Coal ball

Coal balls often preserve a remarkable record of the microscopic tissue structure of Carboniferous swamp and mire plants, which would otherwise have been completely destroyed. Their unique preservation of Carboniferous plants makes them valuable to scientists, who cut and peel the coal balls to research the geological past.

In 1855, two English scientists, Joseph Dalton Hooker and Edward William Binney, made the first scientific description of coal balls in England, and the initial research on coal balls was carried out in Europe. North American coal balls were discovered and identified in 1922. Coal balls have since been found in other countries, leading to the discovery of hundreds of species and genera.

Coal balls may be found in coal seams across North America and Eurasia. North American coal balls are more widespread, both stratigraphically and geologically, than those in Europe. The oldest known coal balls date from the Namurian stage of the Carboniferous; they were found in Germany and on the territory of former Czechoslovakia.

Introduction to the scientific world, and formation

The first scientific description of coal balls was made in 1855 by Sir Joseph Dalton Hooker and Edward William Binney, who reported on examples in the coal seams of Yorkshire and Lancashire, England. European scientists did much of the early research.

Coal balls in North America were first found in Iowa coal seams in 1894, although the connection to European coal balls was not made until Adolf Carl Noé (whose coal ball was found by Gilbert Cady) drew the parallel in 1922. Noé's work renewed interest in coal balls, and by the 1930s had drawn paleobotanists from Europe to the Illinois Basin in search of them.

There are two theories – the autochthonous (in situ) theory and the allochthonous (drift) theory – that attempt to explain the formation of coal balls, although the subject is mostly speculation.

Supporters of the in situ theory believe that close to its present location organic matter accumulated near a peat bog and, shortly after burial, underwent permineralisation – minerals seeped into the organic matter and formed an internal cast. Water with a high dissolved mineral content was buried with the plant matter in a peat bog. As the dissolved ions crystallised, the mineral matter precipitated out. This caused concretions containing plant material to form and preserve as rounded lumps of stone. Coalification was thus prevented, and the peat was preserved and eventually became a coal ball. The majority of coal balls are found in bituminous and anthracite coal seams, in locations where the peat was not compressed sufficiently to render the material into coal.

Marie Stopes and David Watson analysed coal ball samples and decided that coal balls formed in situ. They stressed the importance of interaction with seawater, believing that it was necessary for the formation of coal balls. Some supporters of the in situ theory believe that Stopes' and Watson's discovery of a plant stem extending through multiple coal balls shows that coal balls formed in situ, stating that the drift theory fails to explain Stopes' and Watson's observation. They also cite fragile pieces of organic material projecting outside some coal balls, contending that if the drift theory was correct, the projections would have been destroyed, and some large coal balls are large enough that they could never have been transported in the first place.

The drift theory holds that the organic material did not form in or near its present location. Rather, it asserts that the material that would become a coal ball was transported from another location by means of a flood or a storm. Some supporters of the drift theory, such as Sergius Mamay and Ellis Yochelson, believed that the presence of marine animals in coal balls proved material was transported from a marine to a non-marine environment. Edward C. Jeffrey, stating that the in situ theory had "no good evidence", believed that the formation of coal balls from transported material was likely because coal balls often included material formed by transport and sedimentation in open water.

Contents

Coal balls are not made of coal; they are non-flammable and useless for fuel. Coal balls are calcium-rich permineralised life forms, mostly containing calcium and magnesium carbonates, pyrite, and quartz. Other minerals, including gypsum, illite, kaolinite, and lepidocrocite also appear in coal balls, albeit in lesser quantities. Although coal balls are usually about the size of a man's fist, their sizes vary greatly, ranging from that of a walnut up to 3 feet (1 m) in diameter. Coal balls have been found that were smaller than a thimble.

Coal balls commonly contain dolomites, aragonite, and masses of organic matter at various stages of decomposition. Hooker and Binney analysed a coal ball and found "a lack of coniferous wood ... and fronds of ferns" and noted that the discovered plant matter "appear[ed] to [have been arranged] just as they fell from the plants that produced them". Coal balls usually do not preserve the leaves of plants.

In 1962, Sergius Mamay and Ellis Yochelson analysed North American coal balls. Their discovery of marine organisms led to classification of coal balls were sorted into three types: normal (sometimes known as floral), containing only plant matter; faunal, containing animal fossils only; and mixed, containing both plant and animal material. Mixed coal balls are further divided into heterogeneous, where the plant and animal material was distinctly separated; and homogeneous, lacking that separation.