Kogure Toshihiro, Okunishi Eiji
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan.
J Electron Microsc (Tokyo). 2010;59(4):263-71. doi: 10.1093/jmicro/dfq003. Epub 2010 Feb 17.
Significant improvement of the resolution for electron microscopy by the development of the spherical aberration corrector (Cs-corrector) brings valuable information to understand local atomic structures in silicate minerals. Cation columns separated by ca. 1.5 A, which originate from the ionic radius of oxygen anion or half of the closest oxygen-oxygen distance, are common in silicates and they are easily resolved by a Cs-corrected transmission electron microscope (TEM) or scanning TEM (STEM) with a resolution close to 1 A. High-angle annular dark-field (HAADF) imaging using Cs-corrected STEM was applied to orthopyroxene (Opx) with augite lamellae, and cronstedtite, an iron-bearing sheet silicate. Noisy contrast in the HAADF images was compensated by advanced noise filtering techniques. All cation columns in the pyroxene structure were resolved in the HAADF images from the c-axis. A sub-angstrom difference in the position of the M2 site between Opx and augite, which is caused by the occupation of the site by Fe (Opx) and Ca (augite), was clearly detected in the HAADF image as well as different contrasts of the cation columns related to occupying elements. A pair of tetrahedral cation columns separated by ca. 1.5 A in cronstedtite observed along [100] directions frequently show unequal contrast, suggesting a difference of the amounts of substituted Fe(3+) between the two columns. Comparison between the experimental contrast with a simple simulation suggests the distribution of Fe(3+) in a tetrahedral sheet avoiding linkage of Fe(3+) coordinating tetrahedrons.
球差校正器（Cs校正器）的发展显著提高了电子显微镜的分辨率，为理解硅酸盐矿物中的局部原子结构带来了有价值的信息。由约1.5埃间隔的阳离子柱，源于氧阴离子的离子半径或最近氧-氧距离的一半，在硅酸盐中很常见，并且它们很容易被分辨率接近1埃的Cs校正透射电子显微镜（TEM）或扫描TEM（STEM）分辨出来。使用Cs校正STEM的高角度环形暗场（HAADF）成像被应用于含有普通辉石薄片的斜方辉石（Opx）和含铁层状硅酸盐绿脱石。HAADF图像中的噪声对比度通过先进的噪声过滤技术得到了补偿。从c轴方向的HAADF图像中分辨出了辉石结构中的所有阳离子柱。在HAADF图像中清楚地检测到，由于Fe（Opx）和Ca（普通辉石）占据该位置，导致Opx和普通辉石中M2位点位置存在亚埃级别的差异，以及与占据元素相关的阳离子柱的不同对比度。沿着[100]方向观察到的绿脱石中一对由约1.5埃间隔的四面体阳离子柱经常显示出不相等的对比度，这表明两列之间取代的Fe(3+)量存在差异。将实验对比度与简单模拟进行比较表明，四面体片中Fe(3+)的分布避免了Fe(3+)配位四面体的连接。
