Abstract
Weathered granites were studied in order to understand quantitative mineralogical changes which seriously affect the engineering properties of granite. Nishinomiya granites of different weathering grades were examined by using a X-ray diffractometer (XRD), optical microscope, electron probe micro-analyzer (EPMA). In addition, modal analysis was performed for back-scattered electron images (BSEI) of the granites. XRD and EPMA analyses suggest that Nishinomiya granite weathering can be characterized by the plagioclase and biotite alterations to clay minerals. The mineralogical alteration series in weathered granites are 1) biotite → vermiculite → smectite → kaolin mineral, and 2) plagioclase → smectite → kaolin mineral. These mineralogical changes of the constituent minerals may be one of the major factors controlling physical and chemical properties of granite under weathering environment. Modal analysis were performed by a fitting of probability density functions and a peak decomposition for the gray level histogram of BSE image. This convenient measurement method gives relevant modal values of constituents including alteration products, approximately equivalent to those by the conventional point counting method. It is indicated that the measurement method using BSEI is applicable to highly-resolvable quantitative-analysis of the mineralogical change in granite weathering.
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