Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (P4 2/mnm) highpressure 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 CaCl 2 structure (Pnnm) at Ϸ55 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-CaCl 2 transition in natural stishovite (with 5 wt % Al 2O3) should occur at Ϸ30 GPa or Ϸ1,000-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.hydrogen ͉ phase transition ͉ silica ͉ high pressure ͉ Brillouin scattering S tishovite is the lowest-pressure SiO 2 polymorph with octahedrally coordinated silicon. It adopts the tetragonal rutile structure (P4 2 /mnm) and is stable from Ϸ9 GPa to Ϸ50 GPa at room temperature where it undergoes a displacive phase transition to the orthorhombic CaCl 2 structure (Pnnm), e.g. (1, 2). Estimates for the temperature dependence of this transition (dP/dT) (1, 3, 4) suggested that it might occur at depths of Ϸ1,500 km or more in the mantle (more than Ϸ60 GPa), but the expected seismic signatures have never been observed in that depth range. In fact, the phase relations for pure SiO 2 may not be directly relevant to the mantle because they do not include the possible effects of Al 2 O 3 and H 2 O, which are known to be soluble in stishovite (5, 6). It was previously speculated that impurities might lower the pressure for this transition (7), but clear experimental evidence was lacking. If these effects are significant, they may bring the depth of this phase transition into the region where it could possibly explain seismic reflections observed at 900-to 1,400-km depth (8-11). Fig. 1 shows the angle-dispersive x-ray spectra of the stishovite and post-stishovite (CaCl 2 ) polymorphs. Because these singlecrystal x-ray diffraction (XRD) spectra were obtained by a rotation of only Ϯ12°on the -circle, not all reflections are present. However, the CaCl 2 phase was clearly observed on our x-ray spectra above 24 GPa. The volume change of transition is apparently small (12), and the tetragonal-ort...
The volume thermal expansion of powdered natural orthoenstatite [(Mg 0.994 Fe 0.002 Al 0.004) 2 (Si 0.996 Al 0.004) 2 O 6 ] has been measured to 1473 K using energy dispersive synchrotron X-ray diffraction. Over the temperature range examined, the data are consistent with a volume thermal expansion, , that linearly increases with temperature as given by the expression (T) = 29.7(16) x 10-6 K-1 + 5.7(11) x 10-9 K-2 T. An analysis in terms of the often-used constant expansion coefficient yields 0 = 34.5(17) x 10-6 K-1 , which is in good agreement with several previous experimental results on orthoenstatite (Mg 2 Si 2 O 6) over a similar temperature range. Our results do not support the extreme upper and lower bounds reported in earlier studies for the thermal expansivity of Fe-rich orthoenstatite, but rather suggest that the thermal expansion of this phase is approximately midway between those extreme values.
We describe a new Brillouin spectrometer that has been installed on a synchrotron x-ray beamline for simultaneous measurements of sound velocities ͑by Brillouin scattering͒ and density ͑by x-ray diffraction͒. The spectrometer was installed at the 13-BM-D station ͑GSECARS͒ of the Advanced Photon Source. This unique facility has been tested in studies of transparent single crystal and polycrystalline materials at high pressure and temperature. The equation of state, acoustic velocities, and, hence, elastic moduli of materials as a function of pressure and temperature can now be determined without resort to a secondary pressure standard, such as the ruby fluorescence scale, or the equation of state of standard materials such as Au, Pt, or MgO, thus offering the potential to determine an absolute pressure scale. This article describes the design of the combined Brillouin-x-ray system and the first experimental results obtained. As a general-user facility, the system was designed to require minimal setup time and alignment of sensitive optics, run-time control and adjustments of optics from outside the experimental station, compatibility with powder and single crystal x-ray diffraction measurements, and no interference with other experimental techniques used on the beamline. To satisfy these requirements we adopted a novel optical design for the Brillouin system with a vertical scattering plane. Examples of measurements of acoustic velocities and elastic moduli on single crystal NaCl ͑B1͒ and polycrystalline NaCl ͑B2͒ at high pressure and single crystal velocities and elastic moduli of MgO at high temperature ͑to 600°C͒ and at high pressure are presented.
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