PETROGRAPHY AND MICROANALYSIS OF PENNSYLVANIAN COAL-BALL CONCRETIONS (HERRIN COAL, ILLINOIS BASIN, USA): BEARING ON FOSSIL PLANT PRESERVATION AND COAL-BALL ORIGINS

Coal balls (n=25) from well studied paleobotanical profiles in the Middle Pennsylvanian Herrin Coal (Illinois Basin, USA) and from university collections (n=5), indicate that Herrin coal-ball peats were permineralized by fibrous and non-fibrous carbonates. Fibrous carbonates occur in fan-like to spherulitic arrays in many intracellular (within tissue) pores, and are best developed in relatively open extracellular (between plant) pore spaces. Acid etched fibrous carbonates appear white under reflected light and possess a microcrystalline texture. Scanning electron microscopy, electron microprobe analysis, and x-ray diffraction demonstrate that individual fibers have a distinct trigonal prism form and an elevated magnesium content attributable to abundant microdolomite. Non-fibrous carbonates fill pores within the peat as primary precipitates and neomorphic replacements. In the immediate vicinity of plant cell walls, non-fibrous carbonates cut across fibrous carbonates as a secondary, neomorphic phase attributed to coalification of plant cell walls. Dolomite occurs as diagenetic microdolomite associated with the fibrous carbonate phase, as sparite replacements, and as void-filling cement.

Coal-ball formation in the Herrin Coal began with the precipitation of fibrous high magnesium calcite. The trigonal prism morphology of the carbonate fibers suggests rapid precipitation from super-saturated, meteoric pore waters. Carbonate precipitation from marine waters is discounted on the basis of stratigraphic, paleobotanical, and stable isotopic evidence. Most non-fibrous carbonate is attributable to later diagenetic events, including void-fill replacements, recrystallization, and post-depositional fracture fills. Evidence suggests that CO₂ degassing was important in coal-ball formation in the Herrin Coal, which mainly occurred sequentially upward with peat accumulation in the sites studied.