Petrishcheva E, Rieder M, Predan J, Fischer F D, Giester G, Abart R
1Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria.
2Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia.
Phys Chem Miner. 2019;46(1):15-26. doi: 10.1007/s00269-018-0983-9. Epub 2018 Jul 7.
The chemically driven propagation of interacting parallel cracks in monoclinic alkali feldspar was studied experimentally. Single crystals of potassium-rich gem-quality sanidine were shifted towards more sodium-rich compositions by cation exchange with a NaCl-KCl salt melt at a temperature of and close to ambient pressure. Initially, a zone with elevated sodium content formed at the crystal surfaces due to the simultaneous in-diffusion of sodium and out-diffusion of potassium, where the rate of cation exchange was controlled by sodium-potassium interdiffusion within the feldspar. A chemical shift of potassium-rich alkali feldspar towards more sodium-rich compositions produces highly anisotropic contraction of the crystal lattice. This induced a tensile stress state in the sodium-rich surface layer of the crystals, which triggered the formation of a system of nearly equi-spaced parallel cracks oriented approximately perpendicular to the direction of maximum shortening. Crack propagation following their nucleation was driven by cation exchange occurring along the crack flanks and was controlled by the intimate coupling of the diffusion-mediated build-up of a tensile stress state around the crack tips and stress release by successive crack propagation. The critical energy release rate of fracturing was determined as 1.8-2.2 from evaluation of the near-tip J-integral. The mechanism of diffusion-controlled crack propagation is discussed in the context of high-temperature feldspar alteration.
对单斜碱长石中相互作用的平行裂纹的化学驱动扩展进行了实验研究。通过在温度为且接近常压的条件下，用NaCl-KCl盐熔体进行阳离子交换，将富钾宝石级透长石的单晶转变为钠含量更高的成分。最初，由于钠的向内扩散和钾的向外扩散同时发生，在晶体表面形成了一个钠含量升高的区域，其中阳离子交换速率由长石内部的钠钾互扩散控制。富钾碱长石向钠含量更高的成分的化学转变会导致晶格产生高度各向异性的收缩。这在晶体富含钠的表层中引发了拉伸应力状态，从而触发了一个近乎等间距的平行裂纹系统的形成，这些裂纹大致垂直于最大缩短方向排列。裂纹成核后的扩展是由沿裂纹侧面发生的阳离子交换驱动的，并受裂纹尖端周围拉伸应力状态的扩散介导积累与连续裂纹扩展引起的应力释放之间的紧密耦合控制。通过对近尖端J积分的评估，确定断裂的临界能量释放率为1.8 - 2.2 。在高温长石蚀变的背景下讨论了扩散控制裂纹扩展的机制。
