Oxidative Alteration of Ce-rich Pyrochlore: HRTEM/EELS Investigation
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ABSTRACT Transmission electron microscopy (TEM) and associated electron energy-loss spectroscopy (EELS) study show intergrowth of Ce4-rich pyrochlore (metamict) and Ce 3+-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 Ce". However, the metamict material is Ba-bearing and 4 4 may result from radiation damage by alpha dominated by Ce ,. The Ce +-rich pyrochlore 2 decay that also causes oxidation of Fe + in titanite, and the interaction with a Ba-bearing 3 oxidizing fluid. The oxidation of Ce " in the primary pyrochlore is accompanied by in the loss of REE, Ca, and Pb, a daughter product of U via alpha decay, during the alteration. 4 However, most REE were incorporated in the alteration product, the Ce +-rich pyrochlore. 3 the chemical formulae of the partially metamict Ce +_ Based on EDS and EELS analyses, 4 rich pyrochlore and metamict Ce ,-rich pyroeblore can be written as: (Ca, Ce", U, Pb)2 (Ti, 4 Nb) 20 7 ,(OH),, and (Ba, Ca, Ce +, U)2(Ti, Nb) 20 7 _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.
INTRODUCTION Pyrochlore group minerals commonly display metamict and partially metamict states, because they contain radioactive elements, such as U and Th [1-4]. The REE-rich (or, Ce-rich) pyrochlore is classified as ceriopyrochlore, (Ca, Ce, U) 2(Ti, Nb) 20 7 /(OH)x [1-4]. The oxidation state of Ce in ceriopyrochlore is assumed to be +3. However, there are no reported data about quantification of the oxidation states of Ce in natural Ce-bearing pyrochlore. This paper will describe an integrated transmission electron microscopy (TEM) and associated electron energy-loss spectroscopy (EELS) study of Ce-bearing pyrochlore mineral and its alteration product in an oxidizing environment. The study of Ce- and U-bearing pyrochlore minerals may also help us to study ceramic waste forms of pyrochlore and zirconolite (a derivative structure of pyrochlore), 3 6 4 because these phases can incorporate a variety of radionuclides (e.g., U +, U +, Pu +, 3 4 2 2 6 4 Pu +, Pu +, Ba +, Sr +, Cs+, Rb+) and neutron absorbers (e.g., Hf + and Gd +). The radionuclides are initially incorporated in these phases. However, alteration and corrosion processes of the crystalline phase in geological repositories will result in re-distribution of the radionuclides among alteration products and local fluids. The crystalline high-level waste (HLW) forms will experience radiation damage and corrosion in geological repositories at elevated temperature. In general, corrosion includes dissolution and alteration (i. e., solid-state chemical and structural modification with fluid as a catalyst). Alteration will change the structure and composition of the host phases, affect oxidation states
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