A coal ball |
|
| Composition | |
|---|---|
| Primary | Calcite |
| Secondary | Permineralised life forms |
Coal balls, despite their name, are calcium-rich masses of permineralised life forms, generally having a round shape. Coal balls were formed roughly 300 million years ago (mya), during the Carboniferous Period. They are exceptional at preserving organic matter, which makes them useful 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, discovered coal balls in England, and the initial research on coal balls was carried out in Europe. It was not until 1922 that coal balls were discovered and identified in North America. Since then, coal balls have been discovered in other countries and they have lead to the discovery of over 300 species and 130 genera.
Coal balls can 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 were found in Germany and the former Czechoslovakia.
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In 1855, the first discovery of coal balls was made by Sir Joseph Dalton Hooker and Edward William Binney, who discovered them in the coal seams of Yorkshire and Lancashire, England. European scientists did much of the early work on these objects.[1][2] Coal balls in North America were found in coal seams since the 1890s,[3] although the connection to European coal balls was not made until Adolph Carl Noé (whose coal ball was actually found by Gilbert Cady[3]) made the connection in 1922.[4][2]
Hooker and Binney believed that coal balls were formed in situ – organic matter gently accumulated near a peat bog and was permineralised, a process of fossilisation in which mineral deposits form internal casts of organisms.[5][6] 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.[7] The majority of coal balls are found in bituminous and anthracite coal seams,[8][9] in locations where the peat was not compressed sufficiently to render the material into coal.[7][10]
Besides Hooker and Binney's analysis, Marie Stopes and David Watson analysed their own coal ball samples. Like Hooker and Binney, they decided that coal balls formed in situ, but added that interaction with a marine environment was necessary for a coal ball to form.[11]
Notwithstanding the word "coal" in their name, coal balls are not made of coal (they are nonflammable and useless for fuel),[12][13] but rather calcium-rich permineralised life forms,[10] mostly containing calcium and magnesium carbonate, iron pyrite, silica, and carbonate of lime.[14][15] Coal balls are usually about the size of a man's fist,[16] though their sizes have been known to vary greatly, having been described as ranging from that of a walnut up to three feet in diameter.[17]
Coal balls commonly contain microdolomites, products of aragonite,[10] and masses of organic matter at various stages of decomposition.[7][18][19] Hooker and Binney analysed a sample of a coal ball, finding "a lack of coniferous wood ... and fronds of ferns", and that the discovered plant matter "appeared to [have been arranged] just as they fell from the plants that produced them".[5]
In 1962, Sergius Mamay and Ellis Yochelson discovered signs of marine animal remains in North American coal balls.[20][21]
In 2000, it was noted that the carbonate found in the most concentrated coal balls from coal seams in Herrin, Illinois have 13C/12C ratios as low as –34‰, consistent with CO2 production by anaerobic oxidation of methane.[22] This microbial phenomenon has only been credited since the mid 1990s because, although many factors indicated the process was likely, the energy margin of methane oxidation coupled with sulfate reduction was known to be minuscule. For instance, it was noted [23] that salt dome cap carbonates have isotopes consistent with methane metabolism, though a flood of contemporaneous literature assessing the salt domes as potential waste repositories insisted that the process required more complex hydrocarbon seeps on the basis of thermodynamics. In the mid-1990s, researchers began describing active anaerobic oxidation of methane precipitating carbonates in various anoxic sea and lake floors, such as the Black Sea. The radial fibrous calcite of coal balls is echoed by the radial structure in 1-2 m tall towers built over methane seeps.[24]
The quality of preservation in coal balls varies from no preservation to the point of being able to analyse the cellular structures.[6] Some coal balls have been found to contain preserved root hairs,[13] and described as "more or less perfectly well-preserved"[25] and containing "not what used to be the plant – it is the plant",[26] while others have been described as "[containing] almost no preserved plant remains".[10] Coal balls with well-preserved contents have been used as a means of analysing the geographical distribution of the contained vegetation, providing evidence that Ukrainian and Oklahoman plants of the tropical belt were the same.[27]
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.[13] 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.[7] Coal balls containing quantities of iron sulphide have far lower preservation than coal balls permineralised by magnesium or calcium carbonate,[7] which has earned iron sulphide the title "chief curse of the coal ball hunter".[13]
Coal balls were first found in England,[5] and later in other parts of Eurasia, including Belgium, Holland, former Czechoslovakia, Germany, the former Soviet Union, and more recently, China.[1][10] They were also encountered in North America, where, compared to Europe, they are relatively widespread;[1] in the United States, coal balls have been found from the Illinois Basin[28] to Ohio to the Appalachian region,[7] with ages varying from the later Stephanian (roughly 304 to 299 mya) to the later end of the Westphalian (roughly 313 to 304 mya). European coal balls are generally from the early end of the Westphalian Stage.[1] The age of coal balls generally range from the Permian Period (299 to 251 mya) to the Upper Carboniferous,[29] though the oldest coal balls were of early Namurian age (326 to 313 mya) and were discovered in Germany and former Czechoslovakia.[1]
Thin sectioning was the first procedure used to analyse fossilised material contained in coal balls. The procedure was created and used by Hooker and Binney,[5][3][25][30] and involved cutting a coal ball with a diamond saw, flattening, polishing, and gluing the thin section to a slide, then placing it under a petrographic microscope for examination.[10][31] This process could be done with a machine, although the large amount of time needed and the poor quality of samples produced by thin sectioning gave way to a more convenient method.[1][32]
The thin section technique was superseded by the now-common liquid peel technique in 1928.[1][30] 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.[32] The acid dissolves the mineral matter from the coal ball, and leaves a projecting layer of plant cells.[13] Acetone should be applied and a piece of cellulose acetate.[15] 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.[18][13][33][34] Up to 50 peels can be extracted from 2 millimetres of coal ball with this method.[32]
X-ray powder diffraction has also been used to analyse coal balls.[10] 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.[35]