Thermoelastic parameters of Mg-sursassite and its relevance as a water carrier in subducting slabs

We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of a single crystal of Mg-sursassite, Mg (sub 5) Al (sub 5) Si (sub 6) O (sub 21) (OH) (sub 7) . In situ high-pressure and high-temperature single-crystal diffraction allowed the study of structural variation at non-ambient conditions and the determination of bulk elastic properties. The refined parameters of a second-order Birch-Murnaghan equation of state (BM-II EoS) are V0 = 446.02(1) Aa (super 3) and K (sub T0) = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are alpha (sub 0) = 3.14 (5) X 10 (super -5) K (super -1) , alpha (sub 1) = 2.50(16) X 10 (super -8) K (super -2) , and V0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn (sub 4) Al (sub 6) Si (sub 6) O (sub 22) (OH) (sub 6) . The refined parameters of BM-II EoS [V (sub 0) = 470.2(3) Aa (super 3) , K (sub T0) = 128(4) GPa] indicate that composition has a minimal effect on elastic properties. The similarity of density and bulk properties of Mg-sursassite if compared to olivine and other anhydrous mantle minerals suggests that this phase could be overseen by geophysical methods.