Elasticity of pyroxene-garnet and pyroxene-ilmenite phase transformations in germanates

Ultrasonic data for the velocities of the low- and high-pressure polymorphs of germanate compounds undergoing the pyroxene-garnet (CaGeO₃, CdGeO₃) and pyroxene-ilmenite (MgGeO₃, MnGeO₃) phase transformations have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 60 kbar. These transitions are characterized by the following velocity (υ)-density (ρ) relationships: (1) the velocity jumps are comparable in percentage magnitude to the density jumps, with the exception of shear velocity for the pyroxene-ilmenite transition; (2) the ratio (υ_p/υ_s) of the compressional to shear velocity is constant or increases slightly across the transitions; and (3) low slopes (linear or logarithmic) on υ-ρ diagrams. The observed relationships (1) and (2) are similar to those for the coesite-stishovite transition, but are in marked contrast with those from the olivine-spinel and olivine-beta phase transformations. Coordination changes are thus important factors to be considered in establishing velocity-density systematics governing polymorphic transitions. The υ-ρ changes across the pyroxene-garnet and pyroxene-ilmenite transitions are also distinctly smaller than those produced by compression or thermal expansion of a homogeneous material or by varying composition at constant mean atomic weight. Systematic trends in the elastic properties for isostructural sequences support the concept of germanates as models for the elasticity of their silicate analogues; this scheme is applied to estimate the bulk moduli of the garnet (1.80 Mbar) and ilmenite (2.11 Mbar) polymorphs of MgSiO₃.