We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of single crystal of Mg-sursassite, Mg5Al5Si6O21(OH)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-Murnagham Equation of State (BM-II EoS) are V0= 446.02(1) Å3 and KT0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are α0=3.14 (5) x 10-5 K-1, α1=2.50(16) x 10-8 K-2 and V0=445.94(3). For comparison, the P-V Equation of State is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)6. The refined parameters of BM-II EoS (V0 = 470.2(3) Å3, KT0=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.
