Coal
Coal: the general name given to stratified accumulations of carbonaceous material derived from vegetation. The starting point for coal formation is usually peat or some similar accumulation of partially decayed vegetable matter. By a process of compaction and slight heating during burial, the peat is converted to the familiar black coal. In general, a series of stages are recognised in the process of coalification corresponding to the amount of heating that the rock has undergone. These are:
- Peat
- Lignite (brown coal)
- Sub-bituminous
- Bituminous
- Sub-anthracite
- Anthracite
They are collectively known as the humic coals. This series shows a progressive increase in carbon content and a decrease in volatiles. The percentage of carbon in dry mineral-free coal is called the "rank". The individual constituents of coals are known as "macerals" (derived from the verb "to macerate"). The main groups of macerals (vitrinite, exinite, and inertinite) are recognised. These are in turn subdivided according to the detailed character of the material. The various macerals are best studied in polished surfaces under an immersion medium. The name microlithotype is used for intimate associations of the various macerals. Three main microlithotypes are recognised: vitrite (vitrinite mainly), clarite (vitrinite and exinite) and durite (exinite and inertinite). Types containing all three macerals are also known and other combinations of two macerals occur. The older terms "vitrain", "clarain", "durain", and "fusain" have been superseded by the above terms as a result of more detailed study.
Two other major types of coal are recognised, which may be referred to collectively as the sapropelic coals. They are:
- Cannel coal, which shows many similarities to the durite microlithotype, but is much finer grained. The main constituents appear to be finely divided vegetable matter, spores, algae and fungal material. Resin globules are occasionally found.
- Boghead coals, which consist of algal material, together with some fungal matter. Torbanite, the well-known oil shale of the Midland Valley of Scotland, is a typical boghead coal.
- Coals of various sorts occur throughout the stratigraphic column from the Devonian upwards, with a remarkable maximum in the Carboniferous. Bituminous and anthracite coals occur mainly in the Carboniferous and the lignites and brown coals in the Mesozoic and Tertiary.