29Si MAS NMR spectroscopy and synchrotron XRD study of metamict Cardiff titanite

Metamict titanite from the Cardiff uranium mine (M28696) in Ontario, Canada, has been analyzed using 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). A broad Gaussian shaped NMR signal at -81 ppm occurs at room temperature resulting from the mainly locally ordered metamict structural state. NMR signals were obtained at room temperature and after annealing at 600, 950, 1220 and 1470 K. Because of increasing crystallinity the full width at half maximum (FWHM) decreased from 24 ppm to 20 ppm, respectively using a pseudo-Voigt fit. For comparison highly-crystalline titanite from Rauris showed an NMR signal at -79.3 ppm with FWHM of 4.1 ppm and an almost Lorentzian profile because of its good long range order. Integrating synchrotron X-ray diffraction (XRD) signals of Cardiff titanite show an increase of the crystallographic long range order at annealing temperatures considerably lower than the local ordering seen by NMR.