Date Published: August 01, 2018
Publisher: International Union of Crystallography
Author(s): Jakub Plášil, Anthony R. Kampf, Radek Škoda, Jiří Čejka.
The twinned structure of the new uranium oxide mineral was refined from X-ray diffraction data and was found to contain fluorine and pentavalent uranium. The presence of pentavalent uranium is indicative of the reducing conditions under which the mineral formed. Nollmotzite is the first naturally occurring uranium oxide mineral that contains fluorine.
Uranium oxides, especially those containing UVI as the uranyl (UO22+) ion, are important products of supergene weathering of primary UIV minerals, predominantly uraninite, UO2. Uranium dioxide, both in nuclear fuel and uraninite, readily alters in the presence of water and oxygen resulting in the formation of uranyl-oxide hydroxy-hydrate minerals (UOH) (Finch & Ewing, 1992 ▸; Wronkiewicz et al., 1992 ▸, 1996 ▸; Janeczek et al., 1996 ▸, and others). UOH minerals are among the first phases to form during the oxidation–hydration weathering of UO2 (Finch & Ewing, 1992 ▸; Finch et al., 1996 ▸; Schindler & Hawthorne, 2004 ▸; Krivovichev & Plášil, 2013 ▸; Plášil, 2014 ▸). Because of their importance for nuclear waste disposal and the environmental chemistry of uranium in general (see e.g. O’Hare et al., 1988 ▸; Finch & Murakami, 1999 ▸; Klingensmith et al., 2007 ▸; Kubatko et al. 2006 ▸; Maher et al., 2013 ▸), studies describing their structures and physical–chemical properties, such as solubility and thermodynamic stability, are numerous. Herein, we provide a description of the new mineral nollmotzite, which is the first naturally occurring uranium oxide that contains fluorine. It also is noteworthy for containing uranium as both UV and UVI.
The formation of nollmotzite is a result of solid-state precipitation from U-containing aqueous solutions under partially reducing conditions. Nollmotzite contains ∼20 mol.% F in a structural unit, suggesting that the fugacity of O2 was lower than under fully oxidizing conditions. The reducing environment was most probably enhanced by consumption of oxygen during the oxidation of abundant pyrite present in the gangue (crusts of FeIII oxyhydroxides are abundant on the samples). The major gangue minerals are dark varieties of fluorite (called stinkspath or antozonite), barite and quartz. The high fugacity of fluorine and low fugacity of oxygen led to the incorporation of fluorine into the sheet structure of nollmotzite and a partial reduction of UVI to UV. The discovery of nollmotzite contributes to the large body of radioactive-waste final-disposal research. In designing final repositories for radioactive waste, the paradigm has shifted from using oxidative conditions towards using strongly reducing conditions (Ewing, 2015 ▸; Ewing et al., 2016 ▸; Rojo et al., 2018 ▸). Reducing environments are considered preferable to prevent the corrosion of the stainless steel tanks embedded in clay/cement after the closure of the repository. In spite of efforts to create overall reducing conditions, oxygen atoms will continue to be available from groundwater and/or dissolved oxygen-containing minerals in the backfill. Stainless steel used for the fabrication of tanks in the repository generally does not contain fluorine (even if there are some industrial passivation processes based on fluorination). However, traces of fluorine can be derived from dissolution of phosphate minerals present in the backfill, or it can be present in the groundwater. The corrosion processes take place at the interfaces between radioactive waste and tanks/backfill/surrounding rocks, and they are often bounded onto small (micro) areas with strong geochemical gradients.
The new mineral found at the Clara mine, Black Forest Mountains, Germany, is the first known naturally occurring uranium oxide that contains significant fluorine. Furthermore, the reducing conditions under which this mineral formed led to the partial reduction of the UVI to UV. Therefore, nollmotzite is one of the very few minerals containing pentavalent uranium. The characterization of new supergene uranium minerals with unusual structural and/or compositional features, such as nollmotzite, can provide valuable insights into processes that may occur during the long-term storage of spent nuclear fuel.