How jet is formed: An organic geochemical approach using pyrolysis gas chromatography–mass spectrometry
Introduction
Jet is a black gemstone of organic fossil origin that has been used for decorative purposes for thousands of years. ‘True’ jet, as described by archaeologists, is only found in a few geologically distinct locations. The best known is in the Jurassic outcrops around Whitby in Yorkshire, UK (Watts et al., 1997). The most distinctive characteristics of jet are its very black appearance after polishing, its brittleness upon physical stress and the apparent link to fossil wood material. The general appearance is sometimes described as resembling obsidian. To some extent, jet shares its appearance with other black lithic raw materials such as lignite, cannel coal and some oil shales. Discrimination between these materials is suggested to be based mainly on chemical differences (Hunter et al., 1993).
Jet has been analysed using various methods, including electron spin resonance (ESR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy (Lambert et al., 1992), Py-GC/MS (Watts et al., 1999) and Fourier transform infrared (FTIR) spectroscopy (Traverse and Kolvoord, 1968). These studies indicated that the chemical composition is primarily based on aromatic and phenolic entities. Importantly, Traverse and Kolvoord (1968) report findings of jet in masses resembling tree branches and on occasion observed wood structures, albeit compressed, in hand specimens. These latter observations and the occurrence of jet amongst fossil woody material in outcrops have been used as evidence that jet is directly derived from fossil wood material.
To date, an unambiguous explanation for the formation from wood, referred to as jetification, is not available. Muller (1987) argues that Yorkshire jet is formed from discrete pieces of wood preserved in organic-rich sedimentary deposits. Stagnant water column conditions and an anoxic environment allowed anaerobic decay to soften the wood. This made it responsive to the increasing temperature and pressure that the material would experience during its burial history. This is reflected in the cellular wood structure observed using microscopy, showing extreme disintegration and compression. Muller’s (1987) interpretation implies that jetification occurs over a long period, similar in timescale to coalification. In addition, in a recent paper by Kaelin et al. (2006) jet-like wood, described as vitrified wood, was explained by physical compression of the overlying strata of the wood material following biological removal of the polysaccharides over geological time.
Unpublished observations in the field in 2001 by authors Poole and van Bergen of mummified wood in the Peruvian tropical lowland area, preserved in unconsolidated coarse sand/conglomerate facies, are pertinent to jet formation. These observations showed that uncompressed mummified fossil wood, when exposed to natural Peruvian weather conditions, changed from material with high quality morphological preservation to material resembling obsidian/jet, without any gross shape change. Fossil wood specimens, taken from large branches or entire trees, collected subsequently in 2001 and 2008 from the same geological strata, and stored in the dark in plastic bags in the laboratory, underwent the same morphological changes over a period of 6 months to 1 year. These observations indicate that the processes involved in the morphological transformation of wood to form material resembling jet occur extremely quickly when considered in terms of geological time.
To further understand whether the morphological changes are paralleled by chemical changes, a number of wood specimens with both intact morphology and jet-like appearance, in combination with true jet samples from Whitby and Stockholm, were examined in detail using reflected light microscopy and on line Py-GC/MS. The data can provide insights into the processes governing jetification and as such might explain the abundant presence of jet at a geological site such as Whitby, UK.
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Material
Six samples were studied, including wood and jet-like material from the same mummified wood specimens (Peru and Australia) together with two ‘true’ jet samples, one from the Whitby area and one from Stockholm (Table 1). Representative parts of the jet samples were used for analysis. The Peruvian (ca. 100,000 year) and Australian (Miocene) mummified wood specimens were uncompressed and were sub-divided using a dissecting microscope into morphologically recognisable mummified wood material and
Microscopy
All specimens were evaluated to determine their morphology and anatomy. Both the ‘true’ jet samples consisted of more or less homogeneous material with some recognisable components, in accord with previous observations (Rayner and Hemingway, 1974). The components in the Whitby jet appeared somewhat structured, which could be perceived as remnants of cells or plant tissues. The two true wood specimens, Peru and SM10, showed clear cellular structures in the mummified material, whereas the
Conclusions
Based on the pyrolysis data, the process of jetification is interpreted to be characterised by a rapid change when mummified wood gets re-exposed to sunlight and aerobic conditions. This process bears great resemblance to the synthesis of bakelite. The transformation from mummified wood to jet is probably caused by relatively small chemical changes, yielding extra linkages between the phenolic compounds and causing the structure to become much more rigid, as reflected in increased inertness of
Acknowledgements
We thank E-M. Friis, S. McLoughlin, C. Mitchell and M. Silman for providing supplementary specimens of jet. The Wageningen University is acknowledged for providing access to Py-GC/MS equipment. M. Jones and an anonymous reviewer are thanked for their constructive input.
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