Oxidative alteration of Ce-rich pyrochlore: HRTEM/EELS investigation

Transmission electron microscopy (TEM) and associated electron energy-loss spectroscopy (EELS) study show intergrowth of Ce4+-rich pyrochlore (metamict) and Ce3+-rich pyrochlore (partially metamict) in a Ce-rich pyrochlore from a rare earth element (REE) ore deposit of Inner Mongolia, Northern China. The partially metamict material is Ba-free and dominated by Ce3+. However, the metamict material is Ba-bearing and dominated by Ce4+. The Ce4+-rich pyrochlore may result from radiation damage by alpha decay that also causes oxidation of Fe2+ in titanite, and the interaction with a Ba-bearing oxidizing fluid. The oxidation of Ce3+ in the primary pyrochlore is accompanied by the loss of REE, Ca, and Pb, a daughter product of U via alpha decay, during the alteration. However, most REE were incorporated in the alteration product, the Ce4+-rich pyrochlore. Based on EDS and EELS analyses, the chemical formulae of the partially metamict Ce3+-rich pyrochlore and metamict Ce4+-rich pyrochlore can be written as: (Ca, Ce3+, U, Pb)2(Ti,Nb)2O7-x(OH)x, and (Ba, Ca, Ce4+, U)2(Ti, Nb)2O7-y(OH)y, respectively. Ce is the most abundant element among all REE. It is proposed that the alteration takes place in solid-state with oxidizing fluid as a catalyst. The alteration kinetics is controlled by diffusion processes of aqueous species in metamict pyrochlore