Polarized Raman spectroscopy and lattice dynamics of potassic-magnesio-arfvedsonite

We report polarized Raman spectra from potassic-magnesio-arfvedsonite in all informative scattering configurations. On the basis of the polarization selection rules, several A(g) vibrational modes have been identified. The B-g modes, however, are below the detection limits of the Raman spectrometer. The OH stretching band is situated between 3630 and 3750cm(-1), and its spectral shape is typical of amphiboles with high occupancy of the A site. It is composed of seven overlapping but resolvable subbands, which stem from occupied A-site configurations M(1)M(1)M(3)-OH-(A)(K/Na)-(OH)-O-W and M(1)M(1)M(3)-OH-(A)(K/Na)-F-W, as well as from vacant A-site configurations M(1)M(1)M(3)-OH-(A)-(OH)-O-W, with different Mg and Fe occupancy of the M(1) and M(3) sites. The experimental Raman spectra are compared with the results of theoretical calculations based on a shell-model force-field and a bond polarizability model. The simulated partial Raman spectra allowed us to assign many low-frequency Raman bands to stretching vibrations involving specific cation-oxygen bonds, as well as the higher-frequency modes of the Si-O skeleton. On the basis of our calculations we hypothesize that the Raman bands at 467, 540 and 589cm(-1) are related to a superposition of Fe-M(2)(3+)-O bond stretching and Si-O-Si bending vibrations.