The organic geochemistry of coal: from plant materials to coal

Coalification is a process that transforms plant remains under the influence of time, temperature, and possibly pressure to a black, generally lustrous solid organic fossil fuel having a very complex chemistry. Although much has been learned in the past two centuries on coal chemistry, we still have little knowledge of the way plant materials undergo these transformations. The problem has been the existence of adequate characterization tools. The advent of some new tools for structural elucidation of macromolecules has led to some revised thinking on coalification. Originally thought to be a process involving the full degradation of plant remains and subsequent reconstitution of these remains, the general consensus today is that the process involves a selective preservation of certain resistant plant components followed by some minor reorganization of the biopolymers that survive. The major components of plants do degrade, and the resistant ones often constitute only a small fraction of the original mass of the plant materials. This paper examines the most recent literature concerning coalification and focuses on the chemistry associated with coalification of certain recognizable plant remains that have been at the center of research activity for the past two decades. Thus, we examine the recent knowledge of the chemistry of resistant biopolymers in extant and fossil wood, cuticles, resins, spores, and algae, knowledge that has been assembled to establish the reaction pathways towards coal macromolecules.