Elasticity of some mantle crystal structures: 1. Pleonaste and hercynite spinel

The elasticity of high-pressure mantle phases can be characterized by using data for crystal-chemically similar compounds. The single-crystal elastic constants are determined as a function of pressure and temperature for pleonaste spinel (Mg0.75Fe0.36Al1.90O4) and at room conditions for hercynite spinel (FeAl2O4). The bulk modulus increases from 1.95 Mb for MgAl2O4 to 2.10 Mb for FeAl2O4. The pressure derivative of the shear constant CS = (C11 − C12)/2 is slightly negative for pleonaste. Low or negative values of the pressure derivatives of shear constants are characteristic of the spinel structure and imply a low kinetic barrier to phase transformations and diffusion. Compressional and shear velocities of the spinel phase of olivine are estimated as a function of mean atomic weight by using the pleonaste and hercynite data. In comparison with a velocity and density model of the earth's mantle, the compressional velocity data indicate an increase in mean atomic weight across the transition zone, whereas the shear velocity data indicate no increase.

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