Raman and FTIR Spectroscopy of Synthetic Amphiboles: II. Divalent (Mg,Co) Substitutions at the Octahedrally Coordinated Sites

Raman spectra were recorded for a set of synthetic amphiboles, Na(NaCa)(Mg5–xCox)Si8O22(OH)2 with x = 0–5, that have been previously characterized by Rietveld structure refinement and infrared spectroscopy in the principal OH-stretching region. The chemical compositions derived from the intensities of the O-H stretching Raman peaks are in accord with the nominal compositions, as well as with the compositions previously determined by both Rietveld structure refinement and infrared spectroscopy. Detailed peak assignments have been made of the Raman spectra in the low-wavenumber region (50–1200 cm–1) starting from the work of Leissner et al. (2015) and Waeselmann et al. (2019) and the simulated partial Raman spectra for all the cation-anion pairs in the structure of potassic-magnesio-arfvedsonite given by Ivanov et al. (2018). Most Raman peaks exhibit one-mode behavior and shift toward lower wavenumbers for increasing Co in the amphibole.