On the fundamental difference between coal rank and coal type

• Coal type & coal rank are fundamental aspects of coal systems science.
• Coal type is a result of climate, topography & nutrient availability.
• Coal rank is a function of thermal and pressure history.
• Rank is an overprint on coal type, but does not change type.

This article addresses 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.