Geology Reference
In-Depth Information
The mineral matter in coal, determined by low-temperature ashing or by dissolution in HF, is emplaced during or
after coal formation. Minerals that are an integral part of the organic matrix are considered included minerals, while
those in the cleats and fractures are termed excluded minerals. Although some of the inorganic compounds
originate in the plant material, most are deposited during (syngenetic) or after (epigenetic) coalification. Synge-
netic minerals can be formed by precipitation in an anoxic, aqueous medium during the biochemical stage of
coalification, or they may be detrital clastics transported into the peat swamp by wind or H
2
O. Epigenetic minerals
are deposited within the coal seam, in cracks, fractures, and bedding planes, by migrating fluids. They may also be
produced from syngenetic minerals by increased temperature and pressure. Mackowsky (1968) indicated that most
of the silicates, quartz, and phosphates had been transported into the peat swamp. Carbonates, sulfides, and
chalcedony from the weathering of feldspar and mica, were formed within the swamp. These minerals tended to be
intimately intergrown with the organic matrix, as included minerals. Some carbonates, sulfides, and oxides were
deposited in cleats and fractures; these excluded minerals are independent of the organic portion.
The quartz in 40 samples of a PRB coal was primarily detrital, but trace amounts of
-form quartz, with apatite and
zircon, were attributed to air-fall and reworked volcanic ash deposited in the peat swamp (Brownfield et al., 1999).
In a study of Gulf Coast lignites, enrichment of some elements was attributed to proximity to igneous rocks or to
deposition of volcanic ash (Warwick et al., 1997).
β
Coal
mineral matter includes a variety of minor or trace elements. The concentration of these elements in coal may
be greater than their average concentration in the earth
-
s crust (Table 1.1.4). The distribution of trace elements varies
too widely to be described by a general statement. Coals from different areas may show distinctive trace element
characteristics (Table 1.1.5), and within a single coal seam, the trace element distribution may not be consistent.
This suggests that no single process has been responsible for the accumulation of trace elements in coal. When
compared to the overlying carbonaceous shale, the concentration of trace elements is lower in coal, reflecting the
influx of detrital inorganic sediments that eventually terminated the formation of the peat swamp (Kim, 2002).
'
Ultimate Composition
Ultimate analysis of coal is the determination of the carbon, hydrogen, sulfur, nitrogen, and oxygen (Table 1.1.6)
(ASTM, 2002; Hessley et al., 1986). Carbon includes organic and any mineral carbonate. Hydrogen is present in
the organic portion of the coal and as H
2
O. Nitrogen is assumed to be part of the organic matter, and sulfur may be
Table 1.1.4
Distribution of trace elements in coal ash compared to the average concentration in the earth
s crust and
'
shales (mg/kg).
Element
Crust
Shale
Coal
Coal ash, minimum
Coal ash, maximum
<
Ag
0.7
.07
0.05
1
10
As
1.8
13
<
100
100
900
B
10
100
58
86
5800
Ba
425
580
92
300
3500
Be
2.8
3
1
1
30
Co
25
19
9
30
300
Cr
100
90
20
50
400
Cu
55
45
12
20
500
Mn
950
830
85
200
1000
Mo
1.5
2.6
3
10
200
Ni
75
68
22
50
800
Pb
12.5
20
40
5
700
Sr
375
300
110
80
3500
V
135
5
100
1000
Zn
70
95
34
100
1000
Source: Based on Nicholls (1968).
