Coal is a sedimentary rock composed principally of macerals, subordinately of minerals, and containing water-filled pores. Macerals are solid, organic substances derived from plant tissues and exudates that have been incorporated into the earth's crust, compacted, hardened, and chemically altered by biologic and geologic processes.

Differences in the relative proportions of minerals and macerals, and in the relative proportions of progenitors of the different macerals are established in the peat swamp. In addition, sulfur is emplaced during or shortly after accumulation of peat. Different types of coals result from these depositional or diagenetic variations. Successive layers of a given coal seam may consist of distinctive coal types as a result of altered environmental

Figure

conditions. Thus, it is necessary to sample a coal seam carefully to avoid obtaining a biased sample.

Following deposition, relatively mild diagenetic and metamorphic processes alter the composition of the maceral progenitors, leading to increasing rank of the coal. The metamorphic alteration results largely from thermally induced chemical reactions which increase in severity with increasing depth of burial. Concomitantly, pressure reduces porosity and moisture-holding capacity, and increases hardness. Differences in type are retained irrespective of rank.

Both rank and type influence coal grade (value) orthogonally. Thus, though the rank of a coal may be suitable for a particular use, the type may be entirely unsuitable; the reverse may also be true.

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