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)₃OH, K-dravite KMg₃Al₆Si₆O₁₈(BO₃)₃(OH)₃OH, magnesio-foitite □(Mg₂Al)Al₆Si₆O₁₈(BO₃)₃(OH)₃OH, oxy-uvite CaMg₃Al₆Si₆O₁₈(BO₃)₃(OH)₃O, and olenite NaAl₃Al₆Si₆O₁₈(BO₃)₃O₃OH, where □ 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 (K₀ in GPa) and corresponding pressure derivatives (⁠$K0′$ = ∂K₀/∂P)_T: dravite K₀ = 97(6), $K0′$ = 5.0(5); K-dravite K₀ = 109(4), $K0′$ = 4.3(2); oxy-uvite K₀ = 110(2), $K0′$ = 4.1(1); magnesio-foitite K₀ = 116(2), $K0′$ = 3.5(1); olenite K₀ = 116(6), $K0′$ = 4.7(4). Each tour-maline 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 onlŷ14% 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 K₀ and $K0′$⁠. 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 $K0′$ relative to the alkali and Ca tourmalines. The decrease in $K0′$ 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.