We analyzed the clay mineralogical composition of twelve samples of weathered granitic rocks collected at Saga, Fukuoka and Kumamoto Prefectures. Halloysite was a principal mineral accompanied by mica and vermiculite. A “high-charge” smectite was found in the two of twelve samples but in small quantities. This mineral shows a basal spacing of 18 Å on Mg-saturation and glycerol-solvation and of 10 Å upon K-saturation and air-drying.
The 53-200pm of the samples having “high-charge” smectite did not contain vermiculite, different from the other samples. Mica dominating them was a dioctahedral mineral. Thus, some dioctahedral mica may be transformed into “highcharge” smectite and not into vermiculite.