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3.1 c. High pressure experimental studies of the mechanism of the spinel to modified spinel transformation (D.C. Rubie, in collaboration with A. Brearley/Albuquerque)

We have been investigating the transformation of Mg2SiO4 spinel (γ) to modified spinel (ß) in the multianivil press in order to examine the mechanism of this phase transition. These two phases can potentially undergo phase transitions by a diffusionless, shear-like mechanism in which a reconfiguration the stacking of the basic spinelloid units occurs. Phase transformations which occur by this type of mechanism may result in a significant decrease in shear strength as the transformation occurs, an effect described as transformation plasticity. The operation of this mechanism during phase transformations in subducting oceanic slabs, for example, has the potential to change the rheological properties of the slab in the region where the transformation occurs. The spinel starting material was synthesized by reacting synthetic forsterite at 20.5 GPa and 1200 °C. After quenching to room temperature, this material was recovered and run in a second set of experiments at various conditions of temperature and pressure within the ß-phase stability field. Pressure was increased at room temperature before the sample was heated to the run temperature. Experiments have been carried out at 900 °C, 15 GPa; 1000 °C, 15.5 GPa; 1100 °C, 16 GPa and 1200°C, 16.5 GPa for different periods of time. TEM studies of the starting material show that during pressurization to 15-16 GPa significant dislocation development has occurred in some grains, but in general the samples are undeformed. There is no evidence of transformation to ß-phase during this pressurization step. After 45 minutes at 900 °C, spinel has developed a complex microstructure due to the development of a high density of stacking faults on {110}, which result from a shear-like transformation. Electron diffraction patterns of the spinel show no evidence of discrete diffraction maxima which can be indexed as ß-phase, so this phase appears to be an intermediate disordered spinelloid phase. After 90 minutes at 900 °C, some spinel grains give diffraction patterns which clearly show that they are intergrowths of both spinel and ß-phase, in the crystallographic orientation relationship consistent with a shear transformation. At temperatures 1000 °C, the transformation mechanism is significantly different. After 10 and 60 minutes at 1000 °C, there is no evidence of any disordering of spinel. Instead, discrete ß-phase crystals have nucleated at grain boundaries and triple junctions and in many cases are crystallographically oriented with respect to the spinel. At least 4 distinct orientation relationships have been observed. Experiments carried out at 1100° and 1200 °C show the same type of microstructures except that the number of grains which are crystallographically oriented with respect to the spinel appears to decrease with increasing temperature. These results demonstrate that there is a major change in the mechanism of the γ to ß phase transformation as a function of increasing temperature. At lower temperatures the mechanism is dominated by shear-like transformation, but changes with increasing temperature to a diffusion-controlled nucleation and growth mechanism.

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