Single‐crystal Raman Spectroscopy of Natural Finnemanite and Comparison with Its Synthesised Analogue

The single‐crystal Raman spectra of the natural mineral finnemanite Pb5(AsO3)3Cl• from the Långban 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 (AsO3); 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.