Formation of surface precipitates on tuffaceous rocks near Susobana dam, Nagano City, Nagano Prefecture, Japan

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  • Aikawa Mitsuhiro
    Faculty of Education, Tokyo Gakugei University Asano Junior High School and High School
  • Nakata Masataka
    Faculty of Education, Tokyo Gakugei University
  • Komuro Kosei
    Department of Earth System Science, Faculty of Sustainable Design, University of Toyama

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Other Title
  • 長野県長野市裾花ダム近傍の凝灰質岩表面における表面析出物の生成
  • ナガノケン ナガノシ スソ カ ダム キンボウ ノ ギョウカイシツガン ヒョウメン ニ オケル ヒョウメン セキシュツブツ ノ セイセイ

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Abstract

<p>Surface precipitates are found on the surfaces of tuffaceous rocks near Susobana dam, Nagano City, Nagano Prefecture, Japan. These precipitates were identified, using X-ray powder diffraction with energy dispersive X-ray spectroscopic analysis, as alunogen, gypsum, epsomite, hexahydrite，pickeringite, and tamarugite. Thermodynamic phase diagrams suggest that epsomite and gypsum precipitate under low to high pH conditions, whereas alunogen and pickeringite form under strongly acidic conditions. Model calculations from evaporitic reactions of sulfate-bearing solutions, under equilibrium with wallrock minerals, demonstrates that sulfuric acid formed during oxidation is neutralized by calcium, magnesium, and aluminium in the wallrock minerals. This process leads to the formation of gypsum, epsomite and jurbanite as neutralization products, at pH conditions of 11.2, 7.0, and 3.5, respectively. It is also noted that neither gypsum nor epsomite necessarily form under acidic conditions, and that alunogen does not form under equilibrium with wallrock minerals. Model calculations for evaporitic reactions of sulfate-bearing solutions, under no equilibrium reaction with wallrock Al-bearing phases and precipitated minerals, indicate that the solutions formed are strongly acidified with no formation of alunogen. Model calculations for evaporitic reactions of sulfate-bearing solutions with SO4/Al ratios between 1 and 5, under equilibrium with precipitated minerals but also under no equilibrium reaction with wallrock minerals, suggest that alunogen would form with SO4/Al ratios between 1.5 and 4.7 and an evaporitic enrichment factor of over 1010. The sulfate mineral species of surface precipitates are hence inferred to be related to the leaching of wallrock minerals. For example, gypsum and epsomite form during alkali-earth leaching, pickeringite at the end-stage of alkali-earth leaching, and alunogen when alkali and alkali-earth elements are depleted.</p>

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