Strong hydrogen bonding in a dense hydrous magnesium silicate discovered by neutron Laue diffraction

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.

Hydrogen incorporation mechanism in the lower-mantle bridgmanite

“…The crystal was exposed to the neutron beam for ~2.5 days in total at a TOF Laue diffractometer TOPAZ installed at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory, which was operated at 1.4 MW proton beam power (Schultz et al 2014). The crystal was cooled down to 100 K by cold nitrogen gas, where the signal-to-noise ratio of higher-order reflections was enhanced (Purevjav et al 2016(Purevjav et al , 2018(Purevjav et al , 2020. For covering the full reciprocal space, we sequentially reoriented the crystal to have 25 different orientations with the help of the CrystalPlan software (Zikovsky et al 2011).…”

Temperature Dependence of H2O Solubility in Al‐Free Stishovite

“…Lawsonite (8 GPa) and phengite (6 GPa) are likely the hydrous minerals in the sedimentary and basaltic layers (Poli & Schmidt, 2002; Schmidt & Poli, 1998). In the peridotitic layers, these minerals may include antigorite (∼6 GPa) and dense hydrous magnesium silicates such as phase A (6–14 GPa), phase E (15 GPa), phase D (15–40 GPa), and 10 Å‐phase (6.7 GPa) (Ohtani, 2015; Ono, 1998; Purevjav et al., 2020; Tomioka et al., 2016). However, these hydrous minerals are thermodynamically unstable at relatively high temperatures, i.e., >1300–1400°C (Ohtani, 2021).…”

Single‐Crystal Elasticity of Phase E at High Pressure and Temperature: Implications for the Low‐Velocity Layer Atop the 410‐km Depth

“…Phase E has trigonal symmetry and crystallizes in the rhombohedral space group $$R\bar{3}m$$; its structure consist of brucite‐like octahedral layers with tilted O‐H dipoles that are cross‐linked by statistically distributed Si‐tetrahedra and Mg‐octahedra (Kudoh et al., 1993; Tomioka et al., 2016). It has a nonstoichiometric formula depending on the hydrogen content and composition, and the Mg‐endmember has a general chemical formula of Mg 3−0.5x Si x H 6−3x O 6 , where x varies between 1 and 1.3, with a H 2 O storage capacity of 11–18 wt% (Purevjav et al., 2020; Tomioka et al., 2016). Two previous X‐ray diffraction (XRD) studies reported an isothermal bulk modulus of K T 0 = ∼93 GPa and its pressure derivative of $${K}_{T0}^{\prime }$$ = 5(1) for both iron‐free and iron‐bearing phase E (Crichton & Ross, 2000; Shieh et al., 2000).…”