Two methods to characterize microscopic fractures occurring in rock due to alteration process were developed. One is fracture-mineral composite analysis, which is useful to characterize relationships among frequency, aperture, orientation of fractures, and minerals adjacent to fractures. This method was applied to hydrothermally altered granite samples. Results showed that fractures adjacent to plagioclase, potassium feldspar, or biotite and fractures with apertures less than or equal to

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... mately 36 μm were dominant in strongly altered samples. The results of the analysis were consistent with observations by polarization microscopy. The second method is multifractal analysis of standardized images. Standardization of the analyzed images enabled a comparison of the homogeneity of the fracture distribution based on pixel values. An analysis of weathered gabbro samples showed that the fracture distribution in the samples became more heterogeneous with the progress of weathering. The heterogeneity of strongly weathered gabbro was suggested to be caused as many more fractures developed in the plagioclase particles of the gab bro than in amphibole.