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Metamict aeschynite-(Y) from the Evje-Iveland district (Norway)

¹ Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 1–50121 Firenze, Italy
² Dipartimento di Chimica, Università di Firenze, Via G. Capponi 9, 1–50121 Firenze, Italy

^* e-mail: pbcry{at}steno.geo.unifi.it

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 O^2- 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: Y_0.51REE_0.25Th_0.15U⁴⁺_0.06Ca_0.04)_=1.01(Ti_1.56Nb_0.33Ta_0.05W_0.02Fe²⁺_0.03)_=1.99O_5.52(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.

Key-words: aeschynite-(Y), Ti-Nb-Ta-oxides, rare earth elements, Rietveld structure refinement, metamict minerals, heating treatment, infrared spectroscopy.

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