A new cubic perovskite in PbGeO3 at high pressures
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A new cubic perovskite polymorph of PbGeO3 (Phase II) was synthesized by laser heating in the diamond-anvil cell (DAC) at the pressure of 36 GPa. Fitting the Birch-Murnaghan equation of state against its observed P-V data yields a bulk modulus K0 of 196(6) GPa and the volume V0 of 56.70(13) Å3 when K0′ is assumed being 4. After the pressure is released, the PbGeO3 Phase II changes gradually into an amorphous phase, which contains mainly fourfold-coordinated germanium. It indicates that the PbGeO3 Phase II with a GeO6 octahedron framework transforms to a GeO4 tetrahedron network during the amorphization. The existence of PbGeO3 cubic perovskite Phase II at high pressures indicates that the polarized character of the Pb2+ ion induced by its 6s2 lone pair electrons would be totally reduced in the environment of major silicate perovskites inside the lower mantle, and thus the Pb atom would substitute the Ca atom to enter the CaSiO3 perovskite.Keywords:
Diamond anvil cell
Silicate perovskite
Stishovite
Diamond cubic
Available thermodynamic data and seismic models favor perovskite (MgSiO3) as the stable phase in the mantle. MgSiO3 was heated at temperatures from 1900 to 3200 kelvin with a Nd-YAG laser in diamond-anvil cells to study the phase relations at pressures from 45 to 100 gigapascals. The quenched products were studied with synchrotron x-ray radiation. The results show that MgSiO3 broke down to a mixture of MgO (periclase) and SiO2 (stishovite or an unquenchable polymorph) at pressures from 58 to 85 gigapascals. These results imply that perovskite may not be stable in the lower mantle and that it might be necessary to reconsider the compositional and density models of the mantle.
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Periclase
Diamond anvil cell
Post-perovskite
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Abstract We have investigated the equation of state of Fe‐bearing bridgmanite, (Mg 0.9 Fe 0.1 )SiO 3 , using synchrotron X‐ray diffraction in diamond anvil cells up to 125 GPa and 300 K. Combined with previous synchrotron Mössbauer spectroscopy results, we have found that the occurrence of the low‐spin Fe 3+ in the octahedral sites (B site) of bridgmanite has produced a 0.5(±0.1)% reduction in the unit cell volume at 18–25 GPa and has increased the isothermal bulk modulus to 284(±4) GPa, consistent with recent theoretical calculations. Together with literature results, we note that the addition of Fe can cause an increase in the density, bulk modulus, and bulk sound velocity in both Al‐free and Al‐bearing bridgmanite at lower mantle pressures. The presence of Fe 3+ in the B site of bridgmanite can further enhance this increase. The observed spin transition of B site Fe 3+ in bridgmanite is thus important for understanding the density and velocity structures of the lower mantle.
Silicate perovskite
Spin transition
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Spin states
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Stishovite
Silicate perovskite
Peridotite
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Stishovite
Ultramafic rock
Silicate perovskite
Wüstite
Periclase
Enstatite
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AbstractThree-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 26 GPa. 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.Keywords: high pressurediamond anvil cellthree-dimensional X-ray diffractionmicrostructurestishovitetexture AcknowledgementsThe author wish to thank Masaki Akaogi for providing the stishovite sample, Patrick Cordier for fruitful discussions, Michael Hanfland for sharing his laser heating system, the ESRF high pressure sample environment support for providing diamond anvil cells, and the ESRF for the allocation of beamtime. This work was supported by the ANR "Dislocations Under Pressure (DIUP)" program (ANR-07-5CJC-0136-01), and the Hungarian National Innovation Office, the French Ministry of Research and Education, and the French Ministry of Foreign Affairs through the Partenariat Hubert Curien (PHC) Balaton (projects 19479 PB and 27861 QJ).Notes†This paper was presented at the LIth European High Pressure Research Group (EHPRG 51) Meeting in London (UK), 1–6 September 2013.
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Stishovite
Silicate perovskite
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Silicate minerals
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Stishovite
Ringwoodite
Silicate perovskite
Pyroxene
Silicate minerals
Coesite
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Investigations of electron spin density by magnetic compton scattering with high energy synchrotron radiation, and magnetization density, i.e.Spin plus orbital components, by diffraction at conventional x-ray energies will be presented for a number of rare earth and transition metal ferromagnetic compounds and alloys.The methods for retrieving the weak magnetic signal from the charge scattering will be discussed and the success of the methods separately, and when combined, will be illustrated by reviewing recent results obtained by the warwick group at esrf and collaborators elsewhere.
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Calcium silicate
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Abstract High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24–25 GPa and 1673–1873 K with 5–10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO 3 , (Mg, Fe)SiO 3 , (Mg, Al, Si)O 3 , and (Mg, Fe, Al, Si)O 3 . Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO 2 in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO 2 ratio than the initial material. (Mg + Fe)O‐rich hydrous melt could explain the low‐velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid‐lower mantle and may explain the “stishovite paradox." Since stishovite‐rich materials are formed only when silica‐rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates.
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