南九州に分布する流紋岩質火砕流堆積物中の風化生成物の生成と変化

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  • Formation and evolution of weathering products in rhyolitic pyroclastic flow deposit, southern Kyushu, Japan.

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Weathering reactions of a rhyolitic pyroclastic flow deposit in southern Kyushu, Japan were investigated in order to understand the behavior of elements and the formation and evolution of reaction products. The weathering profile developed in the pyroclastics contains variable amounts of erionite, halloysite and noncrystalline Al-Si-Fe-rich materials with or without noncrystalline Mn-rich material as weathering products. The formation of these products appears to reach depth of more than 10 m below the surface. The mobility of cations in this profile is Na>Ca>K>Si>Fe>Al>Ti>Mn, in which Al, Fe, Mn, and Ti accumulate mostly in the surface horizon during the weathering. The accumulation of these elements is caused by dissolution of volcanic glass, feldspar, and pyroxene, and precipitation of noncrystalline clays enriched in Al, Fe, Mn, and Ti. Aluminum and Si released into solutions from volcanic glass and feldspar tend to combine with each other to produce Al-Si complexes with high reactivity with other metal cations at the initial weathering stages. Fe2+ and Mn2+ derived mainly from pyroxene are incorporated with the Al-Si complexes and precipitate as Al-Si materials containing Fe and Mn exhibiting irregular aggregates of randomly elongated very fine noncrystalline fibers less than 0.1μm in length. These noncrystalline fibers tend to crystallize to halloysite by partial development of a domain structure and elimination of Fe and Mn during crystallization processes. This weathering profile characteristically contains erionite crystals exhibiting hexagonal rods and thin needles normally less than 5μm in length as a major weathering product of the rhyolitic pyroclastic flow deposit. The chemical compositions of erionite crystals show a wide range with negative correlation between numbers of Si and those of Al+Fe3++Ti in which Si content increases continuously with depth. These erionite crystals may have been formed in a micro-environment where high pH and high cation activity conditions have been achieved by heterogeneous dissolution of volcanic glass and feldspar.

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  • 地質学雑誌

    地質学雑誌 105 (10), 699-710, 1999

    一般社団法人 日本地質学会

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