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Theoretical calculations are performed to study transformations in silica as a function of nonhydrostatic stress. Molecular-dynamics calculations reveal a crystalline-to-crystalline transition from alpha-quartz to a phase with five-coordinated silicon (Si-V) at high pressure in the presence of deviatoric stress. The phase, which appears for specific orientations of the stress tensor relative to the crystallographic axes of quartz, is a crystalline polymorph of silica with five-coordinated silicon. The structure possesses P3(2)21 space-group symmetry. First-principles calculations within the local-density approximation, as well as molecular dynamics and energy minimization with interatomic potentials, find this phase to be mechanically and energetically stable with respect to quartz at high pressure. The calculated x-ray diffraction pattern and vibrational properties of the phase are reported. Upon decompression, the Si-V phase reverts to alpha-quartz through an intermediate four-coordinated phase and an unusual isosymmetrical phase transformation. The results suggest the importance of application of nonhydrostatic stress conditions in the design and synthesis of novel materials. NSF

Room temperature investigations on the shear stress and deformation mechanisms of (Mg0.9Fe0.1)SiO3 perovskite are performed in situ up to 32 GPa using radial X-ray diffraction and the diamond anvil cell as a deformation apparatus. The uniaxial stress supported by the perovskite aggregate is found to increase continuously with pressure up to 10.9(+/-1.9) GPa at 32(+/-1) GPa. Our measurements show no development of significant lattice preferred orientations in the sample, which indicates that deformation by dislocation glide is not the dominant deformation mechanism under these conditions. Assuming that the underlying cause for seismic anisotropy in the deep Earth is elastic anisotropy combined with lattice preferred orientation, our results indicate that silicate perovskite deformed under the conditions of this experiment would not be the source of seismic anisotropy. (C) 2003 Elsevier Science B.V. All rights reserved.

Synchrotron x-ray imaging and stress measurements of diamond-anvil cell gaskets revealed large elastic strains at the diamond tip at a pressure of 300 gigapascals. The diamond, generally considered a rigid body, bent 16 degrees over a distance of 300 micrometers without failure. To complement these measurements, a technique was developed that permits x-ray diffraction to be measured through a beryllium gasket. Measurements on tungsten and iron revealed the strain anisotropy, deviatoric stress, and texture and showed that the yield strengths of these materials increase by up to two orders of magnitude at confining pressures of 200 to 300 gigapascals. The results allow identification of the maximum amount of strain accommodated by the anvil tips before failure. Further development of ultrahigh pressure techniques requires relieving stress concentrations associated with this large elastic deformation.

International audience; We use resonant inelastic x-ray scattering to study the electronic structure of nickel oxide, the prototype charge-transfer insulator, as a function of pressure. At ambient pressure, we observe spectral features due to the charge-transfer excitation and the Coulomb correlation energy which progressively smear up to a pressure of 100 GPa. These changes are interpreted as due to increased dispersion of the concerned electronic bands. This opens new perspectives both for the study of electronic structure of correlated materials and for high-pressure research such as a deeper understanding of metal-insulator transitions, as well as the study of deep-earth chemistry.

The differentiation of Earth into a metallic core and silicate mantle left its signature on the chemical and isotopic composition of the bulk silicate Earth (BSE). This is seen in the depletion of siderophile (metal-loving) relative to lithophile (rock-loving) elements in Earth’s mantle as well as the silicon isotope offset between primitive meteorites (i.e. bulk Earth) and BSE, which is generally interpreted as a proof that Si is present in Earth’s core. Another putative light element in Earth’s core is sulphur; however, estimates of core S abundance vary significantly and, due to its volatile nature, no unequivocal S isotopic signature for core fractionation has thus far been detected. Here we present new high precision isotopic data for Cu, a chalcophile (sulphur-loving) element, which shows that Earth’s mantle is isotopically fractionated relative to bulk Earth. Results from high pressure equilibration experiments suggest that the sense of Cu isotopic fractionation between BSE and bulk Earth requires that a sulphide-rich liquid segregated from Earth’s mantle during differentiation, which likely entered the core. Such an early-stage removal of a sulphide-rich phase from the mantle presents a possible solution to the long-standing 1st terrestrial lead paradox.

Recognizing existing materials that can act as proxies for Earth's building blocks, and understanding the accretionary pathway taken during Earth's growth, persist as outstanding issues in need of resolution. In Mahan, Siebert, Blanchard, Badro, et al. (2018, ), we conducted diamond anvil cell (DAC) Cu metal-silicate partitioning experiments and coupled these results with a large complement of literature data to characterize Cu metal-silicate partitioning during Earth's core formation and accretion history. The Comment of Jennings, Wade, and Llovet (2019, ) contends that secondary X-ray fluorescence, originating from the Cu holders that experiments are routinely welded to ("lift-out" grids), compromises the novel Cu partitioning data of Mahan, Siebert, Blanchard, Badro, et al. (2018) beyond utility. To dispel these concerns and further validate our data, we have (i) investigated secondary X-ray fluorescence effects in a Cu-free experiment and provided a matrix-matched data correction, and (ii) rewelded a DAC experiment from a Cu grid to a Mo grid for a comparison of compositional analyses and Cu partitioning results. Secondary fluorescence results, in fact much like the simulated results in Jennings, Wade, and Llovet (2019), indicate that this effect is essentially equal in the metal and silicate phases and therefore has no actual impact on Cu metal-silicate partition coefficients. Moreover, Cu concentrations and partition coefficients determined using the Mo grid are statistically indistinguishable from that determined using the Cu grid. All results decisively illustrate that while secondary X-ray fluorescence must be considered where absolute concentrations are the final objective, it has had no meaningful impact on the partitioning data and observations of Mahan, Siebert, Blanchard, Badro, et al. (2018).

Gallium concentration (normalized to CI chondrites) in the mantle is at the same level as that of lithophile elements with similar volatility, implying that there must be little to no gallium in Earth's core. Metal-silicate partitioning experiments, however, have shown that gallium is a moderately siderophile element and should be therefore depleted in the mantle by core formation. Moreover, gallium concentrations in the mantle (4 ppm) are too high to be only brought by the late veneer; and neither pressure, nor temperature, nor silicate composition has a large enough effect on gallium partitioning to make it lithophile. We therefore systematically investigated the effect of core composition (light element content) on the partitioning of gallium by carrying out metal-silicate partitioning experiments in a piston-cylinder press at 2 GPa between 1673 K and 2073 K. Four light elements (Si, O, S, C) were considered, and their effect was found to be sufficiently strong to make gallium lithophile. The partitioning of gallium was then modeled and parameterized as a function of pressure, temperature, redox and core composition. A continuous core formation model was used to track the evolution of gallium partitioning during core formation, for various magma ocean depths, geotherms, core light element contents, and magma ocean composition (redox) during accretion. The only model for which the final gallium concentration in the silicate Earth matched the observed value is the one involving a light-element rich core equilibrating in a FeO-rich deep magma ocean (>1300 km) with a final pressure of at least 50. GPa. More specifically, the incorporation of S and C in the core provided successful models only for concentrations that lie far beyond their allowable cosmochemical or geophysical limits, whereas realistic 0 and Si amounts (less than 5 wt.%) in the core provided successful models for magma oceans deeper that 1300 km. These results offer a strong argument for an O- and Si-rich core, formed in a deep terrestrial magma ocean, along with oxidizing conditions. (C) 2015 Elsevier B.V. All rights reserved.

The infrared and Raman spectra of CaCO3 calcite substituted with 80% O-18 have been recorded. A detailed mode assignment is proposed for all the observed bands, including combinations and overtones. These data are used to propose a simplified model of the vibrational density of states (VDOS) from which the specific heat, the entropy, and the high-temperature equation of state are calculated. Excellent agreement between calculated and measured values of the thermodynamic properties is obtained when measured vibrational mode anharmonicity is included in the calculations. The model can be used to infer the properties of calcite at high pressures (less than or equal to 3 Gpa) and temperatures (less than or equal to 1200 K). The observed frequency shifts induced by O-18 substitution in both IR and Raman spectra are used to construct the VDOS of (CaCO3)-O-18 calcite. The reduced partition function of calcite is then calculated and the effects of anharmonicity are discussed. Finally the effect of pressure on the reduced partition function is calculated and is shown to be appreciable even at high temperatures.