Investigation of the Long-Term Performance of Betafite and Zirconolite in Hydrothermal Veins From Adamello, Italy
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Betafite and zirconolite occur in Ti-rich hydrothermal veins emplaced within dolomite marble in the contact aureole of the Adamello batholith, northern Italy. Zirconolite contains up to 18 wt% ThO 2 and 24 wt% U0 2 , and exhibits strong compositional zoning. Some zirconolite grains were corroded by the hydrothermal fluid. Betafite, the Ti-rich member of the pyrochlore group, often occurs as overgrowths on zirconolite. The betafite is weakly zoned and contains 29-34 wt% U0 2 . In terms of end-members, betafite contains approximately 50 mole percent CaUTi 2 0 7 and is the closest known natural composition to the pyrochlore phase proposed for use in titanate waste forms. Amorphization and volume expansion of the betafite caused cracks to form in the enclosing silicate mineral grains. Backscattered electron images reveal that betafite was subsequently altered along crystal rims, particularly near the cracks. EPMA data reveal little difference in composition between altered and unaltered areas, except for lower totals, suggesting that alteration is primarily due to hydration. The available evidence demonstrates that both betafite and zirconolite retained actinides for approximately 40 million years after the final stage of vein formation. During this time, betafite and zirconolite accumulated a total alpha-decay dose of 3-4 x 1016 and 0.2-2 x 1016 ux/mg, respectively. INTRODUCTION
The minerals betafite (CaUTi 2 0 7) and zirconolite (CaZrTi 2O7) offer considerable potential as natural analogues for actinide host phases in ceramic nuclear waste forms including Synroc and its various derivatives [1], tailored ceramics [2], and specialized titanate ceramics designed for the storage of weapons plutonium [3]. Betafite, the Ti-rich member of the pyrochlore group, is of particular interest due to the development and testing of pyrochlore based titanate waste forms. Natural analogue studies provide information on the chemical composition and predominant substitution mechanisms, radiation damage effects, and the stability of potential actinide host phases in different geological environments [4,5,6]. In this paper, we report on the occurrence of betafite and zirconolite in Ti-rich veins emplaced within dolomite marble in the contact aureole of the Adamello batholith in northern Italy. This locality is of particular importance because the geology, conditions of vein formation, and mineral chemistry are well established [7,8]. Both betafite and zirconolite have been found in the veins, providing a rare opportunity to examine the long-term behavior of two potential waste form phases together in the same host rock. EXPERIMENTAL
PROCEDURES
Petrographic observations were made by optical microscopy and SEM-EDX on polished sections of Adamello sample Brl9. Secondary and backscattered electron images, semiquantitative analyses, and element distribution maps were obtained with a JEOL 6400 SEM operated at 15 or 25 kV and equipped with a Noran EDX system. Additional X-ray maps were obtained by using a JEOL 8900 Superprobe operated at 15 k
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