Coal balls

(geology) A subspherical mass containing mineral matter embedded with plant material, found in coal seams and overlying beds of the late Paleozoic.

Variously shaped nodules consisting of fossilized peat in which the individual cells and tissue systems of the plant parts are infiltrated by minerals, principally calcium carbonate, along with pyrite, dolomite, and occasionally silica. This type of fossilization, in which the cell walls are filled with minerals, is termed permineralization. Coal balls occur principally in Pennsylvanian (upper Carboniferous) bituminous and anthracite coals, but permineralized plants have also been reported in coals from as early as the Devonian and extending well into the Paleocene.

How coal balls formed is not well understood, but some are believed to represent accumulations of peat in which the plants were growing in low-lying swampy areas close to the sea. According to this model, seawater provided the high source of calcium carbonate in the permineralization processes.

Because the individual plant cells in coal balls have not been crushed, they offer a wealth of information to paleobiologists about the structure, morphology, and biology of the plants. See also Coal paleobotany; Paleobotany.