Formation of Clay Minerals during Low Temperature Hydrothermal Alteration of Obsidian (Part 2). Formation of Clay Minerals in H2SO4 Solution.

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  • KAWANO Motoharu
    Department of Environmental Sciences and Technology, Faculty of Agriculture, Kagoshima University
  • TOMITA Katsutoshi
    Institute of Earth Sciences, Faculty of Science, Kagoshima University

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Other Title
  • 低温水熱反応による黒曜石からの粘土鉱物の生成（その２）　硫酸溶液中での粘土鉱物の生成
  • テイオン スイネツ ハンノウ ニ ヨル コクヨウセキ カラ ノ ネンド コウブ
  • Formation of Clay Minerals in H<SUB>2</SUB>SO<SUB>4</SUB>Solution
  • 硫酸溶液中での粘土鉱物の生成

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Formation and transformation processes of clay minerals during experimental alteration of obsidian in H2SO4 solution were examined by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray analysis. Dissolution mechanism of obsidian in H2SO4 solution was also investigated byX-ray photoelectron spectroscopy (XPS). The experimental alteration was performed in 0.01N and 0.001N H2SO4 solutions at 150°C and 200°C for 1 to 60 days, respectively. Concentrations of dissolvedelements in solutions and their pH values were also measured. At 200°C reaction, alunite, fibrous boehmite, and platy kaolinite were produced in acidic solution, whereas flaky smectite appeared with increasing the pH value above 7. At 150°C reaction, boehmite, allophane, and halloysite were producedat the early stage, and a small amount of spherical kaolinite appeared at later stage. Results of TEM indicated that flaky smectite was formed from fibrous boehmite by phase transition process, likewise halloysite was transformed from aggregates of allophane particles as alteration progressed.Based on stability diagram for the system of Na2O-Al2O3-SiO2-H2O, it was found that kaolinite andsmectite were formed as a stable phase, whereas boehmite, allophane, and halloysite appearedas a metastable phase. It was demonstrated by XPS that Na was preferentially leached from the obsidian surface independent on pH values of solutions. However, the rate of Al/Si leaching increased in solutions at pH<3, and the rate decreased above the value.

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