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Identification of premetamict crystal structure fragments in a metamict mineral using HRTEM and SAED

INTRODUCTION Metamict minerals, although believed to be originally crystalline, are characterised by a breakdown of crystal structure due to radiation damage originating from radioactive elements in structural positions. The  -particles and  -recoil nuclei originating from  -decay series displace atoms from their sites causing defects in crystal structure (1). Accumulation of defects leads to gradual amorphisation of a mineral. Highly metamictised minerals appear to be completely amorphous for X-rays, and it is usually necessary to recrystallise them by heating to obtain structural data. Sometimes recrystallisation does not necessarily result in the crystallisation of the expected structure. Therefore, it is appropriate to investigate a metamict mineral for possibly preserved fragments of the original premetamict structure. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) are suitable tools for such an investigation. EXPERIMENTAL A sample of fergusonite-(Y), ideally defined as YNbO4, originating from granitic pegmatite in Bakkane-Steane, Norway, was studied. The mineral was investigated by X-ray powder diffraction (XRD), HRTEM and SAED. XRD data were obtained by Philips X’ Pert diffractometer with CuK radiation accelerated by 45 kV and 40 mA. HRTEM and SAED experiments were performed using a Philips CM200 TEM operating at 200 kV. The mineral was gradually recrystallised in air by annealing at 400, 500, 650, 800, 1000 and 1300°C for 24 hours in each case. SAED and high resolution images were analysed using Lábár’ s software (2) and DigitalMicrograph 3.6.5 software of GATAN (3). RESULTS AND DISCUSSION X-ray diffraction data indicate that the unheated mineral is completely metamict (Fig. 1). The mineral starts to recrystallise at 400°C with scheelite-type structure ( -fergusonite, s.g. I41/a) (4), being stable up to 800°C. At 1000°C a phase transition to the monoclinic fergusonite with distorted scheelite-type structure ( -fergusonite, s.g. I2) (4) occurs and is completed at 1300°C. By inspection of the unheated mineral using HRTEM and SAED, isolated areas showing lattice fringes are observed in amorphous matrix (Fig. 2A). The corresponding SAEDs are displayed in Fig. 2a1 and 2a2. The obtained d-values vary between 2.948-3.137 Å . From SAED pattern (Fig. 2a1) a set of d-values was obtained, mostly corresponding to those of the  -fergusonite. The observed d-values from HRTEM images are in the range of the most intensive diffraction lines of both  - and  - fergusonite, but SAED data and the sequence of recrystallisation monitored by XRD indicate  -fergusonite as the original phase. This would be in agreement with the expected recrystallisation mechanism by epitaxial growth around preserved crystal structure fragments (5). REFERENCES (1) Ewing, R. C. (1994), Nuclear Instruments and Methods in Physics Research, B91, 22-29 (2) János L. Lábár, Proc. of EUREM 12, July 2000, Brno (L. Frank, F. Ciampor, eds.), Vol. III. pp. I379-380 (3) Digital Micrograph 3.6.5 (1999), Gatan Inc., Pleasanton, CA, USA (4) Komkov, A. I. (1959), Kristallografiya, 4, 836-841 (5) Lian, J., Wang, S.X., Wang, L.M., Ewing, R. C. (2001), Journal of Nuclear Materials, 297, 89-96

fergusonite; metamictization; recrystallization; TEM

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