AND ITS GEOPHYSICAL IMPLICATIONS

Olivine is a relatively loose structure. At low pressure it is stable because unshared octahedral edges may extend far enough to reduce the repulsion force between cations in edge-sharing octahedra and tetrahedra. At high pressure the large volume in olivine makes it unfavorable, so the more compact spinel structure becomes stable. Because the octahedral site in spinel is smaller than in olivine, the crystal field stabilization free energy for Fe2+ ions in spinel is larger than in olivine. Consequently, the pressure for the olivine+ spinel transition in Fe2SiO 4 may be lowered by the crystal field stabilization. Thc rate of the olivine+spirel Wrasition in the Mg2Si 4-Fe2SiO 4 system with Mg/(Mg+Fe)<0.4 has been measured by using a modified diamond anvil press with external heating. The experiments were performed at pressures well above equilibriu