Petrography of Coal
Petrography consists of the micoscopic study and description of coal and rocks. Coal is an organic-rich rock resulting from the alteration or metamorphism of peat. Two general types of coal can be visually differentiated in hand specimens:
• banded coal, is most common, and originates from peat consisting mostly of organic debris that have accumulated in place (autochthonous).
• nonbanded coal is derived from redeposited (allochthonous) resistant plant fragments such as spores or aquatic plants. Nonbanded varieties include:
• cannel coal consisting principally of uniformly sized plant fragments such as spores;
• boghead coal consisting mainly of algae fragments.
Microscopically, thin sections (examined with transmitted light) or polished sections (examined with reflected or incident light) of coal can be used to examine the petrographic composition. Although coal appears to be opaque in hand specimen, thin sections of coal (less than 10 microns thick) reveal both opaque and translucent microscopic components. In reflected light, these components can be differentiated by their various colors and degrees of reflectivity or absorption of light. Some components fluoresce when illuminated by reflected blue light. Petrography of coal is important in considerations of utilization of coal because petrographic composition affects the physical and chemical nature of the coal.