Defect structures at reaction interfaces under deformation

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.

Research on reaction rim formation was embedded in the research network Nanoscale Processes and Geomaterials Properties as a collaboration of GFZ Potsdam (D), FU Berlin (D) and the University of Vienna (A). Reaction rims commonly form in (geo)materials when two adjacent phases chemically react with each other to form a layer of a third phase at their interface. Our research focused on two aspects of reaction rim formation, firstly, on the atomic scale mechanisms occurring at the propagating reaction interfaces, and secondly, on how the microstructure and crystallographic evolution of the newly formed layer depend on the reaction interface orientation with respect to the reactant crystal lattice. We used experimental and natural samples containing spinel (MgAl2O4) reaction rims. Initial reaction stages were studied using thin films with 0 to 200 nanometer thick experimentally grown spinel layers. Advanced reaction stages were investigated using 10 to 60 micrometer thick spinel rims grown between single crystal corundum (Al2O3) and periclase (MgO). To study the effects of varying the interface orientation with respect to the reactant crystal, we used a natural sample of corundum representing a xenocrystal in basalt, where a concentric 120- 280 micrometer thick spinel layer had formed. Atomic scale investigations of the interface defect structures showed that spinel formation at the two opposite reaction interfaces simultaneously proceeds via different mechanisms. At the spinel/periclase interface, regularly spaced misfit dislocations occur. In order to grow spinel, the misfit dislocations have to climb into the layer growth direction. Furthermore, voids develop along the interface as vacancy defects are formed during the reaction. The climb of misfit dislocations and the drag of voids decelerate the spinel/periclase interface propagation with respect to coherent interface portions between the misfit dislocations, generating a nanometer and micrometer scale hill- structure of the reaction surface. At the spinel/corundum interface spinel growth proceeds via a different mechanism, namely the glide of partial dislocations parallel to the reaction interface. Starting from steps in the reaction interface plane, corundum transforms to spinel as partial dislocations run in opposite directions parallel to the reaction interface. Different interface mechanisms proceeding at different synchronously propagating phase boundaries influence overall reaction kinetics during reaction rim growth. The natural sample allowed investigation of spinel formation along differently oriented reaction interface segments. We observed remarkable differences in the microstructure and crystallographic orientation systematics of spinel depending on the interface orientation with the corundum reactant. These new results provide an important basis to identify representative interface orientations when investigating comparable reaction rims.