Origin of coffinite in sedimentary rocks by a sequential adsorption-reduction mechanism

Abstract

Coffinite is the dominant ore mineral in the tabular vanadium-uranium ores of the Tony M mine in the Henry Mountains Mineral Belt of the Colorado Plateau physiographic province of the United States. This ore body formed at a density-stratified solution interface between uranyl ion bearing-meteoric water and a saline fluid which was locally reducing. Uranium localization at this solution interface occurred by adsorption onto the surfaces of detrital minerals. This adsorption was related to the pH difference between the two fluids. New experimental evidence is presented which shows that adsorption facilitated the reduction of uranium to U(IV). This adsorbed, reduced uranium bonded with aqueous silica in the ore zone to form coffinite. Evidence is presented that in order to stabilize coffinite over uraninite, this silica was highly concentrated in the ore-forming solution, and the dominant form of this silica was as a monomeric species.

Response of synthetic coffinite to energetic ion beam irradiation

“…In situ HRTEM images (Fig. 3) of a coffinite particle viewed along [1][2][3][4][5][6][7][8][9][10][11] zone axis clearly show the disruption of the lattice fringes with increasing dose. The lattice fringes are almost entirely lost at a dose of $0.38 dpa.…”

“…Coffinite occurs as a common alteration product of uraninite, UO 2+x , a natural analogue to the UO 2 in spent nuclear fuel [1][2][3]. Coffinite will form as an alteration product of spent nuclear fuel exposed to silica-rich ground water under reducing conditions [4][5][6][7][8][9]. The radiation response of coffinite is of even greater interest if it also incorporates transuranium elements, such as Pu, during the corrosion of UO 2 in spent nuclear fuel, particularly a mixed oxide fuel [10,11].…”

Synthesis and characterization of coffinite

“…In many potential host rocks considered for a repository, silica concentrations are high enough to allow silicate precipitation (for instance, Si $ 10 À4 mol L À1 in the French candidate site -Meuse/Haute-Marne -for a future repository). Coffinite USiO 4 is therefore expected to be one of the potential alteration phases precipitating from SNF alteration under reducing Si-rich conditions [10][11][12][13][14][15]. Indeed, this mineral is widely observed in uranium ore deposits as an alteration phase of uraninite (natural analog of SNF [16]), particularly at Oklo (Gabon) [13,[17][18][19] and Cigar Lake (Canada) [20][21][22][23].…”

“…Conversely, Mumpton and Roy [45] did not synthesize coffinite but uraninite and quartz and suggested coffinite may be metastable. Robit-Pointeau [41,46] suggested that (i) either the domain of thermodynamic stability of coffinite occurs in a narrow Eh/pH range, which would be different from the one predicted by the selected thermodynamic data [9], (ii) and/or coffinite precipitation is a kinetically controlled process and requires an unidentified precursor that could involve organic matter as reported from the uranium deposit [10,47,48]. Other coffinite synthesis protocols based on hydrothermal [49,50,7] or dry methods [51,52] are mentioned in the literature.…”

Uranium secondary phase formation during anoxic hydrothermal leaching processes of UO2 nuclear fuel

“…As an example, the phases coffinite (USiO 4 can form at hydrothermal conditions in geologic media [9]. For the case of coffinite formation, further mechanisms were found to apply [10]. These materials can be seen as potential candidates for uranium(IV) secondary phases formation during anoxic spent fuel alteration.…”