RECRYSTALLIZATION OF METAMICT Nb–Ta–Ti–REE COMPLEX OXIDES: A COUPLED X-RAY-DIFFRACTION AND RAMAN SPECTROSCOPY STUDY OF AESCHYNITE-(Y) AND POLYCRASE-(Y)

Metamict aeschynite-(Y) and polycrase-(Y) were studied by X-ray diffraction and Raman spectroscopy. To restore their crystal structure, the mineral samples were heated at various temperatures in air. The temperatures of 400, 500, 650, 800 and 1000°C were determined from TGA–DTA diagrams to be potentially important for structure restoration or changes, which generally correspond to the data available from previous studies. Raman spectroscopy was used to discover possible vibration bands characteristic of the metamict minerals investigated. The changes of vibration spectra are due to heating, but also reveal possible features of the crystal structure not observable by X-rays. Both X-ray diffraction and Raman spectroscopy data indicate a gradual increase in crystallinity with temperature increase. Transformation from the aeschynite to euxenite structure was observed for both minerals, but up to 1000°C, it is incomplete for aeschynite-(Y) and complete for polycrase-(Y). Raman spectra of both minerals are generally similar, a reflection of their similar chemical and structural properties, but the differences observed probably arise because of the different way the coordination polyhedra are stacked. These differences are the most pronounced in the wavenumber region from 200 to 550 cm–1, which is suitable for fingerprint recognition of the mineral phases with the aeschynite or euxenite structure.