Element transport along grain boundaries of synthetic silicate bicrystals: the effect of tilt orientations on transport properties (TP1)

合成或天然岩石基质中的扩散质量传输可通过体积或晶界扩散，或两者的组合发生。各矿物结构中的许多元素都有大量的体积扩散实验数据，但缺乏关于晶界扩散的数据。对于具有良好特征的晶界结构的单晶界内的质量传输来说，情况尤其如此。在毫米尺度上具有恒定特征的单晶界是进行此类研究的先决条件。这些实验数据是迫切需要的。For 741小组成员在压应力下进行的边缘生长实验表明，多晶尖晶石中的晶界扩散取决于取向错误(Keller等人提交)。在应力作用下，即使整个晶界密度随着应力的增加而减小，增加应力下反应产生的更快的晶界密度也可以促进轮缘的生长。定义晶界的晶体的相对取向对输运速率有重要影响。然而，边缘生长实验只能记录总体效果。在这里，将利用具有明确定义的、变化的错误取向的预制晶界，在地质相关系统中进行更简单的实验。取向的石榴石和橄榄石双晶被产生，溅射或脉冲激光涂覆各种成分，然后在不同的温度和持续时间下加热。沿/跨特定晶界的浓度分布将用分析电子显微镜(ATEM)和同步辐射X射线荧光(SXRF)方法测量，并与同一实验中的体积扩散分布进行比较。应用扩散输运的连续介质模型，通过高分辨电子显微镜(HRTEM)观测来估计晶界宽度，得到了晶界扩散系数，并与体积扩散系数进行了直接比较。实验将得到原子模拟的补充，使用从第一性原理密度泛函计算得出的高级离子相互作用模型。用弛豫结构模型解释了具有原子尺度分辨率的透射电子显微图像。此外，计算还将提供根据实验确定的轮廓建立更大尺度的连续介质模型所需的界面能。最终，分子动力学模拟可能揭示快速扩散的途径。

Diffusional mass transport in a synthetic or natural rock matrix may occur via volume or grain boundary diffusion, or a combination of both. A huge amount of experimental data on volume diffusion exists for many elements within the respective mineral structures; data on grain boundary diffusion are lacking. This is particularly true for mass transport within single grain boundaries with well characterised grain boundary structures. Single grain boundaries with constant characteristics at the mm-scale are a prerequisite for such studies. These experimental data are urgently needed. Rim growth experiments under compressive stress performed by members of the FOR 741 group have shown that grain boundary diffusion in polycrystalline spinel depends on misorientation (Keller et al submitted). An increasing density of "faster" grain boundaries produced by reaction under stress may enhance rim growth even if the overall grain boundary density decreases with increasing stress. The relative orientation of crystals defining the grain boundary has an important effect on the transport rates. Rim growth experiments, however, can only record the overall effect. Here, simpler experiments in geologically relevant systems will be performed making use of pre-fabricated grain boundaries with well-defined, varying misorientations. Oriented garnet and olivine bicrystals are produced, sputtered or pulsed-laser coated with various components and subsequently heated at different temperatures and durations. Resulting concentration profiles along/across the specific grain boundaries will be measured by Analytical Transmission Electron Microscopy (ATEM) and Synchrotron X-Ray Fluosrescence (SXRF) methods and compared with volume diffusion profiles from the same experiment. Applying continuum models for diffusional transport, and estimating grain boundary widths from High Resolution TEM (HRTEM) observations, grain boundary diffusion coefficients are derived and directly compared with volume diffusion coefficients. Experiments will be complemented by atomistic simulations using advanced ionic interaction models that are derived from first-principles DFT calculations. The relaxed structural models are applied to interpret the transmission electron micrographs with atomic scale resolution. Furthermore, the calculations will provide interfacial energies needed to formulate larger scale continuum models derived from the experimentally determined profiles. Eventually, molecular dynamics simulations may reveal pathways for fast diffusion.