Crystal chemistry and compressibility of Fe0.5Mg0.5Al0.5Si0.5O3 and FeMg0.5Si0.5O3 silicate perovskites at pressures up to 95 GPa

Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth’s lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe3+-rich phases, FeMg0.5Si0.5O3 and Fe0.5Mg0.5Al0.5Si0.5O3, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg0.5Si0.5O3 phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe0.5Mg0.5Al0.5Si0.5O3 phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe3+ in a (Fe0.7,Mg0.3)O6 octahedral site of the FeMg0.5Si0.5O3 phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe0.5Mg0.5Al0.5Si0.5O3 phase as all Fe3+ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al0.5,Si0.5)O6

Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth's upper mantle

Majoritic garnet has been predicted to be a major component of peridotite and eclogite in Earth's deep upper mantle (>250 km) and transition zone. The investigation of mineral inclusions in diamond confirms this prediction, but there is reported evidence of other majorite-bearing lithologies, intermediate between peridotitic and eclogitic, present in the mantle transition zone. If these lithologies are derived from olivine-free pyroxenites, then at mantle transition zone pressures majorite may form monomineralic or almost monomineralic garnetite layers. Since majoritic garnet is presumably the seismically fastest major phase in the lowermost upper mantle, the existence of such majorite layers might produce a detectable seismic signature. However, a test of this hypothesis is hampered by the absence of sound wave velocity measurements of majoritic garnets with relevant chemical compositions, since previous measurements have been mostly limited to synthetic majorite samples with relatively simple compositions. In an attempt to evaluate the seismic signature of a pyroxenitic garnet layer, we measured the sound wave velocities of three natural majoritic garnet inclusions in diamond by Brillouin spectroscopy at ambient conditions. The chosen natural garnets derive from depths between 220 and 470 km and are plausible candidates to have formed at the interface between peridotite and carbonated eclogite. They contain elevated amounts (12–30%) of ferric iron, possibly produced during redox reactions that form diamond from carbonate. Based on our data, we model the velocity and seismic impedance contrasts between a possible pyroxenitic garnet layer and the surrounding peridotitic mantle. For a mineral assemblage that would be stable at a depth of 350 km, the median formation depth of our samples, we found velocities in pyroxenite at ambient conditions to be higher by 1.9(6)% for shear waves and 3.3(5)% for compressional waves compared to peridotite (numbers in parentheses refer to uncertainties in the last given digit), and by 1.3(13)% for shear waves and 2.4(10)% for compressional waves compared to eclogite. As a result of increased density in the pyroxenitic layer, expected seismic impedance contrasts across the interface between the monomineralic majorite layer and the adjacent rocks are about 5–6% at the majorite-eclogite-interface and 10–12% at the majoriteperidotite-boundary. Given a large enough thickness of the garnetite layer, velocity and impedance differences of this magnitude could become seismologically detectable

Polymorphism of feldspars above 10 GPa

Feldspars are rock-forming minerals that make up most of the Earth’s crust. Along the mantle geotherm, feldspars are stable at pressures up to 3 GPa and may persist metastably at higher pressures under cold conditions. Previous structural studies of feldspars are limited to ~10 GPa, and have shown that the dominant mechanism of pressure-induced deformation is the tilting of AlO4 and SiO4 tetrahedra in a tetrahedral framework. Herein, based on results of in situ single-crystal X-ray diffraction studies up to 27 GPa, we report the discovery of new high-pressure polymorphs of the feldspars anorthite (CaSi2Al2O8), albite (NaAlSi3O8), and microcline (KAlSi3O8). The phase transitions are induced by severe tetrahedral distortions, resulting in an increase in the Al and/or Si coordination number. High-pressure phases derived from feldspars could persist at depths corresponding to the Earth upper mantle and could possibly influence the dynamics and fate of cold subducting slabs

DataSheet1_Crystal chemistry and compressibility of Fe0.5Mg0.5Al0.5Si0.5O3 and FeMg0.5Si0.5O3 silicate perovskites at pressures up to 95 GPa.pdf

Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth’s lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe3+-rich phases, FeMg0.5Si0.5O3 and Fe0.5Mg0.5Al0.5Si0.5O3, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg0.5Si0.5O3 phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe0.5Mg0.5Al0.5Si0.5O3 phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe3+ in a (Fe0.7,Mg0.3)O6 octahedral site of the FeMg0.5Si0.5O3 phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe0.5Mg0.5Al0.5Si0.5O3 phase as all Fe3+ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al0.5,Si0.5)O6.</p

Front Cover: Chemical Stability of FeOOH at High Pressure and Temperature, and Oxygen Recycling in Early Earth History (Eur. J. Inorg. Chem. 30/2021)

The Front Cover shows a possible deep oxygen cycle in early Earth. FeOOH (“rust”), produced by anoxygenic photosynthesis and accumulated on the ocean floor, was transferred to the lower mantle by subducting slabs. At high pressures and temperatures, FeOOH decomposes into a mixture of complex iron oxides, water, and oxygen. Oxidizing fluids rising to the Earth′s surface could then possibly contribute to (or even be one of the main causes of) the Great Oxidation Event about 2.5 billion years ago. Image credits: Egor Koemets, Timofey Fedotenko, Leonid Dubrovinsky (Bayreuth University). More information can be found in the Full Paper by E. Koemets and co-workers

Chemical Stability of FeOOH at High Pressure and Temperature, and Oxygen Recycling in Early Earth History

Goethite, $α$-FeOOH, is a major phase among oxidized iron species, commonly called rust. We studied the behavior of iron (III) oxyhydroxide up to 81 GPa and 2100 K using in situ synchrotron single-crystal X-ray diffraction. At high pressure-temperature conditions FeOOH decomposes forming oxygen-rich fluid and different mixed valence iron oxides (previously known phases of Fe$_2$O$_3$, Fe$_3$O$_4$, Fe$_5$O$_7$, and novel Fe$_7$O$_{10}$ and Fe$_{6.32}$O$_9$). Rust is known to form as a byproduct of anoxygenic prokaryote metabolism that took place massively from about 3.8 billion years (Ga) ago until the Great Oxidation Event (GOE) ∼2.2 Ga ago. Rust was buried on the ocean floor and was transported into the mantle as a consequence of plate tectonics (started ∼2.8 Ga ago). Our results suggest that recycling of rust in Earth's mantle contributes to redox conditions of the early Earth and formation of oxygen-rich atmosphere

High-Pressure Yttrium Nitride, Y5N14Y_{5}N_{14}, Featuring Three Distinct Types of Nitrogen Dimers

Yttrium nitride, $Y_{5}N_{14}$, was synthesized by direct reaction between yttrium and nitrogen at ∼50 GPa and ∼2000 K in a laser-heated diamond anvil cell. High-pressure single-crystal X-ray diffraction revealed that the crystal structure of $Y_{5}N_{14}$ (space group P4/mbm) contains three distinct types of nitrogen dimers. Crystal chemical analysis and ab initio calculations demonstrated that the dimers [N$_2$]$^{x−}$ are crystallographically and chemically nonequivalent and possess distinct noninteger formal charges (x) that make $Y_{5}N_{14}$ unique among known compounds. Theoretical computations showed that $Y_{5}N_{14}$ has an anion-driven metallicity, with the filled part of its conduction band formed by nitrogen p-states. The compressibility of $Y_{5}N_{14}$, determined on decompression down to ∼10 GPa, was found to be uncommonly high for dinitrides containing +3 cations (the bulk modulus K$_0$ = 137(6) GPa)

Subduction-related oxidation of the lower mantle: evidence from diamond inclusions

Ferropericlase-magnesiowüstite (Mg,Fe)O is the second most abundant mineral in Earth’s lower mantle and the most common inclusion found in lower mantle diamonds. In pyrolitic mantle, it should theoretically contain < 20 mol% FeO. However, where the mineral is found as inclusions in diamond it shows a broad range of Mg# (Mg/(Mg+Fe)) between 12 and 93. Here we use synchrotron Mössbauer source spectroscopy (SMS), complemented by single-crystal X-ray diffraction to determine the structure and iron oxidation state of two magnesiowüstite and three ferropericlase inclusions in diamonds from São Luiz, Brazil. The Mg# of these inclusions varies between 16.1 and 84.5. All three ferropericlase inclusions are monomineralic and contain no ferric iron within the detection limit of SMS. Both magnesiowüstite inclusions show the presence of monocrystalline magnesioferrite (Mg,Fe)Fe$^{3+}$$_2$O$_4$) with an estimated 47-53 wt% of Fe$_2$O$_3$. We argue that the wide range of Fe concentrations observed in (Mg,Fe)O inclusions in diamonds and the appearance of magnesioferrite in Fe-rich inclusions result from oxidation of ferropericlase triggered by the introduction of subducted material into the lower mantle. Subducted carbonates react with Fe-metal to generate diamond and magnesiowüstite. Upon local metal exhaustion, the reaction continues with oxidation of magnesiowüstite into magnesioferrite, forming highly oxidised areas in the lower mantle

Complete agreement of the post-spinel transition with the 660-km seismic discontinuity

The 660-km seismic discontinuity, which is a significant structure in the Earth’s mantle, is generally interpreted as the post-spinel transition, as indicated by the decomposition of ringwoodite to bridgmanite + ferropericlase. All precise high-pressure and high-temperature experiments nevertheless report 0.5–2 GPa lower transition pressures than those expected at the discontinuity depth (i.e. 23.4 GPa). These results are inconsistent with the post-spinel transition hypothesis and, therefore, do not support widely accepted models of mantle composition such as the pyrolite and CI chondrite models. Here, we present new experimental data showing post-spinel transition pressures in complete agreement with the 660-km discontinuity depth obtained by high-resolution in situ X-ray diffraction in a large-volume high-pressure apparatus with a tightly controlled sample pressure. These data affirm the applicability of the prevailing mantle models. We infer that the apparently lower pressures reported by previous studies are experimental artefacts due to the pressure drop upon heating. The present results indicate the necessity of reinvestigating the position of mantle mineral phase boundaries previously obtained by in situ X-ray diffraction in high-pressure–temperature apparatuses