The mechanism of the hydrothermal alteration of cerium- and plutonium-doped zirconolite

Role of vein-phases in nanoscale sequestration of U, Nb, Ti, and Pb during the alteration of pyrochlore

“…Another plausible reason is that the dynamic behavior of Fe-rich fluids, including relatively fast dissolution of pyrochlore, and formation of Fe-rich amorphous material on the surface of pyrochlore grains, prevented TiO 2 precipitation, while promoting the formation of globular, amorphous betafite NPs. In such a case, the Fe-rich amorphous gel would form a kinetic barrier against the precipitation of rutile or anatase as suggested by Pöml et al (2011). The formation of betafite NPs without association with rutile is consistent with processes of betafite alteration in Silver Crater Mine described by Lumpkin and Ewing (1996) who noted precipitation of secondary uranpyrochlore.…”

“…Various studies show that the alteration of Ti-bearing pyrochlore supergroup minerals under natural or laboratory conditions, at temperatures 100-550 o C, led to the formation of various products that include TiO 2 (rutile, anatase), aeschynite, columbite, Nb-goethite, liandratite, uranpyrochlore, Nb-Ta oxide, and Y-REE phases (Lumpkin and Ewing, 1996;Wall et al, 1996;Nasraoui and Bilal, 2000;Xu et al, 2004;Geisler et al, 2005a,b;Pöml et al, 2007;Pöml et al 2011). In the samples analyzed here, no TiO 2 phases were observed.…”

“…Experimental studies investigating the alteration of uranium and plutonium-doped pyrochlore supergroup under hydrothermal conditions (T = 100-300 o C) revealed incongruent dissolution behavior, with different release rates for various elements (Roberts et al, 2000;Xu et al, 2004;Pöml et al, 2011). Furthermore, amorphous pyrochlore is more prone to alteration compared to its crystalline counterpart (Pöml et al, 2011). In most cases, the alteration of pyrochlore results in the formation of porosity, such as micro-cracks at grain boundaries between the original material and secondary phases, which promotes fluid migration.…”

“…Most of these phases form micro-and nanoparticles deposited on the pyrochlore surface, along grain boundaries, pores and along fractures forming secondary veins. In several cases, the alteration of pyrochlore under hydrothermal conditions led to its recrystallization (Xu et al, 2004;Geisler et al, 2005a;Leturcq et al, 2005;Pöml et al, 2007, Pöml et al, 2011.…”

DFT and two-dimensional correlation analysis methods for evaluating the Pu3+–Pu4+ electronic transition of plutonium-doped zircon

“…Brillouin-zone integration was performed using a centered 3 × 3 × 3 Monkhorst-Pack k-point mesh, with a Gaussian smearing broadening of 0.1 eV. Pseudo-atomic calculations were performed for Zr-4s 2 4p 6 4d 2 5s 2 , Si-3s 2 3p 2 , O-2s 2 2p 4 , and Pu-5f 6 6s 2 6p 6 7s 2 [38]. The calculated accuracy was fine.…”