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

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 (H1-H2) pairs and one tetrahedral Si site, consistent with space group Pnn2.