Elasticity of Olivine (α), Beta(β), and Spinel (γ) Polymorphs of Germanates and Silicates

Abstract

Summary Ultrasonic data for the velocities of germanate and silicate compounds in the olivine (a), beta (8) and spinel (y) crystal structures have been determined as a function of pressure to 7-5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 65 kbar. The a-y phase transformations are characterized by the following velocity (u)-density (p) relationships: (a) the velocity jumps are twice the percentage magnitude of the density jumps; (b) the ratio (vp/v,) of the compressional to shear velocity is either approximately constant or increases slightly across the transitions; and (c) the slopes (linear or logarithmic) on v-p diagrams for the a-y transition are comparable to those produced by isothermal compression or isobaric expansion of the low-pressure olivine phase. The behaviour of up/u, is a feature common to many phase transformations. However, the observed relationships (a) and (c), while similar to those for the quartz-coesite transition in Si02, are in marked contrast with those for the coesite-rutile, pyroxene-garnet and pyroxeneilmenite transformations. These latter transitions involve increases in cation-anion co-ordination and nearest-neighbour distances whereas the olivine-spinel and olivine-beta phase transformations do not. Such crystallographic details may be diagnostic for interpretations of the composition and mineralogy of the Earth’s transition zone. 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 modulus of the spinel polymorph of Mg, Si04 (K, = 2-06 0.05 Mbar). Comparison of the new elasticity data with recent Earth models in the vicinity of the 400-km discontinuity reveals that only model 1066B of Gilbert & Dziewonski is compatible with the experimental data for the u-y and a-8 transformations.