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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 and Andrey V. Bobrov
Published/Copyright: September 20, 2020
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American Mineralogist
From the journal American Mineralogist Volume 105 Issue 9

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,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

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.2138/am-2020-7533
Keywords for this article
Mg-sursassite; hydrous dense magnesium silicate; synthesis; microprobe analysis; X‑ray diffraction; crystal structure

Articles in the same Issue

  1. MSA Centennial Review Paper
  2. How American Mineralogist and the Mineralogical Society of America influenced a career in mineralogy, petrology, and plate pushing, and thoughts on mineralogy’s future role
  3. Petrographic and spectral study of hydrothermal mineralization in drill core from Hawaii: A potential analog to alteration in the martian subsurface
  4. Characterizing low-temperature aqueous alteration of Mars-analog basalts from Mauna Kea at multiple scales
  5. Archean to Paleoproterozoic seawater halogen ratios recorded by fluid inclusions in chert and hydrothermal quartz
  6. Metasomatism-controlled hydrogen distribution in the Spitsbergen upper mantle
  7. Phase transformation of hydrous ringwoodite to the lower-mantle phases and the formation of dense hydrous silica
  8. Density and sound velocity of liquid Fe-S alloys at Earth’s outer core P-T conditions
  9. Some geometrical properties of fission-track-surface intersections in apatite
  10. Thermal equation of state of post-aragonite CaCO3-Pmmn
  11. Structure of NaFeSiO4, NaFeSi2O6, and NaFeSi3O8 glasses and glass-ceramics
  12. Raman spectroscopic studies of O–H stretching vibration in Mn-rich apatites: A structural approach
  13. Characterization of modified mineral waste material adsorbent as affected by thermal treatment for optimizing its adsorption of lead and methyl orange
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  19. Inherited Eocene magmatic tourmaline captured by the Miocene Himalayan leucogranites
  20. Memorial of F. Donald Bloss 1920–2020
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