Raman spectroscopy of descloizite and mottramite at 298 and 77 K

The Raman spectra of the isomorphous series descloizite [PbZn(VO4)(OH)] and mottramite [PbCu(VO4) (OH)] were obtained at 298 and 77 K. The Raman band at 844 cm-1, assigned to the 1 symmetric (VO4-) stretching mode for descloizite, is shifted to 814 cm-1 for mottramite. The 3 mode of descloizite is observed as a single band at 777 cm-1 but this mode is more complex for mottramite with three bands observed in the 77 K spectrum at 811, 785 and 767 cm-1. The bending mode (2) is observed at 437 cm-1 for descloizite and at 426 cm-1 for mottramite. The 3 region is complex for both minerals and this is attributed to symmetry reduction of the vanadate unit from Td to Cs. Collecting Raman spectra at 77 K allowed better band separation with observation of additional bands ascribed to the removal of degeneracy. Copyright © 2001 John Wiley & Sons, Ltd.