Zeolites in the Granitic Rocks of Chugoku District, Southwest Japan

Ca-zeolites such as laumontite, stilbite and chabazite were found in the granitic rocks of both magnetite and ilmenite series of late Cretaceous to Tertiary age developed widely in the Chugoku district. These zeolites occur (1) in veins of various scale, (2) replacing plagioclase of the host granitic rocks, and (3) in pegmatitic druse. Mineralogy were investigated by X-ray powder diffraction method, the optical microscope, the scanning electron microscopy and the electron microprobe.
Based on the mode of occurrence and the associated minerals, laumontite and stilbite in veins and replacing plagioclases are concluded to be formed from hydrothermal solutions filling open fractures which have been formed under the paleo-stress field after the solidification stage of the host granitic rocks. Probable formation condition of laumontite and stilbite was estimated as follows; 200-250°C(temperature), 0.5-1.0 Kb(P total) and 20-40 bar(max Pco ).