The nature of radiohaloes in biotite: Experimental studies and modeling

Several micro-techniques (confocal laser-Raman microprobe, optical absorpti on micro-spectroscopy, high-resolution transmission electron microscopy, el ectron microprobe analysis) were employed in the detailed characterization of radiohaloes in biotites from two Variscan rocks from Germany. The studie d biotites are intermediate members of the phlogopite-annite series with Mg /Fe2+ ratios in the range 1.6-1.0. Radiohaloes in biotite resulted from the impact of He-4 cores (alpha -particles) emitted from actinide-bearing incl usions. Monte Carlo simulations yielded alpha (U-238, U-235, and Th-232 ser ies) penetration ranges in biotite between 12.5 and 37.3 mum, which are in reasonable agreement with the observed radii of radiohaloes in natural biot ites. The coloration pattern of a radiohalo closely correlates with the cal culated distribution pattern of point defects generated in displacive event s. Calculated point defect densities in the range from < 10(-5) to at most 10(-2) dpa (displacements per lattice atom) suggest that there are only sca ttered point defects in a mainly pre served biotite lattice. This is consis tent with HRTEM studies that did not reveal any indication for initial volu me amorphization in the haloes. However, general Raman band broadening and intensity loss suggest that the short-range order in radiohaloes is signifi cantly disturbed. The darkened color of radiohaloes. when compared with the un-irradiated host biotite, is caused by increased light absorption over t he complete visible range due to increased point defect density. No additio nal color centers were found, and the absorbances of the Fe-VI(2+), Fe2+-Fe 3+, and Fe2+-Ti(4+)centers seem hardly to be changed. Both Raman and optica l absorption spectra obtained from radiohaloes retain a clear orientational dependence. The results suggest that the formation of point defects rather than ionization is the main process causing the coloration of radiohaloes in natural biotites. The haloes represent an early stage of structural radi ation damage, characterized by significantly disturbed short-range order bu t still widely preserved long-range order of the structure.