Velocity-density systematics for the olivine and spinel phases of Mg2SiO4-Fe2SiO4

Substitution of 3d-transition elements for Mg and Al in the olivine, spinel, and corundum lattices causes a systematic decrease in the compressional wave velocity υp for oxides and silicates crystallizing in these structures at low pressures. The new measurements of υp for Fe2SiO4 spinel by H. Mizutani, Y. Hamano, Y. Ida, and S. I. Akimoto fit the line for spinel compounds, which indicates that the velocity is controlled only by the density p and the crystallographic structure and not by compositional details. In particular, the relationship for spinel compounds is υp = 15.66 - 1.59 p where υp is in km/sec and p is in g/cm3. By analogy, the equation for the shear wave velocity υs is υs = 9.88 -1.17p. These velocity-density systematics suggest that the isotropic elastic properties of the Mg2SiO4-Fe2SiO4 spinel series may be predicted from a knowledge of only the density or the cell volume.