Metamict fergusonite-(Y) and samarskite-(Y) from the Ilmeny mountains (South urals, Russia): secondary alterations and thermal annealing

Abstract

Nb-Ta-Ti-REE (rare earth elements) oxide minerals can serve as petrogenetic indicators in geology, potential materials for nuclear waste immobilization, phosphors for luminescent applications, and other advanced material science uses. The disorder and thermal recovery of their crystal structures are critical properties that determine their suitability for these applications. This study focuses on the metamict fergusonite-(Y) from the Ilmeny Mountains in Russia, which contains inclusions of metamict samarskite-(Y), monazite, xenotime, apatite, and ThSiO₄. To investigate the metamict state, degree of alteration, and thermal recovery of these minerals, we employed scanning electron microscopy (SEM), electron probe micro-analyser (EPMA), Raman spectroscopy, and cathodoluminescence spectroscopy. The fergusonite-(Y) grain, containing mineral inclusions, exhibits variations in chemical composition across grain and pore boundaries, attributed to dissolution-reprecipitation processes under the influence of fluids. Annealing of fergusonite-(Y) at 1250 °C resulted in the formation of β-fergusonite, while the chemical composition remained unchanged. In contrast, samarskite-(Y) transformed into a crystalline state after annealing at 1250 °C, with a compositional shift towards pyrochlore. Our findings demonstrate that fergusonite-(Y) exhibits greater compositional stability compared to samarskite-(Y) under thermal treatment. These results highlight the potential of Nb-Ta-Ti-REE oxide minerals for applications requiring thermal and chemical stability.