An experimental study of grain-scale microstructure evolution during the olivine–wadsleyite phase transition under nominally “dry” conditions

The grain-scale microstructure evolution during the phase transformation of olivine to wadsleyite was investigated using a multi-anvil high-pressure apparatus. We identified three stages in the microstructure evolution during olivine to wadsleyite phase transformation: (1) nucleation of wadsleyite along potential nucleation sites (e.g., grain boundaries (and grain corners), inside of grains (dislocations, stacking faults)), (2) development of an interconnected network of fine-grained wadsleyite along grain boundaries (3) growth of wadsleyite into olivine. Our observations suggest that at low over pressures (<3 GPa), phase transformation takes place mainly via inter-crystalline mechanism. As the over-pressure increases (>3 GPa), intra-crystalline nucleation makes a significant contribution to the volume fraction transformed.