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

反应圈是变质岩中常见的特征，当两个接触的固体相在它们的界面上反应形成新相时，就形成了反应圈。反应环已被广泛用于材料科学和地球科学，从反应系统中推断环境条件和评级信息。深入了解在反应环形成过程中有效的潜在过程，对于正确解释岩石中的微结构、结构和矿物组成是必不可少的。拟议的研究基于研究组第一个资助期的前体项目“运输控制的矿物反应与差异应力之间的反馈”期间所取得的结果。我们的目标是确定和平衡在合成体系中控制反应边生长动力学的有效机制，在确定的实验条件下，在该合成体系中，镁方镁石(方镁石)和氧化铝(刚玉)反应生成镁铝酸镁(尖晶石)。在前身项目期间，出现了新的研究问题，需要通过在方法论方面的扩展完成研究来解决这些问题。我们建议在反应环生长的两个方面进行补充研究：a)将用高分辨率分析方法(扫描电子显微镜、透射电子显微镜)研究两个反应前沿的原子结构，以获得具有确定的反应物和产物的晶体取向关系的界面上的局部缺陷结构的信息。由于我们解释了在渐进反应边缘生长过程中局部缺陷结构的变化，我们比较了来自初始生长阶段的样品和来自长期实验运行的样品。B)此外，我们还研究了外加变形对整体轮缘生长速率以及对微观组织和织构演化的影响。我们打算用Paterson型气体介质装置进行单晶反应扩散偶的扭转实验。由此，我们将获得从核心到边缘暴露于不断增加的有限剪切应变的样品。从高空间分辨率的显微组织和织构分析(扫描电子显微镜、扫描电子显微镜、S电子显微镜)，我们试图解释形变和动态再结晶对反应过程和组织和织构发展的影响。我们还将用EMPA分析直接反应前沿的物相组成，以便将微观结构和织构与偏离局部化学平衡的程度关联起来。以往的研究表明，在静态条件下，生长机制和拓扑关系控制着反应轮缘的微观结构和织构。在动态环境下进行额外的实验有望完成对系统如何响应外部施加的应力、变形和相变过程中的化学反应如何相互作用以及轮缘生长过程中微结构和织构形成的机制的理解。

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. The proposed study is based on results obtained during the precursor project ‘Feedback between transport-controlled mineral reactions und differential stress’ in the first funding period of the Research Group. 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. During the precursor project new research questions have arisen which need to be addressed by completing the study with a methodological extension. We propose complementary 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.