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 Ag vibrational modes have been identified. The Bg modes, however, are below the detection limits of the Raman spectrometer. The OH stretching band is situated between 3630 and 3750 cm−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)–WOH and M(1)M(1)M(3)–OH–A(K/Na)–WF, as well as from vacant A-site configurations M(1)M(1)M(3)–OH–A□–WOH, 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 589 cm−1 are related to a superposition of M(2)Fe3+–O bond stretching and Si–O–Si bending vibrations.