Geochemical and petrographic alteration of rapidly heated coals from the Herrin (No. 6) Coal Seam, Illinois Basin

Reflectance micro-FTIR analysis of collotelinite shows increased aromaticity with rank: both the ratio of the aromatic CH stretching band at ~ 3100-3000 cm− 1 versus the aliphatic CHx stretching bands between 3000 and 2800 cm− 1 (AR1), and the ratio of the aromatic out-of-plane deformation bands between ~ 900-700 cm− 1 versus the aliphatic CHx band (AR2) increase with increasing Rr. Within the 3000-2800 cm− 1 region, there is an increase in the area under the asymmetric CH3 peak at ~ 2960 cm− 1 relative to the asymmetric CH2 peak at ~ 2920 cm− 1 with increased rank. Within the 900-700 cm− 1 region, the overall intensity of the ~ 750 cm− 1 peak (aromatic rings with four adjacent H atoms) relative to the ~ 870 cm− 1 peak (aromatic rings with one isolated H atom) increases up to 2.5% Rr, likely reflecting a lower degree of substitution (DOS) of alkyl groups on aromatic ring sites. The prevalence of the 750 cm− 1 peak at high rank may represent a lower degree of condensation of aromatic rings in the structure of intruded coals compared to normally matured coals.