Coal balls are carbonate concretions formed in peat during the Pennsylvanian and early Permian. Microprobe and microscope analysis reveal that polycrystals of high-Mg calcite (HMC), which are also high in Sr, are the earliest calcium carbonate to form in the Williamson No. 3 coal balls from the Kalo formation in Iowa. This HMC has early diagenetic rims of ferroan and non-ferroan low-Mg calcite (LMC) suggesting diagenesis in meteoric water. The combination of HMC followed by LMC suggests the earliest coal ball carbonate formed in a hydrologically dynamic environment, where saltwater influx into the mire was followed by a return to meteoric pore water. Subsequent generations of carbonate are ferroan and non-ferroan LMC and appear to result from diagenesis of the original HMC fabric with LMC rims. HMC polycrystals from coal balls are among the first abiotic HMC to be reported from the mid-Pennsylvanian; coal balls may be a good source of Pennsylvanian HMC. Coal balls that formed in porous peat (i.e. wood and surficial leaf mats) commonly have abundant radiating arrays of HMC polycrystals. Coal balls that formed in matrix-rich, low porosity peats consist primarily of permineralizing anhedral calcite, which is ferroan LMC. The link between the HMC and porous permeable peat is supported by the distribution of HMC and ferroan LMC in plant cells. Wood cells, which have porous walls, are filled with HMC; fiber cells, which have impermeable walls, are filled with ferroan LMC. This study demonstrates a link between pore volume, porosity, plant cell type, and carbonate fabric.