Kumamoto Akihito, Kogure Toshihiro, Raimbourg Hugues, Ikuhara Yuichi
Institute of Engineering Innovation, School of Engineering, The University of Tokyo.
Department of Earth and Planetary Science, Graduated School of Science, The University of Tokyo.
Microscopy (Oxf). 2014 Nov;63 Suppl 1:i17. doi: 10.1093/jmicro/dfu063.
Dislocations, one-dimensional lattice defects, appear as a microscopic phenomenon while they are formed in silicate minerals by macroscopic dynamics of the earth crust such as shear stress. To understand ductile deformation mechanisms of silicates, atomic structures of the dislocations have been examined using transmission electron microscopy (TEM). Among them, it has been proposed that {100}<001> primary slip system of orthopyroxene (Opx) is dissociated into partial dislocations, and a stacking fault with the clinopyroxene (Cpx) structure is formed between the dislocations. This model, however, has not been determined completely due to the complex structures of silicates. Scanning transmission electron microscopy (STEM) has a potential to determine the structure of dislocations with single-atomic column sensitivity, particularly by using high-angle annular dark field (HAADF) and annular bright field (ABF) imaging with a probing aberration corrector.[1] Furthermore, successive analyses from light microscopy to atom-resolved STEM have been achieved by focused ion beam (FIB) sampling techniques.[2] In this study, we examined dislocation arrays at a low-angle grain boundary of ∼1° rotation about the b-axis in natural deformed Opx using a simultaneous acquisition of HAADF/ABF (JEM-ARM200F, JEOL) equipped with 100 mm2 silicon drift detector (SDD) for energy dispersive X-ray spectroscopy (EDS). Figure 1 shows averaged STEM images viewed along the b- axis of Opx extracted from repeating units. HAADF provides the cation-site arrangement, and ABF distinguishes the difference of slightly rotated SiO4 tetrahedron around the a- axis. This is useful to distinguish the change of stacking sequence between the partial dislocations. Two types of stacking faults with Cpx and protopyroxene (Ppx) structures were identified between three partial dislocations. Furthermore, Ca accumulation in M2 (Fe) site around the stacking faults was detected by STEM-EDS. Interestingly, Ca is distributed not only in these stacking faults but also Opx matrix around the faults. jmicro;63/suppl_1/i17/DFU063F1F1DFU063F1Fig. 1. (a) HAADF and (b) ABF of Opx view of [010] direction with inset simulation images and models of its unit cell (a = 0.52, c = 1.83 nm).
位错作为一维晶格缺陷，虽是微观现象，但却是由地壳宏观动力学如剪切应力在硅酸盐矿物中形成的。为了解硅酸盐的延性变形机制，已使用透射电子显微镜（TEM）研究了位错的原子结构。其中，有人提出斜方辉石（Opx）的{100}<001>主滑移系会分解为部分位错，并且在这些位错之间会形成具有单斜辉石（Cpx）结构的堆垛层错。然而，由于硅酸盐结构复杂，该模型尚未完全确定。扫描透射电子显微镜（STEM）有潜力以单原子柱灵敏度确定位错结构，特别是通过使用配备探测像差校正器的高角度环形暗场（HAADF）和环形亮场（ABF）成像。此外，通过聚焦离子束（FIB）采样技术实现了从光学显微镜到原子分辨STEM的连续分析。在本研究中，我们使用配备100 mm²硅漂移探测器（SDD）用于能量色散X射线光谱（EDS）的HAADF/ABF同步采集（JEM-ARM200F，JEOL），研究了天然变形Opx中围绕b轴约1°旋转的低角度晶界处的位错阵列。图1显示了从重复单元中沿Opx的b轴观察到的平均STEM图像。HAADF提供阳离子位点排列，ABF区分围绕a轴轻微旋转的SiO4四面体的差异。这有助于区分部分位错之间堆垛顺序的变化。在三个部分位错之间识别出两种具有Cpx和原辉石（Ppx）结构的堆垛层错。此外，通过STEM-EDS检测到堆垛层错周围M2（Fe）位点的Ca积累。有趣的是，Ca不仅分布在这些堆垛层错中，还分布在断层周围的Opx基质中。jmicro;63/suppl_1/i17/DFU063F1F1DFU063F1图1.（a）Opx的[010]方向的HAADF和（b）ABF，插图为模拟图像及其晶胞模型（a = 0.52，c = 1.83 nm）。
