American Mineralogist, Volume 102, pages 519–536, 2017SPECIAL COLLECTION: WATER IN NOMINALLY HYDROUS AND ANHYDROUS MINERALSHydrogen incorporation mechanisms in forsterite: New insights from 1H and 29Si NMR
spectroscopy and first-principles calculation
Xianyu Xue1,*, Masami Kanzaki1, Doreen Turner2, and Dominik Loroch3
Institute for Planetary Materials, Okayama University, Misasa, Tottori 682-0193 Japan
2
Institute of Geoscience, University of Jena, Burgweg 11, Jena 07749 Germany
3Institute for Mineralogy, University of Muenster, Corrensstrasse 24, Muenster, 48149 Germany
1Abstract
The presence of water (hydrogen) in nominally anhydrous mantle minerals may have profound effects on their
physical properties (e.g., electrical conductivity, diffusivity, rheology), and these effects are expected to depend on
how water is incorporated in the crystal structure. For olivine, the most abundant upper mantle mineral, despiteextensive studies, mostly using vibrational spectroscopy, the interpretations are still not well constrained. To provide better understanding on this issue, we carried out a comprehensive 1H and 29Si NMR study on an Mg2SiO4
forsterite sample containing about 0.5 wt% H2O synthesized at 12 GPa and 1200 °C, complemented by Raman
measurement and first-principles calculation of the geometry, stability, and NMR parameters of model structures.
The Raman spectra contain relatively sharp O-H stretching bands near 3612, 3579, and 3567 cm–1 and a broader
band near 3547 cm–1, similar to previous reports. The 1H static and MAS NMR data revealed that there are twomain populations of protons in the hydrous forsterite structure, one experiencing strong 1H-1H homonuclear dipolar
couplings and contributing to a broad peak near 2.4 ppm, and another with weaker dipolar couplings and contributing to a narrower peak near 1.2 ppm in the MAS NMR spectrum at 30 kHz. Two-dimensional 1H CRAMPS-MAS
NMR measurements confirmed that the two proton components belong to the same phase and the contrast in MAS
NMR peak width is largely due to difference in the strength of 1H-1H homonuclear dipolar couplings. In addition,
there is also a very weak, narrow 1H MAS NMR peak near 7.3 ppm (contributing to <0.1% of the total intensity)
due to protons that are more remote from the two main components. First-principles calculation confirmed thatthe two main proton components can be attributed to the hydrogarnet-like substitution mechanism of four H ionsfor one Si [(4H)Si] in a tetrahedral site of olivine, but unlike hydrogarnet with one of the protons pointing awayfrom the tetrahedral center and located in an adjacent interstitial site, thus experiencing weaker dipolar couplings
than those in the vicinity of the vacant tetrahedron; the very weak narrow peak near 7.3 ppm can be attributed tothe substitution mechanism of two H ions for one Mg in an M1 site [(2H)M1] of forsterite. The 1H-29Si CP-MAS
NMR spectra revealed both a broad peak encompassing the position for OH defect-free forsterite (–61.7 ppm)
and a narrower peak at higher frequency (–60.9 ppm). First-principles calculation indicates that these peaks areaccountable by the same models as for the 1H NMR data. Thus, this study has provided unambiguous evidence
supporting that hydrogen is incorporated in forsterite at relatively high-pressure dominantly as (4H)Si defects, with(2H)M1 defects playing only a very minor role. The much larger 1H chemical shift for protons associated with thelatter (than the former) is correlated with stronger hydrogen bonding for the latter, which in turn reflects difference
in bonding environments of the OH groups (with the latter bonded to a Si, and the former only bonded to Mg).
Similar correlation applies to the O-H stretching frequency. The (4H)Si defects are responsible for the observedhigh-frequency O-H stretching bands (>3450 cm–1), and the (2H)M1 defects give lower frequencies (undetected
here due to low abundance, but most likely near 3160–3220 cm–1 as previously reported) in vibrational spectra.
These results can serve as a guide for (re-)interpretation of infrared and Raman spectroscopic data on hydrous
olivine produced under different pressure and silica activity conditions, and require reconsideration of any models
for the effects of water on physical properties of olivine based on different interpretations of such data. This study
also demonstrated the usefulness of the combined solid-state NMR and first-principles calculation approach inunraveling the hydrogen incorporation mechanisms in nominally anhydrous minerals.
Keywords: Forsterite, water, structure, Si vacancy, Mg vacancy, NMR, first-principles calculation

Introduction
Water may be incorporated in many nominally anhydrous
minerals as OH defects. The presence of even a small amount of* E-mail: xianyu@misasa.okayama-u.ac.jp
Special collection papers can be found online at http://www.minsocam.org/MSA/
AmMin/special-collections.html.
0003-004X/17/0003–519$05.00/DOI: http://dx.doi.org/10.2138/am-2017-5878

water in minerals may strongly affect the phase relations, melting
temperature, and various physical properties (e.g., thermal andelectrical conductivities, diffusivity, elastic property). Therefore,
many studies have been carried out so far in an attempt to understand the structure and physical properties of nominally anhydrous
minerals (e.g., Keppler and Smyth 2006). To fundamentally understand the effect of water on the physical properties of minerals,
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