Elasticity of MgSiO3 in the Perovskite Structure.

The single-crystal elastic moduli of MgSiO(3) in the perovskite structure, the high-pressure polymorph of MgSiO(3) pyroxene, have been determined. The data indicate that a mantle with either pyrolite or pyroxene stoichiometry is compatible with the seismic models appropriate to the earth's lower mantle, provided that the shear modulus of MgSiO(3) perovskite exhibits a strong negative temperature derivative. Such a temperature derivative falls outside of the range expected for a well-behaved refractory ceramic and could result if the pressure-temperature regime of the earth's lower mantle is near that required for a ferroelastic phase transformation of the perovskite phase.

Elasticity of agr-Cristobalite: A Silicon Dioxide with a Negative Poisson's Ratio.

Laser Brillouin spectroscopy was used to determine the adiabatic single-crystal elastic stiffness coefficients of silicon dioxide (SiO(2)) in the alpha-cristobalite structure. This SiO(2) polymorph, unlike other silicas and silicates, exhibits a negative Poisson's ratio; alpha-cristobalite contracts laterally when compressed and expands laterally when stretched. Tensorial analysis of the elastic coefficients shows that Poisson's ratio reaches a maximum value of -0.5 in some directions, whereas averaged values for the single-phased aggregate yield a Poisson's ratio of -0.16.

Twinning in MgSiO3 Perovskite.

Crystals of MgSiO(3) perovskite synthesized at high pressures and temperatures have orthorhombic symmetry under ambient conditions. Examination by transmission electron microscopy shows that the microstructure of crystals synthesized at 26 gigapascals and 1600 degrees C is dominated by a large number of twin domains that are related by reflection operations with respect to {112} and {110} planes. These twins may be associated with the transformations of MgSiO(3) perovskite from the cubic to tetragonal and tetragonal to orthorhombic phases, respectively, upon decreasing pressure and temperature. These observations suggest that under the experimental synthesis conditions, and perhaps in the earth's lower mantle, the stable phase of MgSiO(3) might have the cubic perovskite structure.

Phase Transition and Thermal Expansion of MgSiO3 Perovskite.

Results from in situ x-ray diffraction experiments with a DIA-type cubic anvil apparatus (SAM 85) reveal that MgSiO(3) perovskite transforms from the orthorhombic Pbnm symmetry to another perovskite-type structure above 600 kelvin (K) at pressures of 7.3 gigapascals; the apparent volume increase across the transition is 0.7%. Unit-cell volume increased linearly with temperature, both below (1.44 x 10(-5) K(-1)) and above (1.55 x 10(-5) K(-1)) the transition. These results indicate that the physical properties measured on the Pbnm phase should be used with great caution because they may not be applicable to the earth's lower mantle. A density analysis based on the new data yields an iron content of 10.4 weight percent for a pyrolite composition under conditions corresponding to the lower mantle. All current equation-of-state data are compatible with constant chemical composition in the upper and lower mantle; thus, these data imply that a chemically layered mantle is unnecessary, and whole-mantle convection is possible.