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.