The rheology of olivine and spinel magnesium germanate (Mg2GeO4):: TEM study of the defect microstructures

Synthetic polycrystals of alpha-Mg2GeO4 (with the olivine structure) and gamma-Mg2GeO4 (with the spinel structure) deformed at high temperature and pressure in their respective stability fields were investigated by analytical transmission electron microscopy. Specimens with a mean grain size of 20-30 mu m deform by dislocation glide and/or climb. The predominance of glide versus climb depends on stress and grain orientation. The defect microstructures of both polymorphs are very similar to those observed in their respective silicate analogues, alpha- and gamma- (Mg,Fe)(2)SiO4, and, in the case of the spinel phase, very similar to those observed in magnesium aluminate spinels. These observations suggest that Mg2GeO4 is a good rheological analogue for the Earth's upper mantle. A spinel specimen deformed under the same conditions of temperature and strain rate as an olivine specimen was approximately three times stronger than olivine. Ln specimens of both phases deformed at or above 1400 K, a thin amorphous film composed of Mg, Ge, and O was detected along some grain boundaries. Grains less than or equal to 10 mu m diameter surrounded by a film of amorphous phase (>10 nm thick) exhibited low dislocation densities, and deformation appeared to have occurred by grain boundary sliding.