Temperature-dependent Raman and infrared spectroscopy study on iron–magnesium tourmalines with different Fe content

Tourmalines with different Fe content have been analyzed by using Raman spectra polarized parallel to b and c-axis at −195 °C, 25 °C, 250 °C, and 450 °C and infrared spectroscopy at 25 °C, 150 °C, and 250 °C, respectively. The Raman spectra show that the intensities of both the stretching bands of FeO5(OH) and the deformation bands of [BO3]3− in b direction increase with the total Fe content, and the spectra polarized along c direction of [Si6O18]12− ring and [OH]− shift to lower frequencies. With increasing temperature, the stretching bands of FeO5(OH) in c direction shift to higher frequencies. Meanwhile, the deformation vibration bands of [Si6O18]12− ring enhance significantly, and the asymmetric stretching bands of [Si6O18]12− ring merge into a broad one. The deformation bands of [BO3]3− and the asymmetric stretch bands of [Si6O18]12− ring are weakened at (bb) polarization. The intensities of infrared absorption peak of tourmalines are weakened, and some peaks shift to lower frequencies with the increase of the total Fe content. With the increase of temperature, most infrared absorption peaks shift to lower frequencies. The variation of the Raman and infrared spectra of tourmalines with temperature and Fe content indicate that more replacement of Fe for Mg in Y–O5(OH) octahedron, leads to the deformation of Y–O5(OH) octahedron, and further the deformation of [BO3]3− polyhedron and [Si6O18]12− ring which are connected with Y–O5(OH) octahedron. Further, Mg(Fe)(OH) bonds extend with increasing temperature, leading to the deformation of [SiO4] tetrahedra connected with Y octahedra.