Compressibility of synthetic Mg-Al tourmalines to 60 GPa

High-pressure single-crystal X-ray diffraction patterns on five synthetic Mg-Al tourmalines with near end-member compositions [dravite NaMg3Al6Si6O18(BO3)(3)(OH)(3)OH, K-dravite KMg3Al6Si6O18(BO3)(3)(OH)(3)OH, magnesio-foitite o(Mg2Al)Al6Si6O18(BO3)(3)(OH)(3)OH, oxy-uvite CaMg3Al6Si6O18(BO3)(3)(OH) (O), and olenite NaAl3Al6Si6O18(BO3)(3)O3OH, where square represents an X-site vacancy] were collected to 60 GPa at 300 K using a diamond-anvil cell and synchrotron radiation. No phase transitions were observed for any of the investigated compositions. The refined unit-cell parameters were used to constrain third-order Birch-Murnaghan pressure-volume equation of states with the following isothermal bulk moduli (K-0 in GPa) and corresponding pressure derivatives (K-0 ' = partial derivative K0/partial derivative P)(T): dravite K-0 = 97(6), K-0 ' = 5.0(5); K-dravite K-0 = 109(4), K-0 ' = 4.3(2); oxy-uvite K-0 = 110(2), K-0"' = 4.1(1); magnesio-foitite K-0 = 116(2), K-0 ' = 3.5(1); olenite K-0 = 116(6), K-0 ' = 4.7(4). Each tourmaline exhibits highly anisotropic behavior under compression, with the c axis 2.8-3.6 times more compressible than the a axis at ambient conditions. This anisotropy decreases strongly with increasing pressure and the c axis is only similar to 14% more compressible than the a axis near 60 GPa. The octahedral Y- and Z-sites' composition exerts a primary control on tourmaline's compressibility, whereby Al content is correlated with a decrease in the c-axis compressibility and a corresponding increase in K-0 and K-0 '. Contrary to expectations, the identity of the X-site-occupying ion (Na, K, or Ca) does not have a demonstrable effect on tourmaline's compression curve. The presence of a fully vacant X site in magnesio-foitite results in a decrease of K-0 ' relative to the alkali and Ca tourmalines. The decrease in K-0 ' for magnesio-foitite is accounted for by an increase in compressibility along the a axis at high pressure, reflecting increased compression of tourmaline's ring structure in the presence of a vacant X site. This study highlights the utility of synthetic crystals in untangling the effect of composition on tourmaline's compression behavior.