engleski

Fergusonite crystallization in altered metamict pyrochlore group mineral

A metamict mineral sample from Ytterby, Sweden, was originally identified as fergusonite. However, detailed heating experiments monitored by X-ray powder diffraction (XRD), Raman Spectroscopy (RS), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED) revealed more complex nature of the mineral sample. XRD shows the natural unheated sample to be almost completely metamict. A few very weak diffraction lines are observed and could be attributed to fergusonite, but also to pyrochlore. Raman spectrum of unheated mineral contains only very broad bands. TEM images of the sample show domains with preserved fragments of the original structure in generally amorphous mineral matrix. These structure fragments are identified as a pyrochlore phase by HRTEM and SAED. On gradual heating in air at 400, 500, 650, 800, 1000°C for 24 hours in each case, and at 1300°C for 6 hours, the mineral recrystallizes, as indicated by successive XRD patterns and Raman spectra. However, the recrystallization is not completely straightforward. Although the mineral starts to recrystallize at 400°C with a pyrochlore structure (s.g. Fd3m, a=10.342(1) &Aring ; , V=1106.3(3) &Aring ; 3), which is continuously present up to 1300°C, a fergusonite phase appears at 1000°C and becomes more crystalline at 1300°C, thus coexisting with the pyrochlore phase. The changes observed by RS in samples heated at temperatures higher than 800°C also suggest crystallization of new phase at 1000°C. Both XRD and SAED patterns indicate the fergusonite phase to be -fergusonite (s.g. I2) with a=5.277(5), b=10.905(7), c=5.021(4) &Aring ; ,  =94.64(6)°, V=288.0(2) &Aring ; 3. Electron microprobe analysis (EPMA) shows significant compositional variation, especially in Ca, Y and Fe content. This could indicate alteration during metamictization characterized by leaching of large A-site cations [G. R. Lumpkin & R. C. Ewing, Am. Mineral. 77 (1992) 179]. Our findings show that the preserved domains of original pyrochlore structure serve as centers of recrystallization of apparently original pyrochlore phase, most likely employing the mechanism of epitaxial growth [W. J. Weber, Nucl. Instr. Meth. B 166-167(2000) 98]. Completely metamict domains are susceptible to alteration resulting in a change of the original mineral stoichiometry. This could result in crystallization of fergusonite phase at higher temperatures. Recrystallization in completely metamictized domains probably employs nucleation-crystal growth mechanism, which is more energetic, and thus occurs at higher temperature [J. Lian et al, J. Nucl. Mater. 297 (2001) 89].

crystallization; metamictization; fergusonite; pyrochlore

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