The influence of defect structures at reaction interfaces and shear-deformation on reaction rim growth kinetics in the MgO-Al2O3 system

Reaction rims are common features in metamorphic rocks, which form, when two solid phases that are in contact react to form a new phase at their interface. Reaction rims have been widely used in Material Sciences and Earth Sciences to infer environmental conditions and rate information from reactive systems. In depth knowledge of the underlying processes that are effective during reaction rim formation is mandatory for properly interpreting microstructures, textures and mineral compositions in rocks. We aim to identify and balance effective mechanisms that govern reaction rim growth kinetics in the synthetic system MgO-Al2O3, where MgO (periclase) and Al2O3 (corundum) react to form MgAl2O4 (spinel) under defined experimental conditions. We propose investigations that focus on two aspects of reaction rim growth: A) Atomic structures at both propagating reaction fronts will be investigated by high resolution analytical methods (SEM, TEM) in order to obtain information on the local defect structure at interfaces with defined crystallographic orientation relation of reactants and product. As we account for changes in the local defect structure during progressive reaction rim growth, we compare samples from initial growth stages with those from long-term experiment-runs. B) In addition, we investigate the effect of externally imposed deformation on the overall rim growth rate and on the microstructure and texture evolution. We intend to perform torsion experiments of single-crystal reaction-diffusion couples using a Paterson-type gas-medium apparatus. Therefrom we will obtain samples exposed to increasing finite shear-strain from the core to the rim. From microstructural and textural analyses at high spatial resolution (SEM-EBSD, SEM-FSD, (S)TEM) we intend to decipher the influence of deformation and dynamic recrystallization on the reaction progress and the microstructure and texture development. We will also analyse the phase compositions at the immediate reaction front by EMPA in order to correlate microstructures and textures with the grade of deviation from local chemical equilibrium. Previous investigations have shown that growth mechanisms and topotactic relations control the microstructure and texture of reaction rims under static conditions. Performing additional experiments in a dynamic setting is expected to accomplish the understanding of how the system responds to externally applied stress, how deformation and chemical reactions interact during phase transformation and which mechanisms underlie microstructure and texture formation during rim growth.