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The crystal structure of a new high-pressure hydrous phase, Si-rich Mg-sursassite, of ideal composition Mg 4 Al 5 Si 7 O 23 (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, Mg 5 Al 5 Si 6 O 21 (OH) 7 , and has space group  P 2 1 / 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) Å 3 , and  Z  = 1. The empirical formula determined by electron microprobe analysis of the same crystal as was used in the X‑ray experiment is [Mg 3.93(3) Fe 0.03(1) ] S3.96 [Al 4.98(3) Cr 0.04(1) ] S5.02  Si 7.02(4) O 23 (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 isM  1  ,  VII    M   g  2         M  2   VIMg   2   2  +   M  3  ,VI(Al    0  .5Si    0  .5)    2        M  4  ,VIAl   2         M  5   ,VIAl   2T  1  ,IVSi   2        T  2  ,IVSi   2        T  3  ,IVSi   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.

Si-rich Mg-sursassite Mg 4 Al 5 Si 7 O 23 (OH) 5 with octahedrally coordinated Si: A new ultrahigh-pressure hydrous phase