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Recrystallisation of samarskite group minerals: characterisation of high-temperature multiphase systems

Samarskite group minerals are complex oxides with general formula ABO4 (A = REE, Ca, Fe2+, Fe3+ and U ; B= Nb and Ta). The crystal structure of samarskite has not been determined precisely due to its metamict nature and complex paths of recrystallisation. Annealing experiments in normal and inert atmosphere yield multiphase systems. Sugitani at al. (1985) obtained a single phase by heating the mineral in a reducing atmosphere, believed to present samarskite structure. However, an investigation of other samarskite samples did not support this finding (Hanson et al., 1998). Five metamict mineral samples predetermined as samarskite and euxenite were heated at 400, 500, 650, 800 and 1000°C for 24 hours in air. X-ray diffraction data were collected to investigate the features of their gradual recrystallisation, but also to reveal the complexity of these high-temperature multiphase systems. All the unheated mineral samples are amorphous to X-rays. Recrystallisation starts in the temperature range between 400-650°C. Three samples clearly show the signatures of euxenite structure (Pbcn, a 14.7, b 5.6, c 5.2 &Aring ; ; ; ), being stable up to 800°C. Since euxenite is considered to be a high temperature phase in REE-Nb-O systems with AB2O6 stoichiometry, its transformation/disintegration to samarskite, petscheckite, pyrochlore and possibly FeNbO4 is not expected. Other samples start to recrystallise as pyrochlore (Fd m, a 10 &Aring ; ; ; ), but at 800°C the phase system becomes more complex. The earliest appearance of samarskite was observed for one sample at 800°C. At 1000°C two multiphase systems were observed: samarskite-pyrochlore-petscheckite-(FeNbO4) and samarskite-pyrochlore. The appearance of petscheckite, U4+Fe2+(Nb, Ta)2O8, (P 1m, a=6.45, c=4.02 &Aring ; ; ; , V = 143.5 &Aring ; ; ; 3) at higher temperatures for some investigated sample indicates uranium enrichment. This is also suggested by d-values for the two strongest samarskite peaks (3.05 and 2.92 &Aring ; ; ; ) having values more close to ishikawaite, (U, Fe, Y, Ce)(Nb, Ta)O4. A significant resemblance of the high-temperature diffraction patterns to  -fergusonite could be also observed, but the patterns fit better to the samarskite high-temperature phase (P2/c, a=5.65, b=9.94, c=5.23 &Aring ; ; ; ,  =93.6°, V=293.1 &Aring ; ; ; 3) (Sugitani et al., 1985). The examined samples appear homogeneous, although an original single phase presence can not be inferred. Obvious chemical complexity and the degree of amorphisation/preservation of original structure could be crucial for such a complicated recrystallisation. Detailed chemical and spectroscopic characterisations are intended for further investigation. References: Hanson, S. L., Simmons, W. B. Jr. & Falster, A. U. (1998): Can. Miner., 36: 601-608 Sugitani, Y. Suzuki, Y. & Nagashima, K. (1985): Am. Miner., 70: 856-866

samarskite; recrystallisation; multiphase systems characterisation

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