Elastic Behavior and Properties of CaSiO3 Wollastonite and Perovskites to 15 GPa and 1200K

Introduction Seismological observations provide information about the properties and processes of the Earth’s interior in the form of profiles of density and velocity as a function of depth. Seismic tomography on a global scale reveals lateral variations in compressional (P) and shear (S) wave velocities and densities at various depths of the Earth’s interior, reflecting differences in chemical composition, mineralogy, and/or temperature. The successful interpretation of these seismic observations in terms of those variables requires experimental and theoretical information on the elasticity of deep Earth materials under elevated pressure and temperature (P-T) conditions. In the last decades, major mantle phases have been investigated by using various techniques, including ultrasonic interferometry, Brillouin scattering, shock wave compression, x-ray diffraction, and, most recently, inelastic scattering. Details can be found in Refs. 1 and 2. CaSiO3 perovskite is the third-most-abundant phase in the lower mantle of the Earth. The assessment of its visibility in the seismic data has been hampered by the lack of reliable elasticity data. The elastic bulk modulus and its pressure derivative have been constrained in previous compression studies only by using diamond anvil cell (DAC) and large-volume apparatus in conjunction with synchrotron x-radiation [3-5]. Moreover, the crystal structure of CaSiO3 in the mantle is still controversial. Most experimental studies have reported that CaSiO3 possesses a cubic structure in the Earth’s mantle, but some theoretical calculations have suggested the possibility of tetragonal and even lower symmetry [6-8]. A recent high-resolution x-ray diffraction study reported evidence for the existence of a symmetry lower than cubic at room temperature from the diffraction line splitting (<0.4%) [9]. An attempt to obtain elastic constants by using the radial x-ray diffraction technique at the lower mantle pressures has also been reported; however, the precision and reliability of this technique still need improvement [10].