Annealing of metamict minerals – spectroscopic study

Author(s)

C. Chanmuang N., M. Zeug, A. Erlacher, K. A. G. Sameera, L. Nasdala

Abstract

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.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

The Open University of Sri Lanka

Publication date

2023

Austrian Fields of Science 2012

104026 Spectroscopy, 105116 Mineralogy, 105113 Crystallography

Portal url

https://ucrisportal.univie.ac.at/en/publications/e26e38e8-7a92-4c0d-b971-cdcd36700013