Surface features and alteration products of natural zirconolite leached in silica-saturated solutions

Zirconolite, CaZrTi2O7, has been proposed as an immobilization phase for the disposition of excess weapons Pu and other actinides (e.g., 235U). Due to actinide incorporation, zirconolite is expected to sustain α-decay event damage and become aperiodic (=metamict) over time. The leaching behavior of metamict zirconolite is, therefore, of interest. Because groundwater in a variety of geologic settings contains up to saturation concentrations of silicic acid, H4SiO4, silica-saturated solutions were used. Natural, metamict (> 1026 α-decay events per mn3) zirconolite grains, nominally (Ca, Th)ZrTi2O7 (U.S. National Museum sample #B20392, Walawada, Sri Lanka) were leached in two separate silica-saturated solutions at 150 °C for 60 days. Surface features and alteration products were examined using scanning electron microscopy (SEM) and quantitative energy dispersive X-ray spectroscopy (EDS). Secondary electron (SEI) images of the surfaces of the leached grains from both experiments revealed pores, probably due to the accumulation of He-bubbles from α-decay events, of approximately 4% as estimated by contrast enhanced gray-scale analysis of digital images. SEI of the zirconolite surface before leaching showed a smooth surface. The pores not only increase the surface area of the metamict zirconolite, but also act as nucleation sites for alteration phase growth. One experiment was conducted in a silica-saturated solution containing approximately 100 ppm P as measured by atomic absorption spectroscopy (AAS). The main alteration phase was euhedral, monoclinic cheralite, (Th, Ca, Ce)(P, Si)O4 (monazite group). The second experiment was conducted in the absence of P. The main alteration phase was subhedral cubic thorianite, ThO2.