The occurrence and origin of selenium minerals in Se-rich stone coals, spoils and their adjacent soils in Yutangba, China

Several Se-bearing minerals have been identified in Se-rich stone coal, spoils, and their adjacent organic-rich soils in Yutangba of Enshi, China, where human Se toxicity occurred in the 1960s. These minerals mainly include native Se (Se ⁰), krutaite (CuSe ₂), klockmannite (CuSe), mandarinoite (Fe ₂(SeO ₃) ₃.6H ₂O), Se-bearing chalcopyrite (CuFe(Se, S) ₂), and pyrite (Fe(Se,S) ₂). The assemblage of native Se, krutaite, klockmannite, and Se-bearing pyrite and chalcopyrite is primarily present in the stone coal near a fault plane, while the assemblage of native Se and krutaite is found in the Se-rich carbonaceous mudstone and organic-rich soils which are 60m away from the stone coal exposure. The assemblage of mandarinoite and native Se is present in abandoned stone coal spoils, where natural combustion occurred. Native Se is quite extensive in the stone coal spoils and nearby soils derived from them. The co-occurrence of Cu-Se minerals and native Se indicates that these minerals could have formed under a relatively acidic and reducing environment below 220°C, and suggests that Cu could play a significant role in fixing reduced Se ions in the acidic, organic-rich surface environment. Furthermore, the occurrence of mandarinoite suggests that iron-oxides constrain the geochemical behavior of Se in oxidizing environments. Our observations provide new insights into the mechanisms of Se fixation and accumulation during weathering of Se-rich rocks.