Thermodynamics of Fe-Mg olivines at mantle pressures: mid- and far-infrared spectroscopy at high pressure

The first far-IR spectra of silicates at high pressure have been obtained by combining various state-of-the-art technologies. Three olivines (Fo₁₀₀, Fo₈₇, and Fo₀) were studied at pressures of up to 425 kbar. In addition, mid-IR data on Fo₁₀₀ were obtained up to 240 kbar. The same pressure responses (ie changes in peak position, area, etc) were seen in the hydrostatic and nonhydrostatic experiments. The response to pressure of bands assigned to SiO₄ vibrations differs fundamentally from that of bands involving M ions in a manner that is consistent with the relative incompressibility of the SiO₄ tetrahedron. A gradual transformation from olivine to a more symmetric structure is supported by the generally continuous but individual responses of peak area and frequency to pressure, by the appearance of new bands at pressure, and by the occurrence of mode softening. -from Authors