High-pressure study of dravite tourmaline: Insights into the accommodating nature of the tourmaline structure

Abstract

The high-pressure behavior of dravite tourmaline [Na(Mg 3)Al 6 (Si 6 O 18) (BO 3) 3 (OH) 3 (OH)] has been studied using luminescence spectroscopy and synchrotron based single-crystal diffraction up to ~65 and ~23.6 GPa, respectively. Two emission bands associated with Cr 3+ /V 2+ substitution are constant in energy up to ~9.0 GPa, and shift to longer wavelength at higher pressures, suggesting that a change in compressional mechanism could occur at this pressure. However, single-crystal diffraction data do not show evidence of any structural changes, nor any resolvable changes in the deformation mechanism at ~9.0 GPa. Near 15 GPa, a splitting of one of the emission bands is observed, suggesting that a phase transition occurs at this pressure and that two unique octahedral sites are present in the high-pressure phase. Hysteresis is not observed on decompression, which indicates that this is a second order transition, and the high-pressure structure appears to be metastable up to ~65 GPa. Singlecrystal diffraction measurements show that a phase transition from rhombohedral R3m to rhombohedral R3 occurs at pressures near 15.4 GPa. The high-pressure phase is characterized