Siewers, Fredrick D.¹, Tom L. Phillips² 
(1) Western Kentucky University, Bowling Green, KY 
(2) University of Illinois, Urbana, IL

ABSTRACT: Carbonate Petrography and Geochemistry of Middle Pennsylvanian Coal Balls (Herrin Coal): Evidence Regarding Coal-Ball Formation And Fossil Plant Preservation

Coal-ball concretions provide important insights into the plant composition and ecological structure of Pennsylvanian coal swamps; however, the processes leading to coal-ball formation and fossil plant preservation are still poorly understood. An investigation of coal balls from the Herrin Coal (Middle Pennsylvanian), Illinois Basin indicates initial coal-ball formation occurred by the precipitation of two distinct carbonate phases: a composite crystalline phase of fibrous Mg-enriched carbonate and a monocrystalline phase of equant, low magnesian calcite. The composite crystalline phase occurs primarily as radiating fans and spherulites of acicular fibers, many of which pass through plant remains without significant disruption of anatomical structures. Individual fibers have a distinctive trigonal prism morphology and a microbulk composition of 9 to 14 mol. % MgCO₃. The monocrystalline phase fills intercrystalline pore spaces, voids within some cell lumens and some of the extra-cellular pore space. This phase occurs as a primary precipitate and as void-fill replacements after precursor fibrous carbonate. 
The composite crystalline phase was typically first to precipitate and was most involved in the initial permineralization of the peat. Although now composed of a cryptocrystalline mixture of low magnesium calcite and dolomite, the composite crystalline phase originally precipitated as high magnesian calcite. Precipitation was likely triggered by CO₂ degassing, flowing porewaters and an abundant supply of chemical reactants. The composite crystals grew by the addition of submicron crystallites along principal growth axes, a growth mechanism that enhanced the preservation of delicate plant structures.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.