Geology Reference
In-Depth Information
Table 3.1.3
Classification and composition of microlithotypes found in coal.
Microlithotype
Maceral composition
Group
Vitrite
Vitrinite (V)
95%
>
Liptite
Liptinite (L)
95%
Monomaceralic
>
Inertite
Inertinite (I)
95%
>
Clarite
V+ L
95%
>
Durite
I + L
95%
Bimaceralic
>
Vitrinertite
V+ I
95%
>
Duroclarite
V
L, I (each
5%)
>
>
Vitrinertoliptite
L
>
V, I (each
>
5%)
Trimaceralic
Clarodurite
I
>
V, L (each
>
5%)
-
Carbargilite
Coal + 20
60% (vol.)
Carbopyrite
Clay
Carbankerite
Coal + 5
-
20% (vol.)
Carbominerite
*
Carbosilicate
Sulfides
Carbopolyminerite
Coal + 20
60% (vol.)
Carbonates
Coal + 20
-
-
60% (vol.)
Quartz
Coal + 20
*
60% (vol.)
Various minerals
-
Source
: Adapted from Stach et al., 1982; Taylor et al., 1998.
*
5% (vol.), if pyrite is high.
There are several systems of megascopic classification. Classification of brown coal lithotypes is based on color,
texture, desiccation pattern, strength, and degree of gelification (Table 3.1.4; after Hagemann, 1980; see Bustin
et al., 1983, for description of use of system). Two systems of the classification of black coal lithotypes are given in
Table 3.1.3 (from Diessel, 1992 and Hower et al., 1990). For black coal (subbituminous rank and higher), texture,
luster, type of fracture, proportion of bright versus dull bands, and mineral matter content are used to distinguish the
coal lithotype. Each individual lithotype must be thicker than 5mm for the Australian system or 3 mm for the US
system. Consequently, thinner bands become part of a thicker lithotype unit.
Coal Rank
C
oal rank is the measure of the degree of organic metamorphism (coalification) of a coal, ranging from low-rank
peat to high-rank meta-anthracite (Table 3.1.5). Rank can be determined through a number of chemical and
physical parameters. In general, no single parameter can be used throughout the entire rank range. For example,
equilibrium moisture is one of the most appropriate parameters at low ranks, but gives way to heating value and
volatile matter at intermediate ranks and hydrogen at the highest ranks. Vitrinite reflectance is a good parameter
for many coal ranks, although it is of questionable value at the lowest ranks where interspecies differences in wood
types and their varying preservation could greatly influence individual values. For bituminous coals, vitrinite
reflectance is a commonly used rank parameter and has advantages over chemical parameters in being based on one
maceral, eliminating the chemical differences between macerals and the interfering effects of mineral matter.
The initial stage of coalification is biochemical degradation which involves the chemical decomposition of
plant material aided by microorganisms. Whereas this process may be rapid in tropical environments, where
conditions are ideal for the bacteria and fungi, rapid plant growth may balance the increased rate of decom-
position. This early decomposition and degradation of organic matter is followed by humification. Humifica-
tion, the biochemical path from woody peat to the huminite macerals, affects the preservation of cellulose,
hemicellulose, and lignin (the most-resistant compound of the three) compounds in plant cell walls. Humifica-
tion begins with the oxidation of plant matter and attack by aerobic organisms such as fungi, insects, and
aerobic bacteria; with consequent extraction of hydrocarbon-rich functional groups and enrichment of the
