The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Article
Formation of surface precipitates on tuffaceous rocks near Susobana dam, Nagano City, Nagano Prefecture, Japan
Mitsuhiro AikawaMasataka NakataKosei Komuro
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2021 Volume 127 Issue 5 Pages 253-267

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Abstract

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.

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