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

  • Luca Bindi ORCID logo , Mark D. Welch , Aleksandra A. Bendeliani und Andrey V. Bobrov
Veröffentlicht/Copyright: 20. September 2020
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Abstract

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 M1,VIIMg2M2VIMg22+M3,VI(Al0.5Si0.5)2M4,VIAl2M5,VIAl2T1,IVSi2T2,IVSi2T3,IVSi2O23(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 diferent from those of Mg-sursassite, and involve other ultrahigh-pressure dense structures such as phase D, rather than phase A.

Acknowledgments and Funding

The paper benefited by the official reviews from Wilson Crichton and an anonymous reviewer. These experiments were a part of the scientific program of the Laboratory of Deep Geospheres, Geological Faculty, Moscow State University and were supported by the Russian Science Foundation, project no. 17-17-01169. A.A.B. thanks the Geodynamics Research Center, Ehime University, Matsuyama, Japan, for support of her visit in 2019.

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Received: 2020-03-31
Accepted: 2020-05-28
Published Online: 2020-09-20
Published in Print: 2020-09-25

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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