It is generally recognized that xylem from trees that are buried in peat swamps is transformed first to huminite macerals in brown coal and then to vitrinite macerals in bituminous coal by processes collectively known as coalification. In order to understand the chemical nature of coalification of xylem and the chemical structures that eventually evolve in coal, we examined a series of gymnospermous xylem samples coalified to varying degrees. The samples included modern fresh xylem, modern degraded xylem in peat, and xylem coalified to ranks of brown coal (lignite B), lignite A, and subbituminous coal. The organic geochemical methods used in this study included solid-state 13C nuclear magnetic resonance (NMR) and pyrolysis/gas chromatography/mass spectrometry. The NMR method provided average compositional information, and the pyrolysis provided detailed molecular information. Although the samples examined include different plants of different geologic ages, they all share a common feature in that they are gymnospermous and presumably have or had a similar kind of lignin. The data obtained in this study provide enough details to allow delineation of specific coalification pathway for the xylem is microbial degradation in peat (peatification), leading to selective removal of cellulosic components. These components constitute a large fraction of the total mass of xylem, usually greater than 50%. Although cellulosic components can survive degradation under certain conditions, their loss during microbial degradation is the rule rather than exception during peatification. As these components of xylem are degraded and lost, lignin, another major component of xylem, is selectively enriched because it is more resistant to microbial degradation than the cellulosic components. Thus, lignin survives peatification in a practically unaltered state and becomes the major precursor of coalified xylem. During its transformation to brown coal and lignite A, lignin in xylem is altered by two important processes. The first involves loss of methoxyl groups, primarily by demethylation (Fig. 1A). The end products of demethylation are catechol-like structures as shown below in Figure 1B. The second transformation process involves increased cross-linking of the aromatic rings. This cross-linking induces increased carbon substitution of the aromatic rings such that the lignin-derived structures become more highly condensed. During its conversion to coalified xylem in subbituminous coal, lignitic xylem, composed primarily of condensed catechol-like structures, is transformed to a macromolecular material primarily composed of phenol-like structures. The catechol-like structures of lignitic xylem loose a hydroxyl group, which is replaced by a hydrogen to form the phenol-like structure as shown in the example in Figure 1B. The pyrolysis data provided only a few clues as to the fate of the C3-side chain of lignin during coalification. However, the NMR data suggest that this side chain is altered, probably by loss of the hydroxyl groups that are attached in modern lignin. Interference in the NMR analysis by aliphatic components of wood, such as resins, precludes definitive determinations of the fate of the C3-side chain during coalification. ?? 1990.
Additional publication details
Organic geochemical studies of the transformation of gymnospermous xylem during peatification and coalification to subbituminous coal