Raman spectroscopy, vibrational analysis, and heating of buergerite tourmaline

Polarized Raman spectra were collected for single crystal buergerite (NaFe₃Al₆(BO₃)₃Si₆O₁₈(O_0.92(OH)_0.08)₃F) from room temperature to near 1,375°C. Vibrational assignments to features in the room temperature spectra were determined by lattice dynamics calculations, where internal BO₃ motions dominate modes near 1,300 cm⁻¹, internal SiO₄ displacements dominate modes between 900 and 1,200 cm⁻¹, while less localized displacements within the isolated Si₆O₁₈ ring mix with motions within Na, Fe, Al, F, and BO₃ environments for fundamental modes below 780 cm⁻¹. At elevated temperatures, most buergerite Raman features broaden and shift to lower frequencies up to 900°C. Above this temperature, the lattice mode peaks evolve into broad bands, while OH stretch modes near 3,550 cm⁻¹ disappear. According to Raman spectroscopy, X-ray diffraction, differential thermal analysis, and scanning electron microscopy, buergerite undergoes a complex transition that starts near 700°C and extends over a 310°C interval, where initially, Al and Fe probably become disordered within the Y- and Z-sites, and most F and all OH are later liberated. A reversible crystal-to-amorphous transition is seen by Raman for buergerite fragments heated as high as 930°C. Buergerite becomes permanently altered when heated to temperatures greater than 930°C; after cooling to room temperature, these altered fragments are comprised of mullite and Fe-oxide crystals suspended in an amorphous borosilicate matrix.