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Metamict thorium orthosilicates from the syenite pegmatites of the Larvic Plutonic Complex, Norway: phase transformations and thermal behaviour

Published online by Cambridge University Press:  25 December 2024

Ruiqi Chen
Affiliation:
Department of Crystallography, St. Petersburg State University, St. Petersburg, Russia Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, Lille, France
Oleg I. Siidra*
Affiliation:
Department of Crystallography, St. Petersburg State University, St. Petersburg, Russia Kola Science Center, Russian Academy of Sciences, Apatity, Murmansk Region, Russia
Vladimir V. Shilovskikh
Affiliation:
Geomodel Resource Center, St. Petersburg State University, St. Petersburg, Russia
Vera A. Firsova
Affiliation:
Institute of Silicate Chemistry, Russian Academy of Sciences, St Petersburg, Russia;
Valery L. Ugolkov
Affiliation:
Institute of Silicate Chemistry, Russian Academy of Sciences, St Petersburg, Russia;
Elena L. Kotova
Affiliation:
Saint-Petersburg Mining Institute, St. Petersburg, Russia
*
Corresponding author: Oleg I. Siidra; Email: o.siidra@spbu.ru

Abstract

Three metamict thorium orthosilicate samples from the syenite pegmatites of the Larvic Plutonic Complex, Norway, were thoroughly examined using Raman spectroscopy, electron probe microanalyses (EPMA), electron back-scatter diffraction (EBSD) and differential scanning calorimetry (DSC). Their thermal evolution upon heating was investigated using in situ powder X-ray diffraction (HTXRD) in the range of 25–1200°C. One of the samples is a colour-zoned metamict thorium silicate with a preserved tetragonal shape. The zonation is due to the increasing hydration and element distribution. The EBSD indicates that the ratio of huttonite to thorite after the crystallisation significantly varies from zone to zone within the same sample. The crystallisation of thorite starts in the range of 420–480°C (lower than reported previously for mineral samples), while the emergence of huttonite peaks in HTXRD patterns occurs at 870–930°C. In contrast to huttonite, no thorite crystallisation peak is observed in the DSC curve. A wide temperature range is observed where both thorite and huttonite can coexist. Several fluorite-type phases form upon heating. Thorianite exists in the range of 810–1140°C. After the cooling, except for huttonite and thorite, the minor crystallised phases vary and may be represented by Ca–Th oxides and rhombohedral CaUO4.

The thermal expansion of the crystalline huttonite and thorite was determined as $\overline\alpha$V = 20.66 × 10–6 deg–1 for huttonite and $\overline\alpha$V = 12.54 × 10–6 deg–1 for thorite in the temperature range 25–1200°C. These findings contribute to a more in-depth understanding of the behaviour of thorium orthosilicates with complex compositions, both metamict and crystalline, at elevated temperatures. They have potential applications in mineralogy, nuclear chemistry and high-level waste management.

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Copyright
© The Author(s), 2024. Published by Cambridge University Press on behalf of The Mineralogical Society of the United Kingdom and Ireland.

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