Radiation damage in biotite; defined by micro XAS and XRD

Radiation damage in mineral structures is of interest because of their consideration as radioactive waste forms. Examples of damage occuring on geological timescales are available in nature. Alpha particle damage around actinidebearing inclusions in silicates is best observed in the mineral biotite where a ca 35 micron damage halo is seen. By using combined synchrotron XAS and XRD at the Diamond Light Source, UK, the changes across the damaged zone around a U/Th-bearing monazite inclusion was investigated. Fe K-edge XANES from a traverse shows that within the damaged zone the biotite Fe3+ is reduced, perhaps as a consequence of radiolysis of the OH groups in the octahedral layers. The Fe K-edge EXAFS show an increase in disorder in the damaged zone. The micro XRD showed major changes in the biotite lattice as the U/Th containing inclusion was approached. The use of a single crystal positioned perpendicular to (001) enhanced the a, b plane reflections. The diffraction spots representing 110 and other sub-parallel planes show evidence of amorphisation of the biotite lattice. Also new reflections appear at d-spacings close to the main reflections, indicating local changes to the structure as a result of atomic displacements during the development of Frenkel pairs.