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An XRD, TEM and Raman study of experimentally annealed natural monazite |
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| Authors: | A M Seydoux-Guillaume R Wirth L Nasdala M Gottschalk J M Montel W Heinrich |
| Institution: | GFZ-Potsdam-Division 4, Telegrafenberg, 14473 Potsdam, Germany, DE Institut für Geowissenschaften – Mineralogie, Johannes Gutenberg-Universit?t, 55099 Mainz, Germany, DE LMTG-UMR 5563, Laboratoire de Minéralogie Université Paul Sabatier, 39 Allées Jules Guesde, 31000 Toulouse, France, FR |
| Abstract: | The healing of radiation damage in natural monazite has been experimentally studied in annealing experiments using XRD, TEM, Raman microprobe and cathodoluminescence analysis. The starting material was a chemically homogeneous monazite from a Brazilian pegmatite with a concordant U–Pb age of 474 ± 1 Ma and a U–Th/He age of 479 Ma. The monazite shows nm-scale defects induced by radioactive decay. The X-ray pattern of the unheated starting material revealed two distinct monazite “phases” A and B with slightly different lattice parameters. Monazite A shows sharp reflections of high amplitudes and slightly expanded lattice parameters (1% in volume) compared to a standard monazite. Phase B exhibits very broad reflections of low amplitudes. Two sets of experiments were performed. First, dry monazite powder was annealed at 500, 800 and 1000 °C for 7 days. Each run product was analysed by X-ray diffractometry. Second, monazite grains were hydrothermally annealed at temperatures from 500 to 1200 °C for 5 to 15 days. TEM observations show that partial healing of the monazite lattice already occurred at 500 °C and increased gradually with temperature, so that after 10 days at 900 °C complete healing was achieved. The observations are interpreted accordingly: the starting material has a mosaic structure consisting of two domains, A and B, which are basically two monazite crystals with different lattice parameters. We suggest that the A domains correspond to well-crystallised areas where helium atoms are trapped. The accumulation of He causes expansion of the A monazite lattice. Diffraction domains B are interpreted as a helium-free distorted monazite crystal lattice, which can be referred to old alpha-recoil tracks. These B domains are composed of “islands” with an expanded lattice, induced by the presence of interstitials, and “islands” of a compressed monazite lattice, induced by presence of vacancies. Both the islands will pose stress on the lattice in the vicinity of the islands. The broadening of the B reflections is due to the expanded or compressed diffraction domains and to the different amount of the distortion. With increasing temperature the unit-cell volume of monazite A decreases, i.e. the position of the A reflections shifts towards smaller d hkl values. This was interpreted as a relaxation of the monazite lattice due to helium diffusion out of the monazite lattice. Simultaneously, the nm-sized defect domains B are healed. At 900–1000 °C only a monazite with well-crystallised lattice and minimum unit-cell volume is observed. Received: 7 May 2001 / Accepted: 11 October 2001 |
| Keywords: | Monazite Annealing experiments Helium XRD TEM |
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