THE RHEOLOGY OF OLIVINE AND SPINEL MAGNESIUM GERMANATE (MG2GEO4) - TEM STUDY OF THE DEFECT MICROSTRUCTURES

Synthetic polycrystals of alpha-Mg2GeO4 (with the olivine structure) a nd gamma-Mg2GeO4 (with the spinel structure) deformed at high temperat ure and pressure in their respective stability fields were investigate d by analytical transmission electron microscopy. Specimens with a mea n grain size of 20-30 mu m deform by dislocation glide and/or climb. T he predominance of glide versus climb depends on stress and grain orie ntation. The defect microstructures of both polymorphs are very simila r to those observed in their respective silicate analogues, alpha- and gamma- (Mg,Fe)(2)SiO4, and, in the case of the spinel phase, very sim ilar to those observed in magnesium aluminate spinels. These observati ons suggest that Mg2GeO4 is a good rheological analogue for the Earth' s upper mantle. A spinel specimen deformed under the same conditions o f temperature and strain rate as an olivine specimen was approximately three times stronger than olivine. Ln specimens of both phases deform ed 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 t o 10 mu m diameter surrounded by a film of amorphous phase (>10 nm thi ck) exhibited low dislocation densities, and deformation appeared to h ave occurred by grain boundary sliding.