Alteration of Uranium-Rich Microlite
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Alteration of Uranium-Rich Microlite R. Gieré1, E. C. Buck2, R. Guggenheim3, D. Mathys3, E. Reusser4 and J. Marques5 1 Earth & Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397, USA; Email: [email protected] 2 Pacific Northwest National Laboratory, P. O. Box 999, Richland, WA 99352, USA 3 SEM Laboratory, University of Basel, CH-4056 Basel, Switzerland 4 Mineralogy & Petrography, ETH-Zentrum, CH-8092 Zürich, Switzerland 5 PO Box 2072, Maputo, Mozambique ABSTRACT Microlite, a Ta-rich member of the pyrochlore group, occurs in 440 Ma old lithium pegmatites in Mozambique and exhibits a pronounced growth zoning, with a U-free core surrounded by a U-rich rim (UO2 ≤ 17 wt%). Subsequent to the uplift of the host rock, microlite was subjected to intense low-temperature alteration during which Na, Ca and F were leached from the microlite crystals. This alteration, resulting from exposure to tropical conditions, also led to localized redistribution of radiogenic Pb (formation of plumbomicrolite) and to hydration of microlite, but U remained immobile. The low-temperature alteration effects are only observed in the U-rich rim, which is characterized by abundant microfractures. As demonstrated by electron diffraction images and powder X-ray patterns, the U-rich rim is largely metamict. Our investigation illustrates the importance of natural analogues in evaluating the durability of pyrochlore-based nuclear waste-form materials over geologic time. INTRODUCTION Ceramic waste forms consisting primarily of pyrochlore are currently under development for the immobilization of excess weapons plutonium [1]. These crystalline nuclear waste forms additionally contain subordinate amounts of brannerite (UTi2O6), zirconolite (CaZrTi2O7), and rutile [2]. Synthetic pyrochlore-group phases are also prominent actinide hosts in polyphase crystalline and certain glass-ceramic waste forms designed for the immobilization of high level nuclear waste [3,4]. Pyrochlore-group minerals have the general formula A2B2X6Y•nH2O, where A = Ca, Na, actinides, rare earth elements (REE), Ba, Sr, Bi, Pb; B = Nb, Ta, Ti, Zr, Sb, W, Fe; X = O, OH; and Y = O, OH, F. On the basis of the B-site cations, three subgroups are commonly distinguished within the pyrochlore group [5]: (1) pyrochlore sensu stricto (Nb-rich), (2) microlite (Ta-rich), and (3) betafite (Ti-rich). The pyrochlore-group minerals provide excellent natural analogues for pyrochlore-based nuclear waste forms, because samples of variable age and with high actinide contents are available. We have examined U-rich microlite samples from Mozambique with scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The primary goal of this study was to investigate the behavior of U during alteration of microlite under tropical conditions.
EXPERIMENTAL PROCEDURES Polished thin sections (30 µm thick) of microlite were examined optically with a polarizing petrographic microscope. The same thin sections were investigated in detail with an
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