A multi-technique study of altered granitic rock from the Krunkelbach Valley uranium deposit, Southern Germany

摘要

Herein, a multi-technique study was performed to reveal the elemental speciation and microphase composition in altered granitic rock collected from the Krunkelbach Valley uranium (U) deposit area near an abandoned U mine, Black Forest, Southern Germany. The former Krunkelbach U mine with 1-2 km surrounding area represents a unique natural analogue site with the rich accumulation of secondary U minerals suitable for radionuclide migration studies from a spent nuclear fuel (SNF) repository. Based on a micro-technique analysis using several synchrotron-based techniques such as X-ray fluorescence analysis, X-ray absorption spectroscopy, powder X-ray diffraction and laboratory-based scanning electron microscopy and Raman spectroscopy, the complex mineral assemblage was identified. While on the surface of granite, heavily altered metazeunerite-metatorbernite (Cu(UO2)(2)(AsO4)(2-x)(PO4)(x)center dot 8H(2)O) microcrystals were found together with diluted coatings similar to cuprosklodowskite (Cu(UO2)(2)(SiO3OH)(2)center dot 6H(2)O), in the cavities of the rock predominantly well-preserved microcrystals close to metatorbernite (Cu(UO2)(2)(PO4)(2)center dot 8H(2)O) were identified. The Cu(UO2)(2)(AsO4)(2-x)(PO4)(x)center dot 8H(2)O species exhibit uneven morphology and varies in its elemental composition, depending on the microcrystal part ranging from well-preserved to heavily altered on a scale of similar to 200 mu m. The microcrystal phase alteration could be presumably attributed to the microcrystal morphology, variations in chemical composition, and geochemical conditions at the site. The occurrence of uranyl-arsenate-phosphate and uranyl-silicate mineralisation on the surface of the same rock indicates the signatures of different geochemical conditions that took place after the oxidative weathering of the primary U- and arsenic (As)-bearing ores. The relevance of uranyl minerals to SNF storage and the potential role of uranyl-arsenate mineral species in the mobilization of U and As into the environment is discussed.