Study on Microstructures of Granite by SEM Secondary-Electron, Cathodoluminescence, and Backscattered-Electron Images

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Microstructures of oriented granitic samples were observed by a scanning electron microscope (SEM) and an optical microscope. Three kinds of images : secondary-, and backscattered-electron and cathodoluminescence images on the same position in a given sample, were able to be clearly obtained by the EM. The sampels were collected from Late Gretaceous to Early Paleogene granitic rock bodies distributed around large-scale faults or including no large-scale faults in Southwest Japan, respectively, (1) Naegi-Agematsu granite distributed around the Atera, Byobusan and Enasan faults, (2) Inagawa granite around Asuke-cho, East Kamo-Gun, Gifu Prefecture, and Hiroshima granite in Kitakijima and Oshima Islands in the Inland Sea of Japan and around Hagi City in San'in Region.The SEM and optical-microscope observations revealed that various types of microstructures existed in the granitic samples. Based on a scale and position of these microstructures, they are classified into three deformation mic … More rostructure types: intergranular type being continuous through multiple crystals, grain boundary type found at contacts between adjacent crystals, and intragranular type existing in a crystal with no extension to the adjacent crystals. The intergranular type includes microbreccia, healed zone, mineral-filled fracture, and intergranular crack. The grain boundary type strongly depends upon the mineralogy of the two adjacent grains. This type has healed, mineral-filled and serrated grain boundaries, grain boundary crack and myrmekite. Occurrence of the intragranular type is related to the kind of the crystals. Healed and carapace cracks are common in quartz, while microfault, cleavage crack, kink bands and mechanical twin commonly exist in feldspars. Undulatory extinction and intragranular cracks are found in both minerals. Biotite includes clevage crack and step, bend, fish and kink bands. Since differences in mineralogy are clearly recognized by contrast in SEM backscattered electron images, these images are good for grain boundary microstructures. SEM cathodoluminescense images clearly display banded structures in quartz and microstructures in myrmekitic and serrated boundary zones, which are impossible to be observed by the optical microscopy. Less