Si-rich Mg-sursassite Mg4Al5Si7O23(OH)5 with octahedrally coordinated Si: A new ultrahigh-pressure hydrous phase

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, beta = 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)] summation3.96[Al4.98(3)Cr0.04(1)] summation5.02 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 M1,VIIMg2 M2,VIMg2 2+ M3,VI(Al0.5Si0.5)2 M4,VIAl2 M5,VIAl2 T1,IVSi2 T2,IVSi2 T3,IVSi2 O23(OH)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 different from those of Mg-sursassite, and involve other ultrahigh-pressure dense structures such as phase D, rather than phase A.