Raman spectroscopic insights into the pressure-induced phase transition in natural elbaite

Tourmaline [XY(3)Z(6)(T6O18)(BO3)(3)V3W] is a potential carrier for water and boron cycles in the Earth's interior. Its high-pressure structural behavior has been extensively investigated by single-crystal or powder X-ray diffraction. Here, we collected Raman spectroscopy data of natural elbaite with two different compositions (Fe-poor elbaite and Fe-rich elbaite) up to 35.1 GPa to complementarily characterize the structural modifications proposed by previous studies. Our results reveal two clear discontinuous changes in high-pressure Raman spectra of both samples. The first change occurred at around 15 GPa, characterized by changes in Raman peak intensity, and the pressure dependence of the frequencies of several lattice vibration modes and hydroxyl stretching vibrations. The second change emerged at around 26 GPa and was marked by the systematic appearance of new Raman peaks. Based on previous X-ray diffraction studies, the change at around 26 GPa is likely caused by a phase transition from rhombohedral R3m to rhombohedral R3, whereas the change at around 15 GPa is likely caused by a change in compression mechanism. Furthermore, the (OH)-O-W stretching vibrations at 3691 cm(-1) in Fe-rich elbaite exhibited a negative pressure coefficient, contrasting with the positive pressure coefficient observed in Fe-poor elbaite, suggesting a significant influence of chemical composition on the evolution of stretching vibration in (OH)-O-W groups under high pressure. After release of pressure, the spectra return to their initial state, indicating the reversibility of all observed changes.