Non-linear diffusion during phase separation and symplectite formation in alkali feldspar: an experimental and modelling approach (TP8)

在前驱体项目中，从实验和理论两方面研究了碱长石在旋多分解过程中的微观结构演变。在后续项目中，我们希望通过(i)将成核生长作为出溶的另一种反应途径，以及（ii）详细研究碱长石中Na+/K+的相互扩散来扩展这项研究。(i)为了通过成核和生长诱导出溶，将在均质碱长石亚稳的条件下进行等温退火。实验将在高压1 ~ 2gpa范围内进行，碱长石二元的亚稳区扩展到高温，整体反应动力学快。利用具有高空间分辨率的电子束显微分析技术，包括FEG-SEM， FEG-EMP和TEM，结合FIB样品制备，研究了exsolution的微观结构和微观化学性质。在前体项目中开发的数值模型将与成核模块相辅相成，通过比较模拟和观察到的微观结构和相组成来提取工艺参数。（ii）从前驱体项目的实验中，我们发现碱长石和NaCl-KCl盐熔体之间的Na+/K+交换产生的浓度曲线的几何形状偏离了简单相互扩散的预期。通过晶体传播的交换前沿显示出锐度，这取决于晶粒的原始域和交换域之间的组成差异，但不随时间变化。我们假设这可能是由于晶格失配（相干应变）导致交换晶体和原始晶体之间的界面发生应变。然而，发生这种情况的机制是难以捉摸的。解决相干应变对阳离子互扩散的影响问题是理解矿物体系中阳离子互扩散和溶出的基础。我们建议使用不同的起始材料进行阳离子交换实验，包括具有明确几何形状的同位素标记的KCl。将利用高分辨率分析技术研究交换前沿的微化学、缺陷密度和晶格畸变，并建立和参数化相干应变对碱长石中Na+/K+相互扩散影响的概念模型。

In the precursor project we studied microstructure evolution during spinodal decomposition of alkali feldspar in experiment and theoretically. With the successor project we want to extend this study by (i) addressing nucleation-growth as an alternative reaction pathway for exsolution and (ii) investigating in detail Na+/K+ interdiffusion in alkali feldspar. (i) To induce exsolution by nucleation and growth, isothermal anneals will be done at conditions where the homogeneous alkali feldspar is metastable. Experiments will be done at high pressure in the 1 to 2 GPa range, where the metastable region in the alkali feldspar binary extends to high temperature, and the overall reaction kinetics is fast. The exsolution microstructure and microchemistry will be investigated using electron beam microanalytical techniques with high spatial resolution including FEG-SEM, FEG-EMP and TEM combined with FIB sample preparation. The numerical model that was developed in the precursor project will be complemented with a nucleation module to extract process parameters from comparing modelled and observed microstructures and phase compositions. (ii) From experiments in the precursor project we find that the geometries of concentration profiles produced from Na+/K+ exchange between alkali feldspar and NaCl-KCl salt melt deviate from what is expected for simple interdiffusion. Exchange fronts propagating through the crystal show a sharpness, which depends on the composition difference between the pristine and the exchanged domains of the grain but does not change with time. We hypothesize that this may be due to strain occurring at the interface between exchanged and pristine crystal due to lattice mismatch (coherence-strain). The mechanisms by which this occurs are, however, elusive. Solving the question of coherence strain effects on cation interdiffusion is fundamental for the understanding of interdiffusion and exsolution in mineral systems. We suggest doing cation exchange experiments using different starting materials including isotopically labelled KCl with well-defined geometry. The microchemistry, defect density and lattice distortion at the exchange fronts will be investigated using high resolution analytical techniques, and conceptual models for the influence of coherency strain on Na+/K+ interdiffusion in alkali feldspar will be developed and parmeterized.