Schäffer Anne-Kathrin, Jäpel Tom, Zaefferer Stefan, Abart Rainer, Rhede Dieter
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria.
Max-Planck Institute for Iron Research, Max-Planck-Strasse 1, 40237 Düsseldorf, Germany.
Phys Chem Miner. 2014;41(10):795-804. doi: 10.1007/s00269-014-0692-y. Epub 2014 Jun 27.
Cation exchange experiments between gem quality sanidine [Formula: see text] and KCl melt produced chemical alteration of alkali feldspar starting at the grain surface and propagating inwards by highly anisotropic Na-K interdiffusion on the alkali sublattice. Diffusion fronts developing in -direction are very sharp, while diffusion fronts within the --plane are comparatively broad. Due to the composition dependence of the lattice parameters of alkali feldspar, the diffusion induced compositional heterogeneity induces coherency stress and elastic strain. Electron back-scatter diffraction combined with the cross-correlation technique was employed to determine the lattice strain distribution across the Na-K interdiffusion fronts in partially exchanged single crystals of alkali feldspar. The strain changes gradually across the broad fronts within the --plane, with a successive extension primarily in -direction conferring to the composition strain in unstressed alkali feldspar. In contrast, lattice strain characterised by pronounced extension in -direction is localised at the sharp diffusion fronts parallel to , followed by a slight expansion in -direction in the orthoclase-rich rim. This strain pattern does not confer with the composition induced lattice strain in a stress-free alkali feldspar. It may rather be explained by the mechanical coupling of the exchanged surface layer and the mechanically strong substratum. The lattice distortion localised at the sharp diffusion front may have an influence on the diffusion process and appears to produce a self-sharpening feedback, leading to a local reduction of component mobilities.
宝石级透长石[化学式：见原文]与氯化钾熔体之间的阳离子交换实验表明，碱性长石的化学蚀变始于晶粒表面，并通过碱金属亚晶格上高度各向异性的钠 - 钾互扩散向内传播。在 - 方向上形成的扩散前沿非常尖锐，而在 - 平面内的扩散前沿则相对较宽。由于碱性长石晶格参数对成分的依赖性，扩散诱导的成分不均匀性会引起相干应力和弹性应变。采用电子背散射衍射结合互相关技术来确定部分交换的碱性长石单晶中钠 - 钾互扩散前沿的晶格应变分布。应变在 - 平面内较宽的前沿上逐渐变化，主要在 - 方向上连续扩展，这与无应力碱性长石中的成分应变一致。相比之下，以 - 方向明显延伸为特征的晶格应变位于与 平行的尖锐扩散前沿处，随后在富含正长石的边缘处 - 方向略有扩展。这种应变模式与无应力碱性长石中成分诱导的晶格应变不一致。这可能是由交换表面层与机械强度高的基体之间的机械耦合来解释。位于尖锐扩散前沿处的晶格畸变可能会对扩散过程产生影响，并且似乎会产生自锐化反馈，导致组分迁移率局部降低。
