On the microscopic level, coal is made up of organic particles called macerals. Macerals are the altered remains and byproducts of the original plant material from which the coal-forming peat originated. Macerals are to coal as sediment grains and cements are to sedimentary rocks.
Coal petrographers (people who study coal under a microscope) separate macerals into three groups, each of which includes several maceral types. The groups are inertinite, liptinite, and vitrinite. These groups were defined according to their grayness in reflected light under a microscope. Liptinites are dark gray, vitrinites are medium to light gray, and inertinites are white and can be very bright.
Different macerals are formed from different plant parts or different residues of bacterially, chemically, or physically broken-down plant parts. Different maceral groups, and individual macerals, have different chemical compositions that, in total, influence the chemical composition of the resulting coal (Stach and others, 1982; Bustin and others, 1983; Teichmüller, 1989).
Liptinites are derived from spores, pollens, cuticles, and resins in the original plant material. These plant parts are more hydrogen-rich than other macerals. They are also fluorescent at some coal ranks.
Vitrinites are composed of partially "gelled" wood, bark, and roots, and contain less hydrogen than liptinites.
Inertinites are mainly oxidation products of other macerals and are consequently richer in carbon than liptinites or vitrinites, because much of the oxygen in the original plant parts or residues has been consumed by oxidation. The inertinite group includes fusinite (most of which is fossil charcoal) derived from ancient fires in the coal-forming peat (see, for example, Scott, 1989). Other inertinite macerals ( for example, macrinite) owe their origin to biological decomposition and decay of plant material (Hower and others, 2009). Still others ( for example, micrinite) result from thermal maturation of the peat.