The molecular structure of the vanadate mineral mottramite [PbCu(VO4)(OH)] from Tsumeb, Namibia - a vibrational spectroscopic study.

Abstract

We have studied a mineral sample of mottramite PbCu(VO4)(OH) from Tsumeb, Namibia using a combination of scanning electron microscopy with EDX, Raman and infrared spectroscopy. Chemical analysis shows principally the elements V, Pb and Cu. Ca occurs as partial substitution of Pb as well as P and As in substitution to V. Minor amounts of Si and Cr were also observed. The Raman band of mottramite at 829 cm_1, is assigned to the m1 symmetric (VO_4 ) stretching mode. The complexity of the spectra is attributed to the chemical composition of the Tsumeb mottramite. The m3 antisymmetric vibrational mode of mottramite is observed as very low intensity bands at 716 and 747 cm_1. The series of Raman bands at 411, 439, 451 cm_1 and probably also the band at 500 cm_1 are assigned to the (VO_4 ) m2 bending mode. The series of Raman bands at 293, 333 and 366 cm_1 are attributed to the (VO_ 4 ) m4 bending modes. The m3, m3 and m4 regions are complex for both minerals and this is attributed to symmetry reduction of the vanadate unit from Td to Cs.