Abstract
Ultrasonic velocities of single crystal MgO (periclase) and MgAl2O4 (spinel) were measured up to 7.78 GPa and 6.16 GPa, respectively, under hydrostatic pressure. The present success was attained by making various technical developments, such as a liquid-solid hybrid system and indium bond between sample and buffer-rod. Analytical procedures were also developed both in required corrections for apparent travel times, and in formulation of pressure effect on elasticity. A distinct shear softening was found in MgAl2O4, and was predicted to occur in MgO beyond 30 GPa. Isothermal equation of state of MgO was derived by combining the present results with shock hugoniot. Heat capacity, Grüneisen constant, and coefficient of thermal expansivity of MgO at high pressures were calculated by means of a new model equation of state. Negative thermal expansivity is expected to take place in MgO above -50 GPa. The pressure derivative of rigidity of γ-Mg2SiO4 is estimated to be 0.56-1.0 by means of a lattice dynamic analogy with MgAl2O4; the lower margin of the estimate leads to the pressure-induced shear softening of γ-Mg2SiO4 in the earth's upper mantle.
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