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输运可通过体积或晶界扩散或两者的组合发生。大量的实验数据存在体积扩散的许多元素在各自的矿物结构;晶界扩散的数据是缺乏的。这是特别真实的单晶界内的质量传输以及表征晶界结构。在毫米尺度上具有恒定特性的单晶界是进行此类研究的先决条件。迫切需要这些实验数据。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差（Keller等人提交）。由应力下的反应产生的“更快”晶界的增加的密度可以增强边缘生长，即使总晶界密度随着增加的应力而减小。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整体效果。在这里，在地质相关的系统中进行简单的实验，利用预制晶界定义明确，不同的取向差。定向的石榴石和橄榄石双晶体被生产，溅射或脉冲激光涂覆有各种组分，随后在不同的温度和持续时间下加热。将通过分析透射电子显微镜（ATEM）和同步加速器X射线荧光（SXRF）方法测量沿沿着/穿过特定晶界的所得浓度分布，并与来自相同实验的体积扩散分布进行比较。应用连续模型的扩散运输，并估计晶界宽度从高分辨率TEM（HRTEM）的观测，晶界扩散系数推导和直接比较体积扩散系数。实验将补充原子模拟使用先进的离子相互作用模型，来自第一原理DFT计算。松弛结构模型被应用于解释透射电子显微镜与原子尺度分辨率。此外，计算将提供界面能需要制定更大规模的连续模型来自实验确定的配置文件。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.