An XRD, TEM and Raman study of experimentally annealed natural monazite

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 Xray 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,I 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 degreesC for 7 days. Each run product was analysed by X-ray diffractometry. Second, monazite grains were hydrothermally annealed Lit temperatures from 500 to 1200 degreesC for 5 to 15 days. TEM observations show that partial healing of the monazite lattice already occurred at 500 degreesC and increased gradually with temperature. so that after 10 days at 900 degreesC complete healing was achieved, The observations Lire 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 or the A monazite lattice. Diffraction domains B Lire 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 Lire healed. At 900-1000 degreesC only a monazite with well-crystallised lattice and minimum unit-cell volume is observed.