A coal ball
|Permineralised plant remains|
A coal ball is a type of concretion that is found in coal and consists of plant debris (peat), which has been permineralised by calcite. Coal balls vary in shape from imperfectly spherical to flat-lying, irregular slabs. These concretions formed by the early permineralisation of peat by calcite in Carboniferous Period swamps and mires prior to its alteration to coal. They derive their name from their association with coal and their often imperfectly spherical shape. Because they formed in prehistoric peats prior to them becoming coalified, they often preserve remarkable record of the tissue structure of Carboniferous swamp and mire plants, which would otherwise be completely destroyed. Their unique preservation of the tissue structures of Carboniferous plants makes them quite valuable to scientists, who cut and peel the coal balls to research the geological past of the Earth.
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 later discovered and identified in 1922. Since then, coal balls have been found in other countries, and they have led 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 relatively widespread, both stratigraphically and geologically, as compared to coal balls from Europe. The oldest known coal balls date from the Namurian stage of the Carboniferous, and 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 of them in the coal seams of Yorkshire and Lancashire, England. European scientists did much of the early research.
Coal balls in North America were found in Iowa coal seams since 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, drew paleobotanists from Europe to the Illinois Basin in search of them.
In the in situ theory, it is believed that in or near 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 of it. Water with a high dissolved mineral content was buried along 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 prevented because of that, 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 their own coal ball samples. They decided that coal balls formed in situ, but stressed the importance of interaction with seawater, believing that it was necessary for a coal ball to form. Some supporters of the in situ theory of coal ball formation 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, stating that the projections would have been destroyed if the drift theory was correct.
The drift theory assumes 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 is evidence of material being 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 is likely because coal balls often include material that is formed by transport and sedimentation in open water.
Coal balls are not made of coal (they are not flammable and useless for fuel), but rather calcium-rich permineralised life forms, mostly containing calcium and magesium 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 have been known to vary greatly, having been described as ranging from that of a walnut up to 3 feet (1 m) in diameter. Some 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 had a coal ball analysed, finding "a lack of coniferous wood ... and fronds of ferns", and 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. Upon their discovery of marine organisms in a coal ball, 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. The mixed coal balls were further divided into heterogeneous, where the plant and animal material was distinctly separated; and homogeneous, which lacked that separation.
The quality of preservation in coal balls varies from no preservation to the point of being able to analyse the cellular structures. Some coal balls have been found to contain preserved root hairs, pollen, and spores, and described as being "more or less perfectly preserved", containing "not what used to be the plant", but rather, the plant itself. Others have been described as "botanically worthless", containing no preserved material at all. Coal balls with well-preserved contents are useful to paleobotanists. They have been used as a means of analysing the geographical distribution of the vegetation they contain, providing evidence that Ukrainian and Oklahoman plants of the tropical belt were once the same. Research on coal balls has also led to the discovery of over 130 genera and 350 species.
Three main factors determine the quality of preserved material in a coal ball: the mineral constituents, speed of the burial process, and the degree of compression before undergoing permineralisation. Generally, coal balls resulting from remains that have a quick burial with little decay and pressure are more well preserved, although plant remains in most coal balls almost always show differing signs of decay and collapse. Coal balls containing quantities of iron sulfide have far lower preservation than coal balls permineralised by magnesium or calcium carbonate, which has earned iron sulfide the title "chief curse of the coal ball hunter".
Coal balls were first found in England, and later in other parts of the world, including Australia, Belgium, the Netherlands, the former Czechoslovakia, Germany, Ukraine, China, and Spain. They were also encountered in North America, where, compared to Europe, they are geographically widespread; in the United States, coal balls have been found from the Illinois Basin to the Appalachian region.
The oldest coal balls in the world were from the early end of the Namurian stage ( ) and discovered in Germany and former Czechoslovakia, but the age of coal balls generally ranges from the Permian ( ) to the Upper Carboniferous. Some coal balls from the United States have ages varying from the later end of the Westphalian (roughly ) to the later Stephanian (roughly ). European coal balls are generally from the early end of the Westphalian Stage.
In coal seams, coal balls are completely surrounded by coal. They are often found randomly scattered throughout the seam in isolated groups, usually in the upper half of the seam. Their occurrence in coal seams can be either extremely sporadic or regular; many coal seams have been found to contain no coal balls at all.
Analytical methods 
Thin sectioning was an early procedure used to analyse fossilised material contained in coal balls. Thin sectioning required cutting a coal ball with a diamond saw, then flattening and polishing the thin section with an abrasive. It would be glued to a slide, and placed under a petrographic microscope for examination. Although the process could be done with a machine, the large amount of time needed and the poor quality of samples produced by thin sectioning gave way to a more convenient method.
The thin section technique was superseded by the now-common liquid peel technique in 1928. In the liquid peel technique, peels are obtained by cutting the surface of a coal ball with a diamond saw, grinding the cut surface on a glass plate with silicon carbide to a smooth finish, and etching the cut and the surface with hydrochloric acid. The acid dissolves the mineral matter from the coal ball, and leaves a projecting layer of plant cells. After applying acetone, a piece of cellulose acetate is placed on the coal ball. This embeds the cells preserved in the coal ball into the cellulose acetate. Upon drying, the cellulose acetate can be removed from the coal ball with a razor and the obtained peel can be stained with a low-acidity stain and observed under a microscope. Up to 50 peels can be extracted from 2 millimetres (0.079 in) of coal ball with this method.
X-ray powder diffraction has also been used to analyse coal balls. In X-ray diffraction, X-rays of a predetermined wavelength are sent through a sample to examine its structure. It reveals information about the crystallographic structure, chemical composition, and physical properties of the examined material. The scattered intensity of the X-ray pattern is observed and analysed, with the measurements consisting of incident and scattered angle, polarisation, and wavelength or energy.
See also 
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