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Single-crystal X-ray structure refinement of Al-bearing superhydrous phase B

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Abstract

Superhydrous phase B is an important dense hydrous magnesium silicate (DHMS) phase in a hydrous peridotite system for understanding the deep water cycle in the Earth’s interior. A large amount of Al can be substituted in the crystal structure of superhydrous phase B, but its position in the crystal structure has yet to be determined. Here, we investigated the crystal structure of Al-bearing superhydrous phase B (Mg8.1Si2.0Al1.9H5.9O18) by single-crystal X-ray diffraction (SC-XRD), together with Raman and Fourier-transform infrared (FT–IR) spectroscopies to determine the positions of Al and predict the positions of H. From crystal structure refinement, we clarified that Al was located in the octahedral Mg1, Mg2, and Si1 sites with a Pnnm space group. Furthermore, the result is consistent with a previously proposed substitution reaction: 2Mg2+ + Si4+ ⇄ 2Al3+ + 2H+ + □Mg. The obtained FT–IR spectra, difference Fourier maps, and bond-valence revealed new H positions at (1) O6–H⋯O4, which is a part of the AlO6 octahedron; (2) O5–H⋯O3, which is the same as the Mg-endmember superhydrous phase B, and (3) O4-H⋯O1, which is the shared edge of the Mg2O6 and Mg4O6 octahedra. By summarizing the present study together with the previous studies, it is predicted that the cause of the phase transition between Pnnm and Pnn2 occurs at ~ 1300 ℃ due to the synthesis temperature, which is not due to the substitution of metal elements.

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Acknowledgements

The authors thank Y. Nishihara for technical support for FT–IR measurements. The comments provided by Associate Editor, anonymous reviewer, and Tetsuya Komabayashi greatly improved the clarity of the manuscript. S. Kakizawa was supported by Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists (DC1). This work was supported by JSPS KAKENHI Grant Numbers 16J0269, 19K23457, 19KK0085, and 20K14572 for S. Kakizawa, and 26247073, 15H05828, 19KK0085 for T. Inoue.

Funding

This study was supported by JSPS KAKENHI Grant Numbers 16J0269, 19K23457, 19KK0085, and 20K14572 for Sho Kakizawa and 26247073, 15H05828, 19KK0085 for Toru Inoue.

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SK and TI conceived and designed the study. SK conducted high pressure and high-temperature experiments and analyzed samples using SEM–EDS, Raman, and FT–IR spectrometer at GRC, Ehime University. SK and TK conducted single-crystal X-ray structure refinement at Tohoku University. All authors wrote and discussed the content of the manuscript.

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Correspondence to Sho Kakizawa.

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Kakizawa, S., Inoue, T. & Kuribayashi, T. Single-crystal X-ray structure refinement of Al-bearing superhydrous phase B. Phys Chem Minerals 48, 29 (2021). https://doi.org/10.1007/s00269-021-01152-8

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  • DOI: https://doi.org/10.1007/s00269-021-01152-8

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