Abstract
A large amount of hydrogen circulates inside the Earth, which affects the long-term evolution of the planet. The majority of this hydrogen is stored in deep Earth within the crystal structures of dense minerals that are thermodynamically stable at high pressures and temperatures. To understand the reason for their stability under such extreme conditions, the chemical bonding geometry and cation exchange mechanism for including hydrogen were analyzed in a representative structure of such minerals (i.e. phase E of dense hydrous magnesium silicate) by using time-of-flight single-crystal neutron Laue diffraction. Phase E has a layered structure belonging to the space group R 3 m and a very large hydrogen capacity (up to 18% H2O weight fraction). It is stable at pressures of 13-18 GPa and temperatures of up to at least 1573 K. Deuterated high-quality crystals with the chemical formula Mg2.28Si1.32D2.15O6 were synthesized under the relevant high-pressure and high-temperature conditions. The nuclear density distribution obtained by neutron diffraction indicated that the O-D dipoles were directed towards neighboring O2- ions to form strong interlayer hydrogen bonds. This bonding plays a crucial role in stabilizing hydrogen within the mineral structure under such high-pressure and high-temperature conditions. It is considered that cation exchange occurs among Mg2+, D+ and Si4+ within this structure, making the hydrogen capacity flexible.
Highlights
Hydrogen can be incorporated into minerals in highly variable amounts
A single crystal of deuterated Dense hydrous magnesium silicates (DHMSs) phase E was synthesized at high pressure and temperature and subsequently analyzed using TOF neutron Laue diffraction
The nuclear density distribution of D+ in the DHMS phase E framework structure at 100 K was obtained with a high spatial resolution of dmin = 0.50 A
Summary
Hydrogen can be incorporated into minerals in highly variable amounts. Once incorporated, the hydrogen is circulated throughout the Earth, from the surface to the deep interior, affecting the long-term evolution of the planet (Iizuka-Oku et al, 2017; Kawakatsu & Watada, 2007; Okuchi, 1997; Thompson, 1992). A major proportion of this hydrogen is currently stored within the crystal structures of dense minerals that are thermodynamically stable under the high pressures and temperatures of the deep mantle of the Earth (Ohtani, 2015; Purevjav et al, 2014, 2016, 2018; Sano-Furukawa et al, 2018). In our previous studies conducted using this combination, reflections with minimum d-spacings (dmin) as low as 0.3 Awere successfully resolved and analyzed (Purevjav et al, 2016, 2018), enabling quantitative determination of site positions and occupancies of deuterium in DHMS phase E. distinctly higher than that typically seen for common hydrogen-bearing minerals of lower density. In order to locate the hydrogen sites, we previously analyzed the structure of deuterated DHMS phase E using powder neutron diffraction at J-PARC, Japan (Tomioka et al, 2016). We concluded that the hydrogen concentration within the mineral structure was reasonably constrained, where the
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