Brillouin scattering and X-ray diffraction of San Carlos olivine: direct pressure determination to 32 GPa

The single-crystal elastic moduli of San Carlos olivine, (Mg_0.9Fe_0.1)₂SiO₄, were determined at seven pressures between 2.5 and 32.5 GPa by Brillouin spectroscopy in a diamond anvil cell. The unit cell volume was also determined at each pressure (and ambient pressure) by single-crystal X-ray diffraction. The shear elastic moduli exhibit non-linear pressure dependencies. In particular, a non-linear dependence is required for the elastic modulus, C₅₅, above 10 GPa. Aggregate bulk and shear moduli were determined from Hashin-Shtrikman averages of the individual constants. Pressures were determined by fitting the bulk modulus (Reuss bound) and volume data to a third-order Eulerian finite strain equation and integrating. The results are compared to independent pressure measurements made using the ruby fluorescence scale. The ruby measurements yield pressures that are in good agreement with the Brillouin results. On average, the ruby fluorescence measurements produce pressures that are larger than the Brillouin determinations by 1.6%.