To study the possible structural-geometrical variations affecting the structure of aeschynite when heated in air, the heat-induced recrystallization process of a metamict sample from Evje-Iveland was monitored by recording X-ray powder diffraction data. The progressive restoring of crystallinity starts at 350°C and appears to be nearly completed at 450°C. During this process the water contained in the untreated amorphous material is released. Structural data were obtained by Rietveld refinements after each successive heat treatment (450–1050°C temperature range, in step of 100°C). With the increase of heating temperature, other phases than aeschynite progressively crystallized (i.e. pyrochlore, rutile, brannerite).The transition to the high-temperature phase (euxenite-type) occurs at approximately 1100°C and appears to be more temperature- than time-dependent. Formula balance requirements and structural evidence lead to the assumption that an appreciable amount of OH- replaces O2- at the O4 site. The presence of OH- groups is confirmed by Fourier Transform Infrared Spectroscopy. Crystal-chemical features and chemical data yielded the following chemical formula: Y0.51REE0.25Th0.15U4+0.06Ca0.04)Σ=1.01(Ti1.56Nb0.33Ta0.05W0.02Fe2+0.03)Σ=1.99O5.52(OH)0.48. Upon heating the sample in air at temperatures higher than 550°C, an oxidation-dehydrogenation reaction takes place, resulting mainly in a lengthening of the donor-acceptor distance and a related shortening of the A-O4 and B-O4 bond distances. The observed variations of the cell parameters appear to be related to the oxidation-dehydrogenation process rather than to changes of the cation site-population.

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