地表近傍の酸化的な水理地質環境における放射性鉱物の安定性―岐阜県南東部，苗木地方の砂鉱床の事例:岐阜県南東部，苗木地方の砂鉱床の事例
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Study of the stability of radioactive minerals in a placer deposit in the Naegi District, southeastern Gifu Prefecture, Central Japan contributes to understanding the modes of nuclide migration under various hydrogeological environments in the tectonically active Japanese Island Arc system. The placer deposit is embedded in basal conglomerates of the lowermost alluvial sequences, exposed to a near-surface, oxidizing hydrogeological environment. The mineral samples, ranging in mesh size from 120 to 250, were identified after sieving and magnetic separation to be mainly cassiterite, thorite, monazite, and topaz, with subordinate amounts of zircon, fergusonite-(Y), xenotime and wolframite. Observations using optical and scanning microscopy indicated that many grains of zircon have well-preserved crystal faces. Most monazite and fergusonite-(Y) grains are partly abraded and corroded whereas thorite grains are highly abraded and corroded. This indicates that under an oxidizing hydrogeological environment, the mechanical durability and geological stability decrease from zircon to monazite/fergusonite-(Y) to thorite, which correlates well with the Mohs's hardness scale. Cut and polished thorite grains display a high degree of alteration. The altered portions have higher Th, Fe and Y contents, and lower U and Si contents in comparison with the unaltered portions, indicating leaching of U and Si. In the fergusonite-(Y) grains, the altered portions have higher Th, Nb and Ta contents, and lower U and Y contents in comparison with the unaltered portions, indicate leaching of U and Y. Thus it is determined that uranium is strongly leached in an oxidizing hydrogeological environment. The leaching behaviour is dependent on mineralogy and is consistent with thermodynamic estimates. The alteration rate of fergusonite-(Y) was calculated to range from 0.05 to 0.000025 μm/year based on the thickness of the external alteration film and the duration of exposure to the oxidizing conditions. The lifetimes of fergusonite, monazite and thorite are estimated to be of the order of 10<SUP>5</SUP>-10<SUP>7</SUP>, 10<SUP>5</SUP>-10<SUP>7</SUP> and 10<SUP>4</SUP> -10<SUP>6</SUP> years, respectively. Although leaching or release of uranium from radioactive minerals to the surface geological environment is estimated to be long duration under an oxidizing hydrogeological environment, total amount of uranium released to surface geological environment is negligible because radioactive minerals are generally rare.
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