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, 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-Murnaghan equation of state (BM-II EoS) are V-0 = 446.02(1) angstrom(3) and K-T0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are alpha(0) = 3.14 (5) x 10(-5) K-1, alpha(1) = 2.50(16) x 10(-8) K-2, and V-0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)(6). The refined parameters of BM-II EoS [V-0 = 470.2(3) angstrom(3), K-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.

Automated summary

In this study, we synthesized and characterized a single crystal of Mg-sursassite, and investigated its structural and thermoelastic properties under high-pressure and high-temperature conditions. The results showed that Mg-sursassite shares similar density and bulk properties with other anhydrous mantle minerals, suggesting that it may be overlooked by geophysical methods.