水硒铁石的再次发现及其环境意义

Second occurrence of mandarinoite at Yutangba and its environmental significance

渔塘坝是湖北恩施地区较为典型的人畜硒中毒区之一。继渔塘坝富硒碳质页岩中首次发现水硒铁石矿物之后,又在渔塘坝废弃石灰窑附近的"石煤"废渣表层松散的堆积物中发现了水硒铁石矿物。同前期发现的微小晶体状水硒铁石相比较,此次发现的水硒铁石矿物粒度大小不等,粒径几μm至上百μm。颗粒主要呈他形,以细小颗粒的团聚体、充填岩石裂隙的脉状和颗粒粗大具有一定矿物晶形的方式产出。自然硒也与水硒铁石伴生,或分布在其内部,或分布在其周围。电子探针的成分扫描表明铁、硒的面分布略有差异,主要体现在自然硒的伴生方面。这表明表生氧化条件下硒阳离子与铁氧化物的强烈结合趋势,或形成低溶解度的矿物、或吸附与络合,在硒的表生迁移转化中起着固定硒的重要作用。

Yutangba was a typical high-Se area in China where a sudden incidence of human Se poisoning occurred in 1963. In this area, after the rare mineral mandarinoite （Fe2（SeO3）3 · 6H2O） was first found in the exposed carbonaceous shale, it occurs again in the surface soil of abandoned stone coal spoils near an abandoned limekiln. In comparison with the first occurrence of mandarinoite, it mainly occurs as three types： small particles aggregates, vein filled in crack or fracture, and crystals with certain forms. Their granularity varies from several μm to more than 100μm. Native Se was generally coexisted with mandarinoite, either distributed along its inner or its boundary and around. The mineral composition scanning from EMPA indicates that there is little difference between the distribution of Fe and Se, especially on the occurrence of native Se. This feature demonstrates that Se oxyanions were highly associated with Fe-oxides in precipitated mineral, adsorbed or complexation forms in a near-surface oxidizing environment. Iron plays a significant role in the geochemical fixation of selenium in the supergene environment of Yutangba.