Metamict fergusonite-(Y) in a spessartine-bearing granitic pegmatite from Adamello, Italy
- Author(s)
- Katja Ruschel, Reto Gieré, Richard Wirth, Terry Williams
- Abstract
A granitic pegmatite associated with the Monte Bruffione granodiorite in the Tertiary Adamello Massif, Italy,consists primarily of albite, potassium feldspar, muscovite, quartz, and spessartine (graphic intergrowthswith quartz). As accessory minerals, the pegmatite contains magnetite, pyrophanite, monazite-(Ce),uraninite, xenotime-(Y), zircon, and fergusonite-(Y). This yttrium niobate mineral contains inclusions of Thrichuraninite, and is itself rich in UO2 (average 7.1±1.1 wt.%, n=34) and ThO2 (average 3.5±1.0 wt.%).Many fergusonite crystals display growth zoning, characterized by a general increase towards the rim in thecontents of Y and rare earth elements at the expense of U and Th. Irregular or patchy zoning as well as sectorzoning are also observed in some of the crystals. Due to the alpha-decay of the U and Th fergusonite ismetamict, as documented by transmission electron microscopy (TEM) and micro-Raman spectroscopy.Nevertheless, the mineral could be identified as fergusonite-(Y) on the basis of a canonical discriminantanalysis of its chemical composition and on the basis of the close similarity of its Raman spectrum with thatof a reference ß-fergusonite. The crystalline-to-metamict transformation was associated with macroscopicswelling, as indicated by microfractures that are arranged radially around fergusonite inclusions inpyrophanite. The TEM data revealed that the amorphous fergusonite contains U-rich nanocrystals (5-15 nmacross), which in most cases are distributed randomly and which probably nucleated after metamictization.The TEM investigations further revealed the ubiquitous presence of nano-sized (typically 5-25 nm across),nearly circular features, which exhibit a low diffraction contrast and which we interpret as nanopores. Wepropose that these nanopores represent former bubbles of radiogenic helium. The data presented hereallowed us to determine that the critical amorphization dose of fergusonite is =0.97×1016 alphas/mg, i.e.,lower than that of other actinide-rich oxide minerals (e.g., pyrochlore). The presence of the U-richnanocrystals indicates that fergusonite is able to retain actinides even when it is entirely metamict and evenif it is surrounded by microfractures, which represent potential fluid pathways. This result suggests thatfergusonite could be a phase suitable to be included in ceramics designed for the immobilization of highlevelnuclear waste.
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- Albert-Ludwigs-Universität Freiburg, Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Natural History Museum London
- Journal
- Chemical Geology
- Volume
- 261
- Pages
- 333-345
- No. of pages
- 13
- ISSN
- 0009-2541
- Publication date
- 2009
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 105113 Crystallography, 1030 Physics, Astronomy
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/e803910a-f1ac-461e-a704-63b08066f04a