Single-crystal Raman spectroscopy of natural finnemanite and comparison with its synthesised analogue

Abstract

The single-crystal Raman spectra of the natural mineral finnemanite Pb 5(AsO3)3Cl• from the LÃ¥ngban locality, Filipstad district, Värmland province, Sweden, are presented for the first time. It is a hexagonal mineral belonging to the ortho-arsenite group, where the [AsO3]3- ion is isolated. The spectra of finnemanite are characterised by a strong band at 734 cm -1 overlying a shoulder at 726 cm-1, and broad overlapping bands in the lower wavenumber region with the strongest band positioned at 174 cm-1. Band assignments were made on the basis of band symmetry, experimental band positions from the literature, the calculated Raman spectrum using density functional theory and spectral comparison with other ortho-arsenite minerals reinerite, cafarsite and nealite as well as with synthetic lead arsenite compounds Pb2(AsO2)3Cl, Pb2As2O5 and PbAs2O4. The band at 734 cm-1 was assigned to ν1 (AsO 3); those at 726 and 640 cm-1 to ν3; those at 372 and 357 cm-1 to ν2, and those at 244, 239 and 207 cm-1 to ν4. The single-crystal spectra of finnemanite showed good mode separation, allowing the bands to be assigned to the symmetry species Ag, E1g or E2g. Copyright © 2011 John Wiley & Sons, Ltd.