Geology and Mineralogy of Clay Deposits in the Naegi District, Gifu Prefecture

Abstract

The lower part of the Pliocene sediments in the Naegi district, Gifu Prefecture, is rich in commercial clays such as 'gaerome' clay, 'kibushi' clay, and white clay, which are considered as fresh-water sediments in small basins on granite.
The present writers have carried out field observation of geological occurrences of the sediments and mineralogical study of the clays by means of X-ray diffraction, electron microscopy, and optical examination.
The clay fractions of the 'gaerome' and 'kibushi' clays are mainly composed of platy crystals of kaolinite accompanied by illite and tubular halloysite. The 'gaerome' clay contains coarser grains of quartz and potash feldspar and the 'kibushi' clay, as well as the larger part of the 'gaerome' clay, is stained dark brown by organic substances. The basement granite, which is considered as the source material of the overlying 'gaerome' and 'kibushi' clays, is weathered and the feldspar is altered. mainly to halloysite with the shape of long tube. Based upon the difference in mineralogy between the 'gaerome' and 'kibushi' clays and the weathered granite, the writers concluded that halloysite was transformed into kaolinite in the course of transportation and deposition of clays.
The white clay is an alteration product of pumice. It is usually composed of spherical or tubular halloysite, although it consists of kaolinite in some cases. This difference in mineralogy seems to be related to the length of time elapsed since the alteration.