Annealing of metamict minerals – spectroscopic study

In the country of Sri Lanka, efforts were recently undertaken to survey the occurrence of radioactive phases (Kuruppu et al. 2020; Sameera et al. 2020), among others as potential Th sources. However, phase identification is hampered in many cases because corpuscular self-irradiation over geologic periods of time, caused by the radioactive decay of Th and U and their instable daughter products, may cause these phases to undergo crystalline-to-aperiodic transition. The final, glassy state is referred to as metamict (Brøgger 1893).
Metamict minerals are difficult to identify, as structural analysis techniques cannot yield specific information. For instance, X-ray powder diffraction patterns of metamict aeschynite–(Y), euxenite–(Y), fergusonite–(Y), polycrase–(Y), pyrochlore, and samarskite–(Y) are widely similar (Erlacher 2021; and references therein). Similarly, Raman spectra of metamict minerals are extremely broadened and hence cannot be reliably distinguished from each other. The initial, crystalline mineral phase may in some cases be identified using annealing experiments. This, however, is only possible if upon dry annealing, the phase under discussion undergoes structural reconstitution but does not decompose. An example for the latter are allanite-group minerals: Already at temperatures as low as ca. 300 °C, potential dehydrogenation and Fe²⁺ oxidation need to be considered (Sobek et al. 2023; and references therein).
The heating-induced recovery of radiation-damaged minerals may be vastly different, depending on the degree of damage. If a specimen consists of aperiodic and remnant crystalline domains, moderate-T growth of the latter may contribute substantially to the reconstitution. In contrast, the recovery of metamict (that is, fully radiation-damaged) phases requires random nucleation in the glassy bulk and, therefore, typically requires more elevated T.
We present herein the identification of two metamict mineral species whose occurrence on the island of Sri Lanka was hitherto unconfirmed, gadolinite–(Y) (Y₂Fe²⁺Be₂Si₂O₁₀) and fergusonite–(Y) (YNbO₄). Stepwise annealings of multiple chips of samples was undertaken to study the recovery processes.