Metamict fergusonite-(Y) in a spessartine-bearing granitic pegmatite from Adamello, Italy

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 intergrowths with 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 Th-rich uraninite, and is itself rich in UO 2 (average 7.1 ± 1.1 wt.%, n = 34) and ThO 2 (average 3.5 ± 1.0 wt.%). Many fergusonite crystals display growth zoning, characterized by a general increase towards the rim in the contents of Y and rare earth elements at the expense of U and Th. Irregular or patchy zoning as well as sector zoning are also observed in some of the crystals. Due to the alpha-decay of the U and Th fergusonite is metamict, 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 discriminant analysis of its chemical composition and on the basis of the close similarity of its Raman spectrum with that of a reference β-fergusonite. The crystalline-to-metamict transformation was associated with macroscopic swelling, as indicated by microfractures that are arranged radially around fergusonite inclusions in pyrophanite. The TEM data revealed that the amorphous fergusonite contains U-rich nanocrystals (5–15 nm across), 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. We propose that these nanopores represent former bubbles of radiogenic helium. The data presented here allowed us to determine that the critical amorphization dose of fergusonite is ≤ 0.97 × 10 16 alphas/mg, i.e., lower than that of other actinide-rich oxide minerals (e.g., pyrochlore). The presence of the U-rich nanocrystals indicates that fergusonite is able to retain actinides even when it is entirely metamict and even if it is surrounded by microfractures, which represent potential fluid pathways. This result suggests that fergusonite could be a phase suitable to be included in ceramics designed for the immobilization of high-level nuclear waste.