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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 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) × 10−5 K−1, α1 = 2.50(16) × 10−8 K−2, and V0 = 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 [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. [ABSTRACT FROM AUTHOR]

The crystal structure of a new high-pressure hydrous phase, Si-rich Mg-sursassite, of ideal composition Mg4Al5Si7O23(OH)5, that was produced by sub-solidus reaction at 24 GPa and 1400 °C in an experiment using a model sedimentary bulk composition, has been determined by single-crystal X‑ray diffraction. The phase was found to be topologically identical to Mg-sursassite, Mg5Al5Si6O21(OH)7, and has space group P21/m and lattice parameters a = 8.4222(7), b = 5.5812(3), c = 9.4055(9) Å, β = 106.793(8)°, V = 423.26(6) Å3, and Z = 1. The empirical formula determined by electron microprobe analysis of the same crystal as was used in the X‑ray experiment is [Mg3.93(3)Fe0.03(1)]S3.96[Al4.98(3)Cr0.04(1)]S5.02 Si7.02(4)O23(OH)5, with hydroxyl content implied by the crystal-structure analysis. The most significant aspect of the structure of Si-rich Mg-sursassite is the presence of octahedrally coordinated Si. Its structural formula is M 1 , VII M g 2 M 2 VI Mg 2 2 + M 3 ,VI ( Al 0.5 Si 0.5 ) 2 M 4 ,VI Al 2 M 5 ,VI Al 2 T 1 ,IV Si 2 T 2 ,IV Si 2 T 3 ,IV Si 2 O 23 (OH) 5. $^{M1,\text{VII}}M{{g}_{2}}{{\,}^{{{M}_{2}}\text{VI}}}\text{Mg}{{_{2}^{2+}}^{\,M3\text{,VI}}}{{\left(\text{A}{{\text{l}}_{\text{0}\text{.5}}}\text{S}{{\text{i}}_{\text{0}\text{.5}}} \right)}_{2}}{{\,}^{M4\text{,VI}}}\text{A}{{\text{l}}_{2}}{{\,}^{{{M}_{5}}\text{,VI}}}\text{A}{{\text{l}}_{2}}^{T1\text{,IV}}\text{S}{{\text{i}}_{\text{2}}}{{\,}^{T2\text{,IV}}}\text{S}{{\text{i}}_{2}}{{\,}^{T3\text{,IV}}}\text{S}{{\text{i}}_{2}}\,\,{{\text{O}}_{\text{23}}}{{\left(\text{OH} \right)}_{\text{5}}}.$ Si-rich Mg-sursassite joins the group of hydrous ultrahigh-pressure phases with octahedrally coordinated Si that have been discovered by experiment, and that may play a significant role in the distribution and hosting of water in the deep mantle at subduction zones. The reactions defining the stability of Si-rich Mg-sursassite are unknown, but are likely to be fundamentally diferent from those of Mg-sursassite, and involve other ultrahigh-pressure dense structures such as phase D, rather than phase A. [ABSTRACT FROM AUTHOR]