9.3 COAL
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in North America the Pennsylvanian coals pass laterally into anthracites as they enter
the Appalachian fold belt.
Coals may be devolatilized by thermal metamorphism where they are cut by igneous
intrusions. Examples of this are seen where Tertiary dykes cut Westphalian coal measures in northeast England. Although the physical appearance of the coals is only affected for a few meters on either side of the intrusions, devolatilization as expressed by
increase in carbon content spreads out regionally.
Most brown coals or lignites are obviously woody, lusterless, and not well jointed; humic coals on the other hand do not normally show a woody appearance in hand specimen. They are well bedded and display various degrees of luster from bright to dull. On
the basis of luster and physical appearance, the humic coals are divided into four "rock"
types: vitrain, clarain, durain, and fusain (Stopes, 1919). Vitrains are the bright coals
which have a vitreous luster. Clarains are less bright with a silky appearance, whereas the
durains are dull. Fusain is sooty black and very friable so that, unlike vitrain, clarain,
and durain, it readily soils the hands like charcoal.
It is important to distinguish between coal type and rank. Coal type is the expression
of the composition of the original plant materials, whereas rank is a measure of the extent of maturation or diagenesis that the vegetable matter has undergone. Some coal
seams consist mainly of one rock type, but many are composite and comprise alternations of layers of vitrain, clarain, durain, and fusain of varying thicknesses. Coal normally breaks in three directions into roughly prismatic blocks. Two of the surfaces are
joints and are termed "cleat" and "end," respectively. The cleat is the more strongly developed joint, and in the days of manual extraction its direction often determined the
layout of the underground workings. The third surface is parallel to bedding and usually
coincides with a fusain layer. It is these bedding surfaces of fusain on a block of coal that
soil the fingers.
As with most other rocks, the "rock" types of coal are each composed of a series of
discrete components, termed "macerals" (the prefix "mac-" implying that they are macerated plant remains, and the termination "-erals" indicating that they are analogous to
minerals). Macerals are named with the suffix "-inite" to distinguish them from the rock
type of coal, termination "-ain." The macerals were all plant tissues or plant degradation products that were modified chemically by diagenetic processes, that is, by maturation. The common ones include vitrinite, the main constituent of the coal type vitrain,
of which there are two varieties -- tellinite and collinite. Under the microscope, tellinite
shows a compressed cellular structure of what was formerly woody tissue or xylem,
whereas collinite which has similar optical properties is structureless. Suberinite, cutinite, and exinite are macerals formed by coalification of bark, leaf cuticles, and spore
jackets, respectively. Fusinite stands apart from other macerals in that it is nearly pure
carbon. It often shows cellular structure under the microscope. The cell walls are usually
preserved as a mass of broken fragments, "bogenstrukture." This contrasts markedly
with cellular tissues that are preserved as tellinite, where the cell walls have been deformed but not broken by compaction. It appears therefore that where cellular tissues
are preserved as fusinite, the alteration must have taken place very early in the maturation history, certainly before significant compaction due to overburden load. It has been
suggested that fusinite may have formed during the peat stage by a process analogous
to "dry rot" or by forest fires. "Micrinite" is the general term applied to aggregates of
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in North America the Pennsylvanian coals pass laterally into anthracites as they enter
the Appalachian fold belt.
Coals may be devolatilized by thermal metamorphism where they are cut by igneous
intrusions. Examples of this are seen where Tertiary dykes cut Westphalian coal measures in northeast England. Although the physical appearance of the coals is only affected for a few meters on either side of the intrusions, devolatilization as expressed by
increase in carbon content spreads out regionally.
Most brown coals or lignites are obviously woody, lusterless, and not well jointed; humic coals on the other hand do not normally show a woody appearance in hand specimen. They are well bedded and display various degrees of luster from bright to dull. On
the basis of luster and physical appearance, the humic coals are divided into four "rock"
types: vitrain, clarain, durain, and fusain (Stopes, 1919). Vitrains are the bright coals
which have a vitreous luster. Clarains are less bright with a silky appearance, whereas the
durains are dull. Fusain is sooty black and very friable so that, unlike vitrain, clarain,
and durain, it readily soils the hands like charcoal.
It is important to distinguish between coal type and rank. Coal type is the expression
of the composition of the original plant materials, whereas rank is a measure of the extent of maturation or diagenesis that the vegetable matter has undergone. Some coal
seams consist mainly of one rock type, but many are composite and comprise alternations of layers of vitrain, clarain, durain, and fusain of varying thicknesses. Coal normally breaks in three directions into roughly prismatic blocks. Two of the surfaces are
joints and are termed "cleat" and "end," respectively. The cleat is the more strongly developed joint, and in the days of manual extraction its direction often determined the
layout of the underground workings. The third surface is parallel to bedding and usually
coincides with a fusain layer. It is these bedding surfaces of fusain on a block of coal that
soil the fingers.
As with most other rocks, the "rock" types of coal are each composed of a series of
discrete components, termed "macerals" (the prefix "mac-" implying that they are macerated plant remains, and the termination "-erals" indicating that they are analogous to
minerals). Macerals are named with the suffix "-inite" to distinguish them from the rock
type of coal, termination "-ain." The macerals were all plant tissues or plant degradation products that were modified chemically by diagenetic processes, that is, by maturation. The common ones include vitrinite, the main constituent of the coal type vitrain,
of which there are two varieties -- tellinite and collinite. Under the microscope, tellinite
shows a compressed cellular structure of what was formerly woody tissue or xylem,
whereas collinite which has similar optical properties is structureless. Suberinite, cutinite, and exinite are macerals formed by coalification of bark, leaf cuticles, and spore
jackets, respectively. Fusinite stands apart from other macerals in that it is nearly pure
carbon. It often shows cellular structure under the microscope. The cell walls are usually
preserved as a mass of broken fragments, "bogenstrukture." This contrasts markedly
with cellular tissues that are preserved as tellinite, where the cell walls have been deformed but not broken by compaction. It appears therefore that where cellular tissues
are preserved as fusinite, the alteration must have taken place very early in the maturation history, certainly before significant compaction due to overburden load. It has been
suggested that fusinite may have formed during the peat stage by a process analogous
to "dry rot" or by forest fires. "Micrinite" is the general term applied to aggregates of