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Akimotoite, ilmenite-type MgSiO3 high-pressure polymorph can be stable in the lower-mantle transition zone along average mantle and subducting slab geotherms. Significant amounts of Al2O3 can be incorporated into the structure, having the pyrope (Mg3Al2Si3O12) composition. Previous studies have investigated the effect of Al2O3 on its crystal structure at nearly endmember compositions. In this study, we synthesized high-quality ilmenite-type Mg3Al2Si3O12 phase at 27 GPa and 1073 K by means of a Kawai-type multi-anvil press and refined the crystal structure at ambient conditions using a synchrotron X-ray diffraction data via the Rietveld method to examine the effect of Al2O3. The unit-cell lattice parameters were determined to be a = 4.7553(7) Å, c = 13.310(2) Å, and V = 260.66(6) Å3, with Z = 6 (hexagonal, R 3 ¯ ). The volume of the present phase was placed on the akimotoite-corundum endmember join. However, the refined structure showed a strong nonlinear behavior of the a- and c-axes, which can be explained by Al incorporation into the MgO6 and SiO6 octahedral sites, which are distinctly different each other. Ilmenite-type Mg3Al2Si3O12 phase may be found in shocked meteorites and can be a good indicator for shock conditions at relatively low temperatures of 1027–1127 K. [ABSTRACT FROM AUTHOR]

Here we report on the discovery of a ternary silicon titanium nitride with the general composition (Si1−x,Tix)3N4 with x = 0 < x < 1 and spinel-type crystal structure. The novel nitride is formed from an amorphous silicon titanium nitride (SiTiN) precursor under high-pressure/high-temperature conditions in a large volume high-pressure device. Under the conditions of 15–20 GPa and 1800–2000 °C, spinel-type γ-Si3N4 and rock salt-type c-TiN are formed. In addition, crystals of the discovered nano-sized ternary phase (Si1−x,Tix)3N4 embedded in γ-Si3N4 are identified. The ternary compound is formed due to kinetically-controlled synthesis conditions and is analyzed to exhibit the spinel-type structure with ca. 8 atom% of Ti. The Ti atoms occur in both Ti3+ and Ti4+ oxidation states and are located on the Si sites. The ternary nano-crystals have to be described as (Si,Ti)3N4 with N-vacancies resulting in the general composition (Si4+1−x Ti4+x-δTi3+δ)3N4-δ. [ABSTRACT FROM AUTHOR]

Ferric iron can be incorporated into the crystal structure of bridgmanite by either oxygen vacancy substitution (MgFeO2.5 component) or charge‐coupled substitution (FeFeO3 component) mechanisms. We investigated the concentrations of MgFeO2.5 and FeFeO3 in bridgmanite in the MgO‐SiO2‐Fe2O3 system at 27 GPa and 1700–2300 K using a multianvil apparatus. The FeFeO3 content increases from 1.6 to 7.6 mol.% and from 5.7 to 17.9 mol.% with and without coexistence of (Mg,Fe)O, respectively, with increasing temperature from 1700 to 2300 K. In contrast, the MgFeO2.5 content does not show clear temperature dependence, that is, ~2–3 and < 2 mol.% with and without the coexistence of (Mg,Fe)O, respectively. Therefore, the presence of (Mg,Fe)O enhances the oxygen vacancy substitution for Fe3+ in bridgmanite. It is predicted that Fe3+ is predominantly substituted following the oxygen vacancy mechanism in (Mg,Fe)O‐saturated Al‐free bridgmanite when Fe3+ is below ~0.025 pfu, whereas the charge‐coupled mechanism occurs when Fe3+ > 0.025 pfu. Plain Language Summary: Bridgmanite, the most abundant mineral of the Earth's lower mantle, can contain Fe3+ although the valance of iron is 2+ in general. An important question is how Fe3+ is substituted in the crystal structure of bridgmanite. It may form the MgFeO2.5 component, in which oxygen anions are partly missing. Or it may form the FeFeO3 component, which has no missing cations or anions. Since bridgmanite is present in the lower mantle together with (Mg,Fe)O, we investigated the MgFeO2.5 and FeFeO3 contents in Al‐free bridgmanite that coexists with and without (Mg,Fe)O under the topmost lower mantle conditions. The results show that the presence of (Mg,Fe)O enhances the formation of MgFeO2.5. The solubility of MgFeO2.5 component is about 2.5 mol.% in bridgmanite that coexists with (Mg,Fe)O, whereas it is nearly zero when (Mg,Fe)O is absent. Key Points: The presence of (Mg,Fe)O enhances the formation of the MgFeO2.5 component in Al‐free bridgmaniteFe3+ substitution predominantly follows the oxygen vacancy mechanism in (Mg,Fe)O‐saturated Al‐free bridgmanite when Fe3+ content is lowThe solubility of the MgFeO2.5 component in Al‐free bridgmanite is about 0.025 pfu, and relatively insensitive to temperature [ABSTRACT FROM AUTHOR]

High pressure and temperature synthesis of compositions made of (Si1-x,Gex)O2 where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7-12 GPa and 1200-1600°C using a Kawai-type high-pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile-type phases in the SiO2-GeO2 system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si0.5,Ge0.5)O2 into SiO2-rich coesite and GeO2-rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite-stishovite phase transition pressure in SiO2. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ionic conductivity in olivine has a positive f O 2 dependence. • Ionic mechanism has large contribution to the upper mantle conductivity profile. • High conductivity in the asthenosphere can be explained by olivine only. The oxygen fugacity (f O 2 ) may affect the ionic conductivity of olivine under upper mantle conditions because Mg vacancies can be produced in the crystal structure by the oxidization of iron from Fe2+ to Fe3+. Here we investigated olivine ionic conductivity at 4 GPa, as a function of temperature, crystallographic orientation, and oxygen fugacity, corresponding to the topmost asthenospheric conditions. The results demonstrate that the ionic conductivity is insensitive to f O 2 under relatively reduced conditions (f O 2 below Re-ReO 2 buffer), whereas it has a clear f O 2 -dependence under relatively oxidized conditions (f O 2 around the magnetite-hematite buffer). The ionic conduction in olivine may contribute significantly to the conductivity anomaly in the topmost asthenosphere especially at relatively oxidized conditions. [ABSTRACT FROM AUTHOR]