抄録
Large amounts of illite/montmorillonite mixed layer minerals occur in Miocene tuffaceous sediments in Waga-Omono district in Akita Prefecture, Northern Japan. It was clarified by mineralogical and chemical examinations of subsurface samples obtained from drilling cores that authigenic minerals of hydrothermal origin are arranged zonally around a Cu-Pb-Zn deposit. The illite/montmorillonite minerals are distributed in the intermediate position between the sericite-chlorite zone and the montmorillonite-zeolites zone.
The mineralogical properties of the illite/montmorillonite minerals change successively with the change in the content of expandable layer. Judging from x-ray patterns, the interstratification of the two minerals may change from random when the content of expandable layer exceeds 60%, to ordered when it is of 25-40% and less than 15%.
The chemical composition of the mixed layer mineral varies systematically in concord with the variation in the content of expandable layer. In the course of the transformation of montmorillonite to illite through their interstratification, Mg is expelled from the octahedral layer, while Al is added to the tetrahedral layer of the minerals.
In the studied area, the above transformation has occurred in the range of about 100 meters. According to Eberl and Hower (1976), the geothermal gradient at the time was estimated to be 20-30°C/100m.
