O'Keefe and 12 co-authors reported in mid-2013 on the fundamental difference between coal rank and coal type. While theoretically settled long ago as being different aspects of coal systems science, the two concepts are still often confounded. In recent years, this has resulted in the publication of several works stating that coal type changes with coal rank. Coal type refers solely to coals' depositional origin and the maceral–mineral admixture resulting from that origin.
Coal types typically fall in to two categories: humic coals, developed from peat, and sapropelic coals, developed from organic mud. Either type may be allocthonous or autochthonous, and within types, further refinement of depositional environment can be made. Coal rank refers to the changes in geochemistry and resultant changes in reflectance caused by increasing thermal maturity of the coal. Thus, it provides an overprint of maturity on existing coal types. With proper techniques, such as use of crossed polars and etching, maceral forms can be differentiated even at high ranks, and the original coal type determined.
The authors concluded that coal rank and type are fundamentally separate parameters. Coatype is directly related to the environment of peat formation, the climate of peat formation, and the decompositional history of the mire being studied. In general, coal type can be defined as being either humic or sapropelic.
Humic coals are those that develop from primarily helophytic plant materials and may be either autochthonous, forming in place, as is the case of many domed peats; or allochthonous, transported to the site of deposition, such as may be seen in coals formed in logjams. Sapropelic coals develop from primarily aquatic plant materials, or from helophytic remains that have undergone significant decomposition (e.g., detrohuminite/detrovitrinite and macrinite). Both humic and sapropelic facies of a single coal are present in the rock record. It is not uncommon for coals to begin as a collection of alginite, sporinite, and humo-/vitro- and inertodetrinite in a depression, and as the depression fills to become increasingly humic as the mire transitions from being
topogenous to being ombrogenous.
Coal rank provides a reflectance overprint upon coal type. In general, vitrinite macerals become increasingly reflective as rank increases, as do liptinite and inertinite macerals. By the top end of low volatile bituminous, it becomes increasingly difficult to differentiate vitrinite and liptinite macerals. By the anthracite rank, this becomes challenging.
Etching of high-rank coals allows for individual maceral identifications to be made, and thus for coal type to be identified. The use of white light with crossed polars also allows the differentiation of liptinite macerals. Both humic and sapropelic coals occur as anthracites, notably in the Zhejiang Province of China.
The authors' understanding of the depositional settings of peat and coal are robust, especially in North America, Australia, China, and Europe. Significant work remains to be done in Africa, central Asia, and in South America. Beyond specific locations, there are numerous questions remaining about the effect of Milankovitch and other orbital cyclicity on the initiation, accumulation, and preservation of peats, as well as on coals' occurrence within existing sequence stratigraphic models.
Existing models for peat formation need to be reexamined and possibly revised in light of new data on the formation and preservation of tropical, temperate, and cool-climate peats.
Managed & Built by I2M Consulting, LLC | © 2010-
