Raman mapping of coal halos induced by uranium mineral radiation

Raman mapping microspectroscopy was used as an advantageous high spatial resolution method for detailed assessment of the structure of radiation-induced halos in bituminous coal (Upper Paleozoic) with numerous inclusions of uraninite and coffinite. The uranium content in inclusions in the samples studied ranged from 40 to 50 wt%. Raman structural parameters such as full width at half maximum, the positions of the D-band and G-band peaks and their area ratios were calculated, and these correlated well with vitrinite reflectance. Using linear profiles across the entire halos, changes in the degree of radiolytic alteration of coal matter caused by ionising radiation resulting from the decay of uranium and its daughter products, were described. Using micro-ATR-FTIR, oxidative radiolytic alteration of coal was identified in halos, with oxidisation to alcohols, ketones and carboxyl groups, which were then converted to COO^- ions bound to the cations present, including UO₂²⁺. From our data, we conclude that the conversion of the original coal to a type of anthracite occurred during the process of CH and CC cleavage, dehydroaromatisation of naphthenic rings and oxidation resulting in the transformation of aliphatic structures into aromatic clusters. Generally, radiolytic alteration increased the structural organisation of coal. Monte Carlo simulations of the observed radiation alteration by the ionising energy loss and non-ionising energy loss were performed.