Experimental and calculated Raman spectra in Ca-Zn pyroxenes and a comparison between (CaxM2+1-x)M2+Si2O6 pyroxenes (M2+= Mg, Co, Zn, Fe2+)

The Raman spectra of the end member pyroxenes CaZnSi2O6 and Zn2Si2O6 are calculated by quantum mechanical modeling and compared with the experimental ones measured in synthetic (CaxZn1−x)ZnSi2O6 pyroxenes with x = 0, 0.2, 0.3, 0.5, 0.7, 1. The calculated spectra correctly predict the intensity and peak positions of the spectra recorded on the end members. The model provides also useful hints for the mode assignment at the intermediate compositions. The experimental peak positions are compared in (CaxM²⁺1−x)M²⁺Si2O6 pyroxenes, with M²⁺ = Mg, Co, Zn, Fe²⁺. These pyroxenes share a common charge and different mass and ionic radius; the relative contributions of the mass and ionic radius in the experimental spectrum are discussed in four of the most intense peaks. We found that the positions of the strongest peaks are related to the average bond distances of the polyhedra which most affect a given mode. Ca–Zn pyroxenes provide an exception, whereas the CaZnSi2O6 end member fits quite well in the bond-distance/peak positions relations found in other pyroxenes, and the same does not occur as Zn exchanges for Ca. Peak broadening occurs in Zn pyroxenes in intermediate compositions; it is related to the presence of polyhedral local configurations around Zn and Ca atoms in the M2 polyhedron. The broadening is higher in the ~ 1010 cm⁻¹ peak (ν19), which, among the strongest peaks, shows the highest difference in the Raman wavenumber between end members. The different behaviours of Zn pyroxenes with respect to Mg, Co, and Fe²⁺ ones are likely related to the partially covalent bonding in the M2 cavity shown by Zn pyroxenes.