Composition Changes of Tokyo Pumice Bed by Weathering
1983 Volume 23 Issue 3 Pages 106-118
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1983 Volume 23 Issue 3 Pages 106-118
The samples from Tokyo pumice bed, which was erupted from Volcano Hakone and covered South Kanto, Central Japan widely about fifty thousands years ago, were collected from 19 sampling points. Mineralogical and chemical alterations of them by weathering were investigated by the chemical analysis, X-ray powder diffraction, DTA, grain size distribution analysis and transmission electron microscopy. It was found that halloysite, allophane and imogolite were the major species of clay minerals formed by weathering. Composition change in SiO2, Al2O3 and ΣFe2O3 system of bulk samples showed the trends of relative condensation of Al2O3 and Fe2O3 components and leaching of SiO2 and FeO components in which trends were generally observed in the alteration under neutral condition. The linear correlation was observed between the amount of H2O component of each sample and that of each chemical component. With the increment of the amount of H2O component, components of SiO2, FeO, CaO, MgO, and Na2O showed the trends of decreasing while those of Al2O3, Fe2O3, TiO2 and MnO showed the trends of increasing by relative condensation. It was also found that these chemical composition changes show not always direct correlation with the species of clay minerals formed by weathering. By examining the local distribution of the species of clay minerals, predominant species of clay minerals were found to change from halloysite to allophane and further to imogolite according to the thickness of loam beds above Tokyo pumice bed became thinner and these phenomena were considered to closely correlate to the amounts of SiO2 component provided from the above beds.
