American Mineralogist, Volume 93, pages 1099–1111, 2008

Dense hydrous magnesium silicates, phase D, and superhydrous B: New structural
constraints from one- and two-dimensional 29Si and 1H NMR
Xianyu Xue,* Masami Kanzaki, and Anton Shatskiy
Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori, 682-0193 Japan

Abstract
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. Thesedata have revealed that phase D is characterized by disordered and varying local structures aroundboth 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 1Hand 29Si NMR results have clearly revealed that it contains dissimilar hydrogen (H1-H2) pairs andone tetrahedral Si site, consistent with space group Pnn2.
Keywords: Phase D, superhydrous B, NMR, Raman, order/disorder, space group

Introduction
The potential storage of water in the Earth’s deep interior
is an issue of continued pursuit because the presence of water
may significantly affect the phase relations and various physicalproperties such as viscosity, density, and electrical conductivity(Prewitt and Parise 2000). In the MgO-SiO2-H2O system, which
represents a simplified model system for mantle minerals, several
high-pressure hydrous phases (so-called dense hydrous magnesium silicates, DHMS) have been discovered experimentally,including phase A, B, superhydrous B (=C), E, and D (=F, G)(cf. Angel et al. 2001). These phases may potentially serve aswater carriers along cold subduction slabs.
Among the DHMS phases, phase D (=F, G) (with an ideal
chemical formula of MgSi2H2O6) has the highest-pressure stability (at least up to 50 GPa), and thus may play a key role intransporting water to the lower mantle (Liu 1987; Ohtani et al.
1997, 2000; Frost and Fei 1998; Kuroda and Irifune 1998; Shiehet al. 1998). The crystal structure of phase D has been solved
independently by Kudoh et al. (1997) and Yang et al. (1997),
both in space group P31m, using single-crystal X-ray diffraction(XRD). It is the only DHMS phase in which all Si (and Mg) arein octahedral coordination (Fig. 1). Phase D has been reported
to show varying Mg/Si ratios (0.56~0.71, generally greater thanthe value of 0.5 for the ideal composition), and varying watercontents (10~18 wt% H2O; 2.0~3.4 H per formula unit) (Ohtani etal. 1997; Frost and Fei 1998). In the crystal structure refinements,
the high Mg/Si ratios were assumed to result from excess Mg
substituting for Si, with the deficit charge compensated by H (Si4+= Mg2+ + 2H+) (Kudoh et al. 1997; Yang et al. 1997). However,* E-mail: xianyu@misasa.okayama-u.ac.jp0003-004X/08/0007–1099$05.00/DOI: 10.2138/am.2008.2751

there were also suggestions from compositional variations that
Si4+ = 4H+ may be another important substitution mechanism(Frost and Fei 1998). As XRD is intrinsically insensitive to thedistribution of Mg and Si, because of similar scattering factors,
additional information is necessary. The H sites in phase D
have been located by difference Fourier map and bond valence
analysis (Kudoh et al. 1997; Yang et al. 1997). They are only
partially occupied, as revealed by Rietveld refinement of powder
neutron diffraction data and also expected from the chemicalcomposition (Suzuki et al. 2001). There is, however, no direct
information concerning the H distribution and its relation tothe Mg-Si distribution, although these factors would affect thestability and physical properties of phase D.
Solid-state nuclear magnetic resonance (NMR) and vibrational (Raman and infrared) spectroscopy are ideal tools to address
the above issues. 29Si magic-angle spinning (MAS) NMR has
been known to be particularly useful in revealing and quantifying cation disorder involving Si, because Si atoms with different
next nearest neighbors often yield distinct peaks with intensities
proportional to site abundances (cf. Kirkpatrick 1988). Both 1H
MAS NMR and vibrational spectroscopy are sensitive probesof hydrogen bonding (cf. Nakamoto et al. 1955; Novak 1974;
Eckert et al. 1988; Libowitzky 1999; Xue and Kanzaki 2004;
Xue et al. 2006). Recently, we have applied these techniques tohigh-pressure minerals in the Al2O3-SiO2-H2O system (Xue et al.
2006; Xue and Kanzaki 2007b). These studies have revealed partial Al-Si disorder in phase egg (AlSiO3OH), and also provided
additional constraints on the H distributions and their hydrogenbonding distances in phase egg, δ-AlOOH, δ-Al(OH)3, and topazOH [Al2SiO4(OH)2] (Xue et al. 2006; Xue and Kanzaki 2007b).
A unique aspect of NMR is its capacity to directly probe thespatial proximities of nuclei of the same or different types through1099