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Dense hydrous magnesium silicates, phase D, and superhydrous B: New structural constraints from one- and two-dimensional ²⁹Si and ¹H NMR

Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori, 682-0193 Japan

Correspondence: ^* E-mail: xianyu{at}misasa.okayama-u.ac.jp

To gain new structural insights into phase D and superhydrous B, two phases of potential mantle water reservoir, we have applied a range of one- (1D) and two-dimensional (2D) ¹H and ²⁹Si NMR techniques, as well as Raman spectroscopy, to samples synthesized at 24 GPa and 900~1100 °C. These data have revealed that phase D is characterized by disordered and varying local structures around both Si and H. The ²⁹Si NMR spectra of phase D contain a nearly symmetric, broad peak near –177.7 ppm, attributable to octahedral Si with local structural disorder. The high-resolution ¹H CRAMPS spectra of phase D contain a main broad peak near 12.6 ppm with shoulders near 10 and 7 ppm, suggesting a distribution of hydrogen bonding distances. For superhydrous B, our comprehensive 2D ¹H and ²⁹Si NMR results have clearly revealed that it contains dissimilar hydrogen (H1-H2) pairs and one tetrahedral Si site, consistent with space group Pnn2.

Key Words: Phase D • superhydrous B • NMR • Raman • order/disorder • space group

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