Elasticity of some mantle crystal structures: 2. Rutile GeO2

Abstract

The single-crystal elastic constants are determined as a function of pressure and temperature for rutile structure germanium dioxide (GeO2). The data are qualitatively similar to those of rutile TiO2 measured by Manghnani (1969). The compressibility in the c direction is less than one-half that in the a direction, the pressure derivative of the shear constant Cs = (C11 − C12)/2 is negative, and the pressure derivative of the bulk modulus ∂K/∂P has a relatively high value of about 6.2. The high ∂K/∂P values for rutile GeO2 and rutile TiO2 are not consistent with the Slater or Dugdale-MacDonald equations relating ∂K/∂P to the Gruneisen parameter. The ∂K/∂P for stishovite (which has the rutile structure) is probably high also. According to an elastic strain energy theory, the negative shear modulus derivative implies that the kinetic barrier to diffusion decreases with increasing pressure. Kinetic data on the quartz GeO2 → rutile GeO2 transformation appear to support the applicability of the theory.