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- W2079397768 abstract "Previous investigations on the olivine-spinel transformation are reviewed. The present investigation was carried out on synthetic (MgFe)2SiO4 olivine solid solutions using a Bridgman-anvil apparatus equipped with an internal heater. The experimental method and methods of calibration are described and the precision evaluated. More than 140 runs have been carried out in the pressure range 50–200 kb and approximately at 1000 °C. After quenching, the phases produced were examined by X-ray diffraction and optical methods. A continuous series of spinel solid solutions from Fe2SiO4 to (Mg0.8Fe0.2)2SiO4 was synthesized, and lattice parameters and refractive indices determined. The density of pure Mg2SiO4 was determined by extrapolation to be 3.56 g/cm3, some 10.6% denser than forsterite. A phase diagram for the Fe2SiO4Mg2SiO4 system was constructed on the basis of the experimental results. In compositions between Fe2SiO4 and (Mg0.8Fe0.2)2SiO4, olivines transform to spinels with a large two-phase loop of coexisting olivine plus spinel. Between (Mg0.8Fe0.2)2SiO4 and pure Mg2SiO4, olivines transform to a new orthorhombic phase, β-Mg2SiO4 which is 8% denser than forsterite. This phase is believed to be stable in its synthesis field. The olivine to beta phase transformation at 1000 °C occurs at approximately 120 kb. Reconnaissance investigations were also carried out on the Co2SiO4Mg2SiO4 system, in which the beta phase possesses a more extensive field of occurrence. The application of the experimental phase diagram to the constitution and seismic structure of the mantle is discussed. Assuming that the olivine of the mantle has an Mg/(Mg+Fe) ratio of 0.89, olivine would partially transform with increasing depth first to spinel. At a greater depth spinel reacts to produce beta phase, and at still greater depths, the remaining olivine transforms to beta phase. The depth interval over which these transformations occur is 27 km and contains a first order density discontinuity at the spinel-beta phase reaction point. Assuming a temperature in the mantle of 1600 °C at a depth of about 400 km, and a gradient for the transformations of 30 b/°C, the olivine-spinel-beta phase transformations would occur over an interval of about 27 km, with a median depth of 397 km and with a first order discontinuity at 403 km. This agrees closely with the depth of a major seismic discontinuity in the mantle. The experimental data thus provide a satisfactory explanation of this important geophysical feature of the mantle." @default.
- W2079397768 created "2016-06-24" @default.
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- W2079397768 date "1970-01-01" @default.
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- W2079397768 title "The system Mg2SiO4Fe2SiO4 at high pressures and temperatures" @default.
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- W2079397768 doi "https://doi.org/10.1016/0031-9201(70)90046-4" @default.
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