Nollmotzite, Mg[U^V(U^VIO₂)₂O₄F₃]·4H₂O, the first natural uranium oxide containing fluorine

Abstract

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.

Crystal structure of uranyl-oxide mineral wölsendorfite revisited

“…4) and bond-valence sums incident upon those sites (Tab. 5) are consistent with all Pb present as Pb 2+ , as is the case for all known Pb-containing uranyl-oxide hydroxy -hydrate minerals (Plášil 2018). According to site-scattering refinement results, some of the sites (Pb7 and Pb8) are considerably low-populated, <0.5.…”

“…The third most complex UOH has to be newly considered wöl-sendorfite. This study revealed additional H 2 O sites, and both richetite and schoepite that had been considered in the past as far more complex turned out to be less-complex actually as they have been structurally revisited (Plášil 2017(Plášil , 2018. For comparison, the most complex mineral in Nature is the uranyl carbonate ewingite (Olds et al 2017b), Mg 8 Ca 8 (UO 2 ) 24 (CO 3 ) 30 O 4 (OH) 12 (H 2 O) 138 .…”

“…7) is vandendriesscheite, Pb 1.5 [(UO 2 ) 10 O 6 (OH) 11 ] (Burns 1997). This mineral appears relatively soon in the alteration sequence after uraninite (Finch, Ewing 1992;Plášil 2018) and is characteristic for the presence of Pb 2+ , usually of the radiogenic origin, derived from weathered pitchblende. The second most complex mineral is gauthierite, KPb[(UO 2 ) 7 O 5 (OH) 7 ](H 2 O) 8 (Olds et al 2017a), which seems to appear relatively soon in the sequence after vandendriesscheite.…”

“…Uranyl-oxide hydroxy-hydrates (further on abbreviated as UOHs) represent one of the most structurally and chemically complex families of naturally occurring U 6+ phases (Plášil 2018), extremely important for the nuclear fuel waste management (see, e.g., O'Hare et al 1988;Finch, Murakami 1999;Klingensmith et al 2007;Kubatko et al 2006;Maher et al 2013;Ewing 2015). The onset of oxidation and hydration of uranium oxide, UO 2 (as pitchblende or synthetic in the nuclear fuel), often produces minerals consisting of electroneutral sheets of uranyl pentagonal bipyramids (of the α-U 3 O 8 type) with substantial H 2 O in the interlayer region, and typically little, if any, additional metal cations (Burns 2005;Krivovichev, Plášil 2013;Lussier et al 2016;Plášil 2018a;Plášil et al 2020).…”

“…The onset of oxidation and hydration of uranium oxide, UO 2 (as pitchblende or synthetic in the nuclear fuel), often produces minerals consisting of electroneutral sheets of uranyl pentagonal bipyramids (of the α-U 3 O 8 type) with substantial H 2 O in the interlayer region, and typically little, if any, additional metal cations (Burns 2005;Krivovichev, Plášil 2013;Lussier et al 2016;Plášil 2018a;Plášil et al 2020). There are also particularly interesting rare cases when UOHs contain mixtures of U(IV), U(V), or U(VI) oxidation states as well (Burns et al 1997;Burns, Finch 1999;Plášil 2017;Plášil et al 2018). Continued alteration of uranium oxide, alteration in more chemically diverse aqueous fluids, and alteration of geologically old uranium oxide that contains substantial radiogenic lead result in the formation of uranyl-oxide hydroxy-hydrates with anionic sheets of uranyl polyhedra also containing uranyl square pyramids (β-U 3 O 8 type of sheets) charge-balanced by cations in the interstitial regions of the structures.…”