APPLICATIONS OF ABERRATION-CORRECTED Z-CONTRAST IMAGING AND DENSITY FUNCTIONAL THEORY (DFT) IN MINERALOGICAL AND PETROLOGICAL STUDIES

Understanding crystal structures of nano-phases, vacancies and impurities in minerals are important to elucidate formation mechanism and subsolidus reactions of minerals and their host rocks. Aberration-corrected Z-contrast imaging can provide chemical images with sub-Å resolution. Scanning transmission electron microscopy (STEM) uses a high-angle annular dark-field (HAADF) detector to give the most highly localized 1s Bloch state imaging. Z-contrast images are HAADF images with atomic resolution. Multiple diffraction effects that appear in high-resolution transmission electron microscopic (HRTEM) imaging can be eliminated or minimized in Z-contrast imaging, because Z-contrast imaging uses non-coherent elastically scattered electrons at high scattering angle. Using a HAADF detector and annular bright-field (ABF) detector, both a Z-contrast image and an annular bright-field image can be obtained simultaneously. Bright spots in the Z-contrast image correspond to positions of atom columns. The contrast is roughly reversed in ABF image. The intensity of Z-contrast images is dependent on the atomic number of atom and occupancy in the site through Zⁿ. The power n (~ 2) depends on experimental conditions. Local composition and occupancy may be obtained from measured intensities. We can obtain positions of atoms directly over a large range of thickness, with Z-contrast to help distinguish columns of different atoms and their occupancies along the beam direction. Crystal structures of nano-minerals and nano-precipitates can be solved by combining the Z-contrast images and ab-initio calculation using density functional theory (DFT) method. Crystal structures for polymorphs of jimthompsonite and chesterite (clinojimthompsonite, Ca-clinojimthompsonite, proto-jimthompsonite, clino-chesterite, Ca-clino-chesterite, and proto-chesterite) are calculated. Vacancies, impurities, adsorbed heavy atoms can be also revealed directly. Examples of vacancy ordering in Fe-sulfides and Fe-bearing olivine, crystal structure of G. P. zones within OPX, stacking faults in anisotropic sphalerite, and cation orderings in biopyribles and intermediate plagioclase feldspars will be presented. New results show that intermediate plagioclase feldspars do not have inversion center due to Ca-Na ordering.