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

Luca Bindi; Mark D. Welch; Aleksandra A. Bendeliani; Andrey V. Bobrov

doi:10.2138/am-2020-7533

Published by De Gruyter September 20, 2020

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

Luca Bindi , Mark D. Welch , Aleksandra A. Bendeliani and Andrey V. Bobrov

From the journal American Mineralogist

https://doi.org/10.2138/am-2020-7533

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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 [Mg_3.93(3)Fe_0.03(1)]_S3.96[Al_4.98(3)Cr_0.04(1)]_S5.02 Si_7.02(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 M2VIMg22+ M3,VI(Al0.5Si0.5)2 M4,VIAl2 M5,VIAl2T1,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 diferent from those of Mg-sursassite, and involve other ultrahigh-pressure dense structures such as phase D, rather than phase A.

Keywords: Mg-sursassite; hydrous dense magnesium silicate; synthesis; microprobe analysis; X‑ray diffraction; crystal structure

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

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

Abstract

Acknowledgments and Funding

References cited

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