Cs-corrected HAADF-STEM imaging of silicate minerals

Abstract

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.