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Three-dimensional study on the interconnection and shape of crystals in a graphic granite by X-ray CT and image analysis

S. Ikeda^1,*,, T. Nakano² and Y. Nakashima³

¹ Geological Institute, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
² Geological Information Centre, Geological Survey of Japan, Higashi, 1-1-3, Tsukuba, Ibaraki 305-8567, Japan
³ Geophysics Department, Geological Survey of Japan, Higashi, 1-1-3, Tsukuba, Ibaraki 305-8567, Japan

^* E-mail: ikeda{at}chem.s.u-tokyo.ac.jp

The technique of investigating 3-dimensional interconnections and the shapes of crystals in a rock by X-ray computerized tomography (CT) and image analysis was developed using a graphic granite specimen as an example. Fifty 2-dimensional tomographic images (slices) of the graphic granite were obtained ‘non-destructively’ using a medical X-ray CT scanner. Since a CT value of the specimen was decreased with increasing cross-sectional sample area by the effect of beam-hardening, the CT value was corrected using the area of each slice. Binary images of the slices were made comparing one of them with a thin-section of the slice. Using the binary images, connection analysis of quartz rods in the graphic granite specimen was performed on the basis of percolation theory (cluster labelling). This analysis showed that at least 89.9% of the quartz rods were connected in three dimensions. Furthermore, the 3-dimensional shape of the quartz rods was analysed using the 2-point correlation function calculated from the binary images. The average shape of the quartz rods was obtained by fitting an ellipsoid to the high-value region of the 2-point correlation function. The elongation axis of the ellipsoid agreed well with the crystallographic c-axes of the quartz rods.

KEYWORDS: X-ray CT, 3-dimensional image analysis, graphic granite, cluster labelling, 2-point correlation function

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