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

Pressure tuning of charge ordering in iron oxide

A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (Fe$_4$O$_5$) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in Fe$_4$O$_5$ and strongly affect the ordering temperature. We report two charge-ordered phases, the first of which may comprise both dimeron and trimeron units, whereas, the second exhibits an overall dimerization involving both the octahedral and trigonal-prismatic chains of iron in the crystal structure. We link the dramatic change in the charge-ordering pattern in the second phase to redistribution of electrons between the octahedral and prismatic iron chains, and propose that the average oxidation state of the iron cations can pre-determine a charge-ordering pattern

The S<SUB>3</SUB><SUP>-</SUP> Ion Is Stable in Geological Fluids at Elevated Temperatures and Pressures

International audienceThe chemical speciation of sulfur in geological fluids is a controlling factor in a number of processes on Earth. The two major chemical forms of sulfur in crustal fluids over a wide range of temperature and pressure are believed to be sulfate and sulfide; however, we use in situ Raman spectroscopy to show that the dominant stable form of sulfur in aqueous solution above 250°C and 0.5 gigapascal is the trisulfur ion S3-. The large stability range of S3- enables efficient transport and concentration of sulfur and gold by geological fluids in deep metamorphic and subduction-zone settings. Furthermore, the formation of S3- requires a revision of sulfur isotope-fractionation models between sulfides and sulfates in natural fluids

High Temperatures ^ High Pressures, 2003/2004, volume 35/36, pages 47 ^ 53 DOI:10.1068/htjr073 Pressure-induced phase transformations in tetragonal and rhombohedral C60 polymers

Abstract. Tetragonal and rhombohedral 2D polymers of C 60 were studied by Raman spectroscopy in situ under high-pressure conditions. Unlike previous reports, no new phase was observed upon compression of the tetragonal polymer to 32 GPa. Above 20 GPa both tetragonal and rhombohedral polymers undergo irreversible amorphisation. The same results were obtained for samples studied without a pressure-transmitting medium and when a methanol ^ ethanol mixture was used. In the pressure range below 20 GPa, both tetragonal and rhombohedral polymers partially decompose to chains and monomers during compression.

Polymorphs of the Gadolinite-Type Borates ZrB2O5 and HfB2O5 Under Extreme Pressure

Based on the results from previous high-pressure experiments on the gadolinite-type mineral datolite, CaBSiO4(OH), the behavior of the isostructural borates beta-HfB2O5 and beta-ZrB2O5 have been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction experiments. On compression to 120 GPa, both borate layer-structures are preserved. Additionally, at approximate to 114 GPa, the formation of a second phase can be observed in both compounds. The new high-pressure modification gamma-ZrB2O5 features a rearrangement of the corner-sharing BO4 tetrahedra, while still maintaining the four- and eight-membered rings. The new phase gamma-HfB2O5 contains ten-membered rings including the rare structural motif of edge-sharing BO4 tetrahedra with exceptionally short B-O and B...B distances. For both structures, unusually high coordination numbers are found for the transition metal cations, with ninefold coordinated Hf4+, and tenfold coordinated Zr4+, respectively. These findings remarkably show the potential of cold compression as a low-energy pathway to discover metastable structures that exhibit new coordinations and structural motifs.Funding Agencies|Federal Ministry of Education and Research, Germany (BMBF)Federal Ministry of Education &amp; Research (BMBF) [05K19WC1]; Deutsche Forschungsgemeinschaft (DFG)German Research Foundation (DFG) [DU 954-11/1, DU 393-9/2, DU 393-13/1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]</p

Polymorphs of the Gadolinite‐Type Borates ZrB2_2O5_5 and HfB2_2O5_5 Under Extreme Pressure

Based on the results from previous high-pressure experiments on the gadolinite-type mineral datolite, CaBSiO$_4$(OH), the behavior of the isostructural borates β-HfB$_2$O$_5$ and β-ZrB$_2$O$_5$ have been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction experiments. On compression to 120 GPa, both borate layer-structures are preserved. Additionally, at ≈114 GPa, the formation of a second phase can be observed in both compounds. The new high-pressure modification γ-ZrB$_2$O$_5$ features a rearrangement of the corner-sharing BO$_4$ tetrahedra, while still maintaining the four- and eight-membered rings. The new phase γ-HfB$_2$O$_5$ contains ten-membered rings including the rare structural motif of edge-sharing BO$_4$ tetrahedra with exceptionally short B−O and B⋅⋅⋅B distances. For both structures, unusually high coordination numbers are found for the transition metal cations, with ninefold coordinated Hf$^{4+}$, and tenfold coordinated Zr$^{4+}$, respectively. These findings remarkably show the potential of cold compression as a low-energy pathway to discover metastable structures that exhibit new coordinations and structural motifs

Compressibility of hingganite-(Y): high-pressure single crystal X-ray diffraction study

Behaviour of hingganite-(Y), Y$_2\square$Be$_2$Si$_2$O$_8$(OH)$_2$, on compression to 47 GPa has been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction at room temperature in a diamond anvil cell. In the studied pressure range no obvious phase transitions have been observed. The compression of hingganite-(Y) crystal structure is anisotropic, with b axis showing the maximal compressibility. A fit of the experimental pressure–volume data by the Birch-Murnaghan third-order equation of state yielded the bulk modulus of 131(2) GPa and its pressure first derivative of 3.5(2). The difference between high-pressure behaviour of hingganite-(Y) and structurally related datolite is governed by the different chemical nature of interlayer cations

Tuning of the stoichiometry of Fe1−xO wüstite by compression

International audienceWe report the results of the investigation of the thermoelectric powe