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Now showing items 1 - 7 of 7

  • Raman spectroscopy of water-rich stishovite and dense high-pressure silica up to 55 GPa

    Nisr, Carole   Shim, Sang-Heon   Leinenweber, Kurt   Chizmeshya, Andrew  

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  • Three-dimensional X-ray diffraction in the diamond anvil cell: application to stishovite

    Nisr, Carole   Ribárik, Gábor   Ungár, Tamás   Vaughan, Gavin B.M.   Merkel, Sébastien  

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  • High resolution three-dimensional X-ray diffraction study of dislocations in grains of MgGeO\r 3\r post-perovskite at 90 GPa

    Nisr, Carole   Ribárik, Gábor   Ungár, Tamás   Vaughan, Gavin B. M.   Cordier, Patrick   Merkel, Sébastien  

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  • Large H2O solubility in dense silica and its implications for the interiors of water-rich planets

    Nisr, Carole   Chen, Huawei   Leinenweber, Kurt   Chizmeshya, Andrew   Prakapenka, Vitali B.   Prescher, Clemens   Tkachev, Sergey N.   Meng, Yue   Liu, Zhenxian  

    Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in H2O-rich setting at high pressures and temperatures (P-T) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between SiO2 and H2O as archetypal materials for rock and ice, respectively, at high P-T. We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above similar to 24 GPa where the CaCl2-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of (Si1-xH4x)O-2 (x =3D 0.12). At pressures above 60 GPa, H2O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.
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  • High resolution three-dimensional X-ray diffraction study of dislocations in grains of MgGeO3 post-perovskite at 90 GPa

    Nisr, Carole   Ribarik, Gabor   Ungar, Tamas   Vaughan, Gavin B. M.   Cordier, Patrick   Merkel, Sebastien  

    MgGeO3 post-perovskite (pPv) is a lower pressure analogue of MgSiO3-pPv, which is believed to be the main constituent of the Earth's D ''. layer. Understanding the physical properties of this phase is critical to explain seismological observations as seismic anisotropy is likely linked to lattice preferred orientation in post-perovskite, which is governed by the motion of defects such as dislocations. Here, we apply in-situ three-dimensional X-ray diffraction to a polycrystalline sample of MgGeO3-pPv at 90 GPa. We demonstrate how the method can be used to follow individual grains within the material, including their individual orientations, positions, and strain tensors. We then use X-ray line profile analysis to characterize dislocations in the grains. The most probable slip systems are 1/2 < 110 > gliding on {(1) over bar 10} or (001) and [110](001), with a smaller contribution of [100](001) and [001] dislocations. These slip systems are shown to be consistent with observations of seismic anisotropy in D ''.
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  • Reliability of multigrain indexing for orthorhombic polycrystals above 1 Mbar:application to MgSiO3 post-perovskite

    Langrand, Christopher   Hilairet, Nadege   Nisr, Carole   Roskosz, Mathieu   Ribarik, Gabor   Vaughan, Gavin B. M.   Merkel, Sebastien  

    This paper describes a methodology for characterizing the orientation and position of grains of an orthorhombic polycrystalline material at high pressure in a diamond anvil cell. The applicability and resolution of the method are validated by simulations and tested on an experimental data set collected on MgSiO3 post-perovskite at 135 GPa. In the simulations, similar to 95% of the grains can be indexed successfully with similar to 80% of the peaks assigned. The best theoretical average resolutions in grain orientation and position are 0.02 degrees and 1.4 mu m, respectively. The indexing of experimental data leads to 159 grains of post-perovskite with 30% of the diffraction peaks assigned with a 0.2-0.4 degrees resolution in grain orientation. The resolution in grain location is not sufficient for in situ analysis of spatial relationships at high pressure. The grain orientations are well resolved and sufficient for following processes such as plastic deformation or phase transformation. The paper also explores the effect of the indexing parameters and of experimental constraints such as rotation range and step on the validity of the results, setting a basis for optimized experiments.
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  • Three-dimensional X-ray diffraction in the diamond anvil cell: application to stishovite

    Nisr, Carole   Ribarik, Gabor   Ungar, Tamas   Vaughan, Gavin B. M.   Merkel, Sebastien  

    Three-dimensional X-ray diffraction can be used for characterizing the orientation, position, and strain tensor of single grains in a polycrystalline aggregate. Here, we show how the method is well suited for diamond anvil cell data with heterogeneous grain sizes, with an application to two samples of stishovite at 15 and 26GPa. For each grain, we obtain a well-defined orientation matrix and cell parameters. Center of mass position can also be adjusted to the experimental data, with errors in the present experiment. Finally, strain tensors are adjusted for the individual grains. The stress distribution obtained is in agreement with expectations from the diamond anvil cell geometry and previous measurements of stishovite strength. Advantages and potential for improvement of the method are then discussed.
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