Vermiculitization of chlorite through weathering was studied in the Myoukurasawa landslide area by means of optical microscopy, X-ray diffractometry (XRD), electron microprobe method (EPMA), analytical transmission electron microscopy (ATEM) and infrared spectroscopy.
FeMg-chlorite widely occurs with mica clay mineral in primary Miocene sedimentary rocks in the Myoukurasawa landslide area. The chlorite in the altered zone around the slip surface of landslide was transformed partly to vermiculitic minerals. EPMA and ATEM analysis of the chlorite indicated that the FeO-and MgO-contents decrease but the SiO₂-contents increase corresponding to the vermiculitization of chlorite. XRD analysis showed that the chlorite was transformed to two vermiculitic minerals: chlorite/vermiculite mixed-layer mineral evaluated to be of interstratification (g=1) with chlorite-layer percent of 60-70%, and low-charge vermiculite mineral. ATEM observations for the chlorite showed that particle size decreased corresponding to the chlorite transformation, and that the shape changed from polygonal particles to rounded ones correspondingly. It is considered that the slip surface was formed in chlorite-bearing rocks by early rock slide, and the late circulation of meteoric water facilitated the vermiculitization of chlorite in various degrees of transformation along the slip surface.