GEOCHEMICAL AND PETROGRAPHIC ALTERATION OF RAPIDLY HEATED COALS FROM THE HERRIN (NO. 6) COAL SEAM, ILLINOIS BASIN

A petrographic, geochemical, and molecular assessment has been performed on a series of Illinois Basin coal samples, collected at various distances from a Permian-age igneous dike. Vitrinite reflectance, proximate, and ultimate analyses were coupled with reflectance micro-FTIR analysis to provide a deeper understanding of the molecular changes that occur in the coal structure during relatively short-lived, intensive heating events.

With increasing proximity to the intrusion, coals have higher mean random vitrinite reflectance values (Rr) within the dike alteration zone. Geochemical data indicate an overall loss of H, O, and N and an increase in C approaching the dike. Carbonate minerals contribute significantly to geochemical data, but even after their removal with HCl, coals have higher VM contents at high rank than expected compared to normal coalification trends. When plotted on a van Krevelen diagram or Seyler chart, intruded coals follow different coalification trends than coals matured through normal burial metamorphism.

FTIR analysis shows increased aromaticity with rank, with the ratio of the aromatic CHx band at 3100-3000 cm^-1 versus the aliphatic CHx band at 3000-2800 cm^-1 (AR1), and the ratio of the aromatic out-of-plane deformation band at 900-700 cm^-1 versus the aliphatic CHx band (AR2) becoming greater with increasing Rr. Within the 3000-2800 cm^-1 bandwidth, 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. The intensity of the aromatic out-of-plane deformation modes at 900-700 cm^-1 increase relative to aliphatic CHx bandwidths at high rank as well. Within the 900-700 cm^-1 band, the overall intensity of the ~750 cm^-1 peak (aromatic rings containing 4 adjacent H atoms) relative to the ~870 cm^-1 peak (aromatic rings containing 1 isolated H atom) increases with rank, likely reflecting a lower degree of substitution (DOS) of alkyl groups on aromatic ring sites in high rank coals. This trend likely also represents the building of large aromatic clusters at high rank.