Alpha particle damage in biotite characterised by microfocus X-ray diffraction and Fe K-edge X-ray absorption spectroscopy

Combined microfocus XAS and XRD analysis of α-particle radiation damage haloes around thorium containing monazite in Fe-rich biotite reveals changes in both short and long range order. The total α-particles flux derived from the Th and U in the monazite over 1.8Ba was 0.022 α particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short range order disruption revealed by Fe K-edge EXAFS is manifest by a high variability in Fe-Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K-edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe3+ components of the biotite by interaction with the (_2^4)He2+, the result of low and thermal energy electrons produced as a result of the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionisation interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing expansion in the c direction and contraction in the a,b direction. The high amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.