Masayuki Nishi

Crystal chemistry of dense hydrous magnesium silicates: The structure of phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C

Abstract The crystal structure of the dense hydrous magnesium silicate phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C, was investigated by single-crystal X-ray diffraction. Although showing a deterioration process under the X-ray beam, the compound was found to be orthorhombic, space group Pnnm (CaCl2- type structure), with lattice parameters a = 4.733(2), b = 4.3250(10), c = 2.8420(10) Å, V = 58.18(3) Å3, and Z = 1. The structure was refined to R1 = 0.0387 using 53 observed reflections [2s(I) level]. Magnesium and silicon were found to be disordered at the same octahedral site (with a mean bond distance of 1.957 Å). Hydrogen was not located in the difference Fourier maps, but it is very likely disordered at a half-occupied 4g position. The centrosymmetric nature of the structure of phase H is examined in relation to that reported for pure d-AlOOH at ambient conditions (non-centrosymmetric, P21nm), and the possibility that these two compounds can form a solid solution at least at high pressure is discussed.

The pyrite-type high-pressure form of FeOOH

Water transported into Earth’s interior by subduction strongly influences dynamics such as volcanism and plate tectonics. Several recent studies have reported hydrous minerals to be stable at pressure and temperature conditions representative of Earth’s deep interior, implying that surface water may be transported as far as the core–mantle boundary. However, the hydrous mineral goethite, α-FeOOH, was recently reported to decompose under the conditions of the middle region of the lower mantle to form FeO2 and release H2, suggesting the upward migration of hydrogen and large fluctuations in the oxygen distribution within the Earth system. Here we report the stability of FeOOH phases at the pressure and temperature conditions of the deep lower mantle, based on first-principles calculations and in situ X-ray diffraction experiments. In contrast to previous work suggesting the dehydrogenation of FeOOH into FeO2 in the middle of the lower mantle, we report the formation of a new FeOOH phase with the pyrite-type framework of FeO6 octahedra, which is much denser than the surrounding mantle and is stable at the conditions of the base of the mantle. Pyrite-type FeOOH may stabilize as a solid solution with other hydrous minerals in deeply subducted slabs, and could form in subducted banded iron formations. Deep-seated pyrite-type FeOOH eventually dissociates into Fe2O3 and releases H2O when subducted slabs are heated at the base of the mantle. This process may cause the incorporation of hydrogen into the outer core by the formation of iron hydride, FeHx, in the reducing environment of the core–mantle boundary.

Melting temperatures of MgO under high pressure by micro-texture analysis

Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earths lower mantle, and its melting behaviour under pressure is important to constrain rheological properties and melting behaviours of the lower mantle materials. Significant discrepancies exist between the melting temperatures of MgO determined by laser-heated diamond anvil cell (LHDAC) and those based on dynamic compressions and theoretical predictions. Here we show the melting temperatures in earlier LHDAC experiments are underestimated due to misjudgment of melting, based on micro-texture observations of the quenched samples. The high melting temperatures of MgO suggest that the subducted cold slabs should have higher viscosities than previously thought, suggesting that the inter-connecting textural feature of MgO would not play important roles for the slab stagnation in the lower mantle. The present results also predict that the ultra-deep magmas produced in the lower mantle are peridotitic, which are stabilized near the core–mantle boundary.

Partition of Al between Phase D and Phase H at high pressure: Results from a simultaneous structure refinement of the two phases coexisting in a unique grain

Abstract The crystal structure of the two dense hydrous magnesium silicates Phase D, MgSi2H2O6, and Phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C and coexisting in the same micrometer-sized grain, was investigated by single-crystal X‑ray diffraction to study the preferential partition of Al between the two structures. In agreement with the literature, Phase D was found to be trigonal, space group P3̅1m, with lattice parameters a = 4.752(2), c = 4.314(2) Å, V = 84.37(6) Å3 (R1 = 0.020), and Phase H was found to be orthorhombic, space group Pnnm, with lattice parameters a = 4.730(2), b = 4.324(2), c = 2.843(2) Å, V = 58.15(5) Å3 (R1 = 0.024). The estimated proportion (vol%) of the two phases from the refinement is 27(2)PhD - 73PhH. The analysis of the geometric details of the two structures shows that Phase D hosts almost all the Al available, whereas Phase H is nearly identical to pure MgSiH2O4. Overexposed electron-microprobe X‑ray maps of the same grain used for the X‑ray diffraction study together with WDS spots on the two phases confirmed the structural results. Thus, our results suggest that when Phase D and Phase H coexist, Al is strongly partitioned into Phase D at the expense of coexisting Phase H. At pressure above ~50 GPa, where Phase D is no longer stable, Phase H is able to incorporate the high aluminum contents present in hydrous peridotitic compositions in the deep lower mantle and be stabilized at the expense of Phase D and magnesium silicate perovskite.