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Identification of Coal Components

Coal can be examined on both macroscopic (observable with the unaided eye or a hand lens) and microscopic levels. When attempting to describe a coal specimen, starting out on the macroscopic level is the best option. At this level, coal will appear banded or nonbanded. Nonbanded massive coals include cannel and boghead coal, both of which are dull and blocky. Cannel coal is composed of spores, whereas boghead coal consists mainly of algal matter. Cannel is derived from the word "candle," because pencil-shaped pieces were used as candles in the past.

Banded coals grade from dull banded ("splint coal") to bright banded, depending on whether dull bands or bright bands are dominant. The bands are divided into four major lithotypes: durain (dull, grainy texture, tough), fusain (dull black, charcoal texture, hands get dirty), clarain (bright, satiny texture, brittle), and vitrain (bright, black, glassy, brittle).

A graphic displaying banded and non-banded coals

Bright coals have lots of vitrain and clarain; dull coals are rich in durain bands. Fusain generally occurs only in thin and sporadic bands. Splint coals are durain-rich and can be massive (nonbanded) or banded. Most vitrain-and clarain-rich banded coals break into small blocky pieces along joints, called cleats. Vitrain and clarain are brittle and break easily. "Block coals" are dull coals that break into large blocks because they have fewer vitrain and clarain bands, but have a composition higher in liptinite macerals, which are tough.

"Bone" and "bone coals" have a high ash content in the form of clays and silts; they form part of a continuum between dark shale and dull (banded or nonbanded) coal in the following sequence: dark shale, bone (greater than 50 percent ash), boney coal (less than 50 percent ash), dull coal (cannel, boghead, or splint).

a graphic displaying the constituents of coal

On the microscopic level, coal is made up of organic grains called macerals. Coal petrographers (people who study coal under the microscope) separate the macerals into three maceral groups, each of which includes several maceral types. The groups are liptinite, vitrinite, and inertinite. Macerals are defined according to their grayness in reflected light: liptinites are dark gray, vitrinites are medium to light gray, and inertinites are white and can be very bright. Liptinites are composed of hydrogen-rich hydrocarbons derived from spores, pollens, cuticles, and resins in the original plant material. Vitrinites are composed of "gelified" 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. The inertinite group includes fusinite, most of which is fossil charcoal, derived from ancient peat fires.

A microscopic view of  vitrinite, inertinite, and liptinite

 

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