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High-pressure single-crystal X‑ray diffraction patterns on five synthetic Mg-Al tourmalines with near end-member compositions [dravite NaMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 OH, K-dravite KMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 OH, magnesio-foitite ☐(Mg 2 Al)Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 OH, oxy-uvite CaMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 O, and olenite NaAl 3 Al 6 Si 6 O 18 (BO 3 ) 3 O 3 OH, where o 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 ʹ = ∂ K  0 /∂ P ) T : dravite  K  0  = 97(6),  K  0 ʹ = 5.0(5); K-dravite  K  0  = 109(4),  K  0 ʹ = 4.3(2); oxy-uvite  K  0  = 110(2),  K  0 ʹ = 4.1(1); magnesio-foitite  K  0  = 116(2),  K  0 ʹ = 3.5(1); olenite  K  0  = 116(6),  K  0 ʹ = 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 ~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 ʹ. Contrary to expectations, the identity of the  X -site-occupying ion (Na,  0  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 ʹ relative to the alkali and Ca tourmalines. The decrease in  K  0 ʹ 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.

Compressibility of synthetic Mg-Al tourmalines to 60 GPa