Research Article: Determination of hydrogen site and occupancy in hydrous Mg2SiO4 spinel by single-crystal neutron diffraction

Date Published: February 01, 2018

Publisher: International Union of Crystallography

Author(s): Narangoo Purevjav, Takuo Okuchi, Xiaoping Wang, Christina Hoffmann, Naotaka Tomioka.

http://doi.org/10.1107/S2052520618000616

Abstract

A single-crystal neutron diffraction study was performed on hydrogen incorporation in ringwoodite, which is the most important host mineral of water in the Earth’s deep mantle. Its hydrogen incorporation mechanism, bonding geometry and occupancy at the relevant hydrogen site were unambiguously revealed.

Partial Text

Ringwoodite [γ-(Mg,Fe2+)2SiO4] with cubic spinel structure (space group ) is one of the most abundant mineral phases in the Earth’s mantle (Ringwood & Seabrook, 1962 ▸, Ringwood & Major, 1966 ▸). Based on high-pressure phase equilibrium studies, it was confirmed that ringwoodite is thermodynamically stable at pressures between 17 and 23 GPa at temperatures along the geotherm (Ito & Katsura, 1989 ▸), which corresponds to depths between 520 and 660 km. This depth range is defined as the lower half of the mantle transition zone (MTZ) within the Earth (Dziewonski & Anderson, 1981 ▸; Shearer, 1990 ▸). Subsequently, it was reported that a significant quantity of hydrogen cations could be dissolved into the structure of ringwoodite, which corresponds to at least 2.7 wt% of H2O, even if the chemical environment of the crystal growth was not saturated with water (fH2O < P) (Kohlstedt et al., 1996 ▸). Recently, natural ringwoodite included in a diamond coming from the lower MTZ was discovered for the first time, which contained about 1 wt% of H2O within its structure (Pearson et al., 2014 ▸). Based on this data, the amount of water contained in hydrous ringwoodite in the Earth is about the same as the sum of all the oceans (Keppler, 2014 ▸). Thus, hydrous ringwoodite is now considered the main water reservoir in the Earth’s deep mantle. Table 1 ▸ shows crystal data and details of the structure refinement at dmin = 0.50 Å. Refined structure parameters of each atom at dmin = 0.50 Å are shown in Table 2 ▸. The H+ is located at the 192i position which is along the shortest O—O edge of the M site. This geometry is qualitatively consistent with our previous powder neutron diffraction study of deuterated ringwoodite (Purevjav et al., 2014 ▸). The O—H bond length of 1.10 (4) Å was 0.20 Å shorter than the O—D bond length. The H⋯O distance was 1.79 (3) Å, which was longer than the D⋯O distance by 0.16 (3) Å. The O—H⋯O angle of 162 (3)° was 9° larger than the O—D⋯O angle. These differences are possibly due to the different numbers of obtained reflections and different spatial resolutions between the powder and single-crystal neutron diffractograms. In the current single-crystal study, the number of reflections is more than 40 times that in our previous powder study. We also enhanced the reciprocal-space resolution in the single crystal compared with the powder study by decreasing dmin by 0.10 Å. In addition, because of the low-temperature condition of the present study, the statistics were improved to help collecting the reflections of the weaker intensities at the lower d-spacings, which was a good contrast to the powder study conducted at room temperature. This obtained O—H distance was nearly comparable to the O—H separation in hydrous wadsleyite determined using single-crystal diffraction [0.999 (5) Å at 100 K] (Purevjav et al., 2016 ▸), and it was also consistent with the previously reported first-principles calculation result on hydrous ringwoodite (1.02 Å at 300 K) (Panero, 2010 ▸).   Source: http://doi.org/10.1107/S2052520618000616

 

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