Showing papers by "Stanislav V. Sinogeikin published in 2007"

The post-stishovite phase transition in hydrous alumina-bearing SiO2 in the lower mantle of the earth

Abstract: Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (P42/mnm) high-pressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase in the lower mantle and at the core–mantle boundary. Pure stishovite undergoes a displacive phase transition to the CaCl2 structure (Pnnm) at ≈55 GPa. Theory suggests that this transition is associated with softening of the shear modulus that could provide a significant seismic signature, but none has ever been observed in the Earth. However, stishovite in natural rocks is expected to contain up to 5 wt % Al2O3 and possibly water. Here we report the acoustic velocities, densities, and Raman frequencies of aluminum- and hydrogen-bearing stishovite with a composition close to that expected in the Earth mantle at pressures up to 43.8(3) GPa [where (3) indicates an uncertainty of 0.3 GPa]. The post-stishovite phase transition occurs at 24.3(5) GPa (at 298 K), far lower than for pure silica at 50–60 GPa. Our results suggest that the rutile–CaCl2 transition in natural stishovite (with 5 wt % Al2O3) should occur at ≈30 GPa or ≈1,000-km depth at mantle temperatures. The major changes in elastic properties across this transition could make it visible in seismic profiles and may be responsible for seismic reflectors observed at 1,000- to 1,400-km depth.

Instrument for x-ray magnetic circular dichroism measurements at high pressures.

Abstract: An instrument has been developed for x-ray magnetic circular dichroism (XMCD) measurements at high pressures and low temperatures This instrument couples a nonmagnetic copper-beryllium diamond anvil cell featuring perforated diamonds with a helium flow cryostat and an electromagnet The applied pressure can be controlled in situ using a gas membrane and calibrated using Cu K-edge x-ray absorption fine structure measurements The performance of this instrument was tested by measuring the XMCD spectra of the Gd5Si2Ge2 giant magnetocaloric material

Distinct thermal behavior of GeO2 glass in tetrahedral, intermediate, and octahedral forms.

Abstract: One fascinating high-pressure behavior of tetrahedral glasses and melts is the local coordination change with increasing pressure, which provides a structural basis for understanding numerous anomalies in their high-pressure properties. Because the coordination change is often not retained upon decompression, studies must be conducted in situ. Previous in situ studies have revealed that the short-range order of tetrahedrally structured glasses and melts changes above a threshold pressure and gradually transforms to an octahedral form with further pressure increase. Here, we report a thermal effect associated with the coordination change at given pressures and show distinct thermal behaviors of GeO2 glass in tetrahedral, octahedral, and their intermediate forms. An unusual thermally induced densification, as large as 16%, was observed on a GeO2 glass at a pressure of 5.5 gigapascal (GPa), based on in situ density and x-ray diffraction measurements at simultaneously high pressures and high temperatures. The large thermal densification at high pressure was found to be associated with the 4- to 6-fold coordination increase. Experiments at other pressures show that the tetrahedral GeO2 glass displayed small thermal densification at 3.3 GPa arising from the relaxation of intermediate range structure, whereas the octahedral glass at 12.3 GPa did not display any detectable thermal effects.

Effect of Al3+ and H+ on the elastic properties of stishovite

Abstract: X-ray diffraction, Brillouin, and Raman scattering measurements were performed on Al 3+ and H + -bearing stishovite at ambient conditions. Samples with different Al 3+ and H + contents were used to examine the effects of these minor constituents on the density, acoustic velocities, single-crystal and aggregate elastic moduli. The X-ray diffraction and compositional data suggest that the incorporation mechanism of Al 3+ into stishovite involves the formation of oxygen vacancies, in addition to the incorporation H + in the structure. Our data show an overall linear decrease of the acoustic velocities, single crystal ( c ij ) and aggregate ( K 0 , G 0 ) elastic moduli as a function of Al 3+ concentrations. For the sample of stishovite containing 6.07(5) wt% Al 2 O 3 and 0.24(2) wt% H 2 O we obtained: zero-pressure adiabatic bulk modulus, K S0 = 290(3) GPa, and the shear modulus, G S0 = 207(2) GPa, with a calculated density of ρ = 4.16(1) g/cm 3 based on X-ray diffraction. Stishovite containing 4.37(12) wt% Al 2 O 3 and 0.29(3) wt% H 2 O possesses higher aggregate moduli: K S0 = 296(3) GPa and G S0 = 213(2) GPa, with a calculated density of ρ = 4.21(1) g/cm 3 based on X-ray diffraction. We conclude that the formation of oxygen vacancies has a stronger effect on the density and thus elastic properties of stishovite than does the incorporation of hydrogen.

Cubic boron nitride as a primary calibrant for a high temperature pressure scale

Abstract: We present results establishing a new high pressure scale at high temperature based on the thermal equation of state and elastic properties of cubic boron nitride (cBN). This scale is derived from simultaneous measurements of density and sound velocities at high pressure and temperature independent of any previous pressure scale. The present results obtained at room temperature to 27 GPa suggest the validity of the current ruby scale (within±4% at 100 GPa). At high temperature, data obtained at 16 GPa to 723 K are in fair agreement with the thermal equation of state of cBN reported in our previous work. We have also shown that cBN can serve as a convenient pressure gauge in X-ray and optical studies using the laser heated diamond anvil cell.

Single-crystal elastic properties of Ca0.07Mg1.93Si2O6 orthopyroxene

Abstract: The single-crystal elastic properties of Ca 0.07 Mg 1.93 Si 2 O 6 orthopyroxene (space group Pbca ) have been investigated by Brillouin spectroscopy at ambient conditions. The aggregate bulk and shear moduli, K 0 ,S = 102.5 GPa (1.5) and μ = 74.2 GPa (1.1), respectively, are ~5% and ~3% lower than commonly accepted values for MgSiO 3 end-member ( K 0 ,S = 107.6, μ = 76.8 GPa). These results indicate that the incorporation of small amount of Ca in the orthoenstatite structure does not greatly affect its elastic properties. As a consequence, the increase in bulk modulus reported in natural orthopyroxenes relative to the Mg-end-member is not related to the substitution of Ca in the M2 octahedral sites, but more probably to the substitution of Al in tetrahedral sites.