Raman spectroscopic study of the mineral finnemanite Pb5(As3+O3)3Cl

The arsenite mineral finnemanite Pb-5(As3+O3)(3)Cl has been studied by Raman spectroscopy. The most intense Raman band at 871 cm(-1) is assigned to the nu(1)(AsO3)(3-) symmetric stretching vibration. Three Raman bands at 898, 908 and 947 cm(-1) are assigned to the nu(3)(AsO3)(3-) antisymmetric stretching vibration. The observation of multiple antisymmetric stretching vibrations suggest that the (AsO3)(3-) units are not equivalent in the molecular structure of finnemanite. Two Raman bands at 383 and 399 cm(-1) are assigned to the nu(2)(AsO3)(3-) bending modes. Density functional theory enabled calculation of the position of AsO32- symmetric stretching mode at 839 cm(-1), the antisymmetric stretching mode at 813 cm(-1) and the deformation mode at 449 cm(-1). Raman bands are observed at 115, 145, 162, 176, 192, 216 and 234 cm(-1) as well. The two most intense bands are observed at 176 and 192 cm(-1). These bands are assigned to PbCl stretching vibrations and result from transverse/longitudinal splitting. The bands at 145 and 162 cm(-1) may be assigned to Cl-Pb-Cl bending modes. Copyright (C) 2009 John Wiley & Sons, Ltd.