Partitioning of Actinides, Rare Earth Elements, and Other Trace Elements In Titanium-Rich Veins From Adamello, Italy
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Partitioning of Actinides, Rare Earth Elements, and Other Trace Elements In Titanium-Rich Veins From Adamello, Italy Gregory R. Lumpkin1, Reto Gieré2, T.E. Payne1, P.J. McGlinn1 and K.P. Hart1 Materials Division, ANSTO, Private Mail Bag 1, Menai, NSW 2234, Australia 2 Dept. of Earth and Atmospheric Sciences, Purdue Univ., West Lafayette, IN 47907-1397, USA 1
ABSTRACT Extensive mineralogical and chemical studies have been carried out on the Ti-rich hydrothermal veins emplaced within the contact aureole of the Adamello batholith. In addition to other actinide and rare earth element host phases, the veins contain both zirconolite and betafite and provide information relevant to ceramic wasteforms designed for the disposal of actinide-rich nuclear wastes. In this paper, we describe the results of element partitioning studies based on dissolution experiments using 9M HCl. Generally, the acid-resistant minerals include allanite, baddeleyite, betafite, chalcopyrite, geikielite, titanite, spinel, and zirconolite. We also found that the major silicate minerals forsterite, phlogopite, and titanian clinohumite and the sulfide mineral pyrrhotite are partially dissolved by the acid treatment, whereas calcite and apatite are highly soluble (as expected). In particular, the distributions of Th and U between the acid-resistant and acid-soluble fractions indicate that they partition mainly between zirconolite, titanite, betafite, and apatite. However, there is a considerable increase in the amounts of Zr, Nb, Th, and U released in certain actinide-rich samples that may result from enhanced dissolution following radiation damage. INTRODUCTION During the 1990s, much of the wasteform research and development activities focused on the design and production of pyrochlore- and zirconolite-rich titanate ceramics suitable for the disposal of actinide-rich wastes [1-4]. An important milestone during this period was the recommendation that pyrochlore be selected as the basis of a wasteform for the dispositioning of weapons Pu [5]. In parallel with the laboratory studies, investigations of the Ti-rich hydrothermal veins at Adamello, Italy have yielded important data on the long-term behavior of potential actinide host phases such as zirconolite and betafite (Ti-rich member of the pyrochlore group) in nuclear wasteforms [6-9]. One of the major aspects of the work at this locality is the transport and deposition of high field strength elements (HFSE, e.g., Ti, Zr, Nb), rare earth elements (REE), and actinides (ACT) in hydrothermal fluids [10-14]. In this report, we describe the partitioning behavior of selected major elements, HFSE, REE, and ACT elements present in one of the Ti-rich veins. We have used an aggressive leaching technique because of the need to discriminate between a number of host phases having a range of solubilities. The results of this study are intended to complement the development of an extended model of vein formation, including aspects of the complexation and deposition of the HFSE, REE, and ACT in the system. Some aspect
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