz and saponite in oxidation zones of copper ore deposits. Aurichalcite occurs also as fibrous crystals and has a more fixed composition (Cu : Zn ratio is 1.3 : 3.7). Aurichalcite sample also contains prismatic hemimorphite. The nature of the copper ion and its site symmetry in rosasite and aurichalcite were investigated using a combination of optical absorption and EPR spectroscopy. Optical absorptions of rosasite exhibited three bands due to Cu 2 + in tetragonally elongated octahedral site and the parameters evaluated from the observed band positions are Dq = 1480, Ds = 2330 and Dt = 410cm - 1 . The EPR features observed in rosasite (g⊥>g∥) is an indication of predominant interactions within the copper ions (Cu > Zn). Optical and EPR spectra of aurichalcite sample account for Cu 2 + ion in the distorted octahedron site. The molecular structures of the two copper zinc carbonate hydroxide minerals were also studied by FTIR spectroscopy. The infrared spectra of the two minerals are different in line with differences in crystal structure and composition. The shift towards lower wave numbers of the carbonate bands and splitting of bands are more in aurichalcite than the other. This indicates that the symmetry of CO 3 2- ion in both the minerals is lowered from D 3 h to C 2 v or C s and distortion is more in aurichalcite.