Macerals

Macerals are best described as inorganic minerals. A particular suite of macerals determines the coal type and reflects the coal rank (or degree of geochemical alteration, degree of organic metamorphism). Certain compositions of initial plant material lead to a specific composition of the coal in terms of maceral content. Important macerals are exinite (largely derived from spores), vitrinite (from wood), micrinite (highly decomposed humus) and fusinite (partly charcoal). Macerals have an inorganic component which is referred to as the ash content. As the chemical composition of certain minerals may change with pressure and temperature (allowing the use of composition as a geo-barometer and geo-thermometer) so the chemical composition of macerals changes with the coalification process. Thus, the petrographic examination of certain macerals following accepted definitions of a set of standards, allows the ready determination of coal rank.

Reflectance microscopy of vitrinite is the internationally agreed technique for the classification of coal rank. The determination of coal rank gives an indication of the calorific value and is therefore important for the economic assessment of coal deposits.

Properties such as percentage volatiles, percentage water, trace element and sulphur concentration and inorganic concentration (ash content) are important in classifying the value of a particular deposit, and the suitability of a certain coal type for different commercial outlets (e.g. cooking coal, domestic use, etc.)

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