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Silicon-bearing rutile has been found in chromitite from the Luobusa (罗布莎) ophiolite, Tibet. However, the extent of SiO2 solubility in rutile and the nature of its origin are still unclear. At high pressure, SiO2 takes a rutile structure with Si in 6-fold coordination. Thus, high pressures may enhance its solubility in rutile because of possible isovalent exchange in the oetabedral site. In this study, we report new experimental results on SiO2 solubility in rutile up to 23 GPa and 2 000 ℃. Starting materials were mixtures of powdered pure rutile and pure quartz, with compositions of (Ti0.5Si0.5)O2,(Ti0.93Si0.07)O2, and (Ti0.75Si0.25)O2. The mixtures were loaded into either platinum capsules (for a 10/5 assembly) or rhenium capsules (for an 8/3 assembly). The experiments were carried out using multi-anvil high-pressure apparatus with a rhenium resistance heater. Sample temperatures were measured with a W5%Re-W26%Re thermoeouple and were controlled within ±1 ℃ of the set temperature. TiO2-rich and SiO2-rich phases were produced in all the quenched samples. Microprobe analyses of the phases show that the solubility of SiO2 in rutile increases with increasing pressure, from 1.5 wt.% SiO2 at 10 GPa to 3.8 wt.% SiO2 at 23 GPa at a temperature of 1 800 ℃. The solubility also increases with increasing temperature from 0.5 wt.% SiO2 at 1 500 ℃ to 4.5 wt.% SiO2 at 2 000 ℃ at a pressure of 18 GPa. On the other hand, the solubility of TiO2 in coesite or stishovite is very limited, with an average of 0.6 wt.% TiO2 over the experimental P-T ranges. Temperature has a much larger effect on the solubility of SiO2 in rutile than pressure. At high pressure, the melting point of SiO2 is defmitely higher than that of TiO2 and the eutectic point moves towards SiO2 in the TiO2-SiO2 system. Lower oxygen fugacity decreases the solubility of SiO2 in rutile, whereas water has little effect on the solubility. Our experimental data are extremely useful for determining the depth of origin of the SiO2-bearing ruffle found in nature. 相似文献
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The charge density and bond character of the rutile-type structure of SiO2 (stishovite) under compression to 30 GPa were investigated by X-ray diffraction study using synchrotron radiation and AgKα rotating anode X-ray generator through a newly devised diamond-anvil cell. The valence electron density was determined by
least-squares refinement including the κ parameter and the electron population in the X-ray atomic scattering parameters.
The oxygen κ-parameter of SiO2 is 0.94 under ambient conditions and 1.11 at 29.1 GPa and the silicon valence changes from +2.12(8) at ambient pressure to
+2.26(15) at 29.1 GPa. These values indicate that the electron distributions are more localized with increasing pressure.
The difference Fourier map shows the deformation of the valence electron distribution and the bonding electron population
in residual electron densities. The bonding electron observed from the X-ray diffraction study is interpreted by molecular
orbital calculations. The deformation of SiO6octahedra and the bonding electron density of stishovite structures are elucidated from the overlapping electron orbits. The
O–O distances of shared and unshared edge of SiO6 octahedra change with the cation ionicity. The repulsive force between the two cations in the adjacent octahedron makes its
shared edge shorter. The pressure changes of the apical and equatorial Si–O interatomic distances are explained by the electron
density of state (DOS) of Si and electron configuration.
Received: 7 January 2002 / Accepted: 6 May 2002 相似文献
3.
Xi Liu Norimasa Nishiyama Takeshi Sanehira Toru Inoue Yuji Higo Shizue Sakamoto 《Physics and Chemistry of Minerals》2006,33(10):711-721
In order to constrain the high-pressure behavior of kyanite, multi-anvil experiments have been carried out from 15 to 25 GPa, and 1,350 to 2,500°C. Both forward and reversal approaches to phase equilibria were adopted in these experiments. We find that kyanite breaks down to stishovite + corundum at pressures above ∼15 GPa, and stishovite + corundum should be the stable phase assemblage at the pressure–temperature conditions of the transition zone and the uppermost part of the lower mantle of the Earth, in agreement with previous multi-anvil experimental studies and ab initio calculation results, but in disagreement with some of the diamond-anvil cell experimental studies in the literature. The Al2O3 solubility in nominally dry stishovite has been tightly bracketed by forward and reversal experiments; it is slightly but consistently reduced by pressure increase. Its response to temperature increase, however, is more complicated: increases at low temperatures, maximizes at around 2,000°C, and perhaps decreases at higher temperatures. Consequently, the Al2O3 solubility in dry stishovite at conditions of high temperature–high pressure is very limited. 相似文献
4.
Sheng-Nian Luo Tahir Ça?inAlejandro Strachan William A. Goddard IIIThomas J. Ahrens 《Earth and Planetary Science Letters》2002,202(1):147-157
A Morse-stretch potential charge equilibrium force field for silica system has been employed to simulate the thermodynamics of stishovite with the molecular dynamics (MD) method. The equation of state, thermal expansivity and melting curve of stishovite have been obtained. This simple force field yielded results in accordance with the static and dynamic experiments. The stishovite melting simulation appears to validate the interpretation of superheating of the solid along the Hugoniot in the shock melting experiments. MD simulations show that the thermal expansivity of stishovite at lowermost mantle conditions is a weak function of temperature. The phase diagram of silica up to the mega bar regime is proposed based on the experimental and theoretical studies. The related physical and geophysical implications are addressed. 相似文献
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Dmitry L. Lakshtanov Carine B. Vanpeteghem Jennifer M. Jackson Jay D. Bass Guoyin Shen Vitali B. Prakapenka Konstantin Litasov Eiji Ohtani 《Physics and Chemistry of Minerals》2005,32(7):466-470
We have determined the P-V equation of state of Al-rich H-bearing SiO2 stishovite by X-ray powder diffraction at pressures up to 58 GPa using synchrotron radiation. The sample contained 1.8 wt%
Al2O3 and up to 500 ppm H2O, and had a composition that would coexist with Mg-silicate perovskite in a subducted slab. By fitting a third-order Birch-Murnaghan
equation of state to our compression data, we obtained a bulk modulus K
T0=298(7) GPa with K′=4.3(5). With K′ fixed to a value of 4, the bulk modulus K
T0=304(3) GPa. Our results indicate that Al3+ and H+ have a small effect on the elastic properties of stishovite. Compared with data obtained up to 43.8 GPa, peak intensities
changed and we observed a decreased quality of fit to a tetragonal unit cell at pressures of 49 GPa and higher. These changes
may be an indication that the rutile↔CaCl2 transition occurs between these pressures. After laser annealing of the sample at 58.3(10) GPa and subsequent decompression
to room conditions, the cell volume is the same as before compression, giving strong evidence that the composition of the
recovered sample is also unchanged. This suggests that Al and H are retained in the sample under extreme P-T conditions and
that stishovite can be an agent for transporting water to the deepest lower mantle. 相似文献
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