長野県伊那地方の風化堆積火山灰の組成変化

概要

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Chemical and mineralogical changes of volcanic ash beds which were erupted from the Volcano Ontake around Ina district, Nagano Prefecture by weathering were examined by means of the chemical analysis. X-ray powder diffraction, differential thermal analysis and grain size distribution analysis. In the Middle Loam unit, clay mineral formed by weathering was mainly allophane in the pumice beds of Pm-II. II'. III, whereas it was mainly halloysite 10Å in the Pm-I and the other loam beds. In the Younger Loam unit, clay mineral formed was mainly allophane and halloysite 10Å was not observed. By weathering the compositions of SiO_2, CaO. MgO, Na_2O and K_2O decreased, whereas those of Al_2O_3 and ΣFe_2O_3 increased by the relative condensation, and H_2O increased by the formation of clay minerals. It was found in the chemical composition changes of SiO_2, Al_2O_3 and ΣFe_2O_3 that SiO_2 and ΣFe_2O_3 leached and Al_2O_3 relatively condensed in the lower part of the outcrop, whereas SiO_2 leached and Al_2O_3 and ΣFe_2O_3 relatively condensed in the middle and upper parts. These chemical changes were understood by assuming the existences of reducing, intermediate and oxidizing zones by the reaction of underground water and/or by considering the changes of the chemical compositions of the host rocks from rhyolitic to andesitic.
日本粘土学会の論文
1983-03-25