Raman and Fourier transform infrared spectroscopic study of pyromorphite-vanadinite solid solutions

Due to the great range of the application fields for apatites, there is a strong need to complete the data set determining the properties of these minerals. In this study, Raman and Infrared spectra of the phases from pyromorphite Pb₅(PO₄)₃Cl - vanadinite Pb₅(VO₄)₃Cl series were investigated. Totally, 9 samples (2 end-members and 7 solid solutions of the series) were synthesized at 25°C and pH=3.5, and analyzed. In the Raman and Infrared spectra of the studied Pb-apatites, the bands typical for the vibrations in the PO₄ and the VO₄ tetrahedra appeared. The bands attributed to the stretching vibrations (ν₁, ν₃) occurred in the 1050-910cm^-1 and 830-720cm^-1 regions, whereas the bending vibrations (ν₂, ν₄) were visible at the 580-540cm^-1, 430-380cm^-1 and 370-290cm^-1 range. The position of the bands depended on the P/(P+V) ratio in the analyzed solid, since there are differences in the ionic radii and the atomic mass of P⁵⁺ and V⁵⁺, which affect the bong lengths, bond forces and the banding energies of the substituting tetrahedra. The analysis allowed observing gradual shifts of the bands caused by the replacement of phosphorous with vanadium in the studied phases. The positions and the intensities of selected bands are proposed to serve as a semi-quantitative estimation of the chemical composition of the phases from the pyromorphite - vanadinite series.