Wednesday, October 12, 2011

The Identification and Characterization of Mineral in Bukit Asam

The identification and characterization of mineral in Bukit Asam, coal seams based on XRD, XRF, SEM-EDAX and thin section analysis.

THE IDENTIFICATION AND CHARACTERIZATION OF MINERAL IN BUKIT ASAM COAL SEAMS BASED ON XRD, XRF, SEM-EDAX AND THIN SECTION ANALYSISByS.S. Rita Susilawati
Coal Division

1. INTRODUCTION
The Bukit Asam coalfield is located at Tanjung Enim, South Sumatra Province. The coal deposit extends over an area approximately 4 km wide and 6 km long, and is worked by the state owned coal company Perusahaan Tambang Batubara Bukit Asam (PTBA). In general, the coal at Bukit Asam,  has a sub-bituminous rank, but there are also coals with rank up to semi anthracite due to local heating resulted from igneous intrusions of Plio-Pleistocene age. Although much is known about the chemical and physical properties of Bukit Asam coal, as with other coal deposit areas in Indonesia, comprehensive coal properties studies are limited. Furthermore, those studies deal mainly with the organic properties (eg: Daulay,1985; Daulay and Cook, 1988; Sarangih, 1985; Waluyo (1992) and Pujobroto (1997), while the inorganic constituents do not appear to have been well studied.This paper will explain the crystalline mineral presence in Bukit Asam coal and associated strata, based on XRD, XRF, SEM-EDAX and thin section analysis, which is presented on the basis of grouping. This paper does not include the explanation about the nature, distribution and significance of those minerals which will be discussed later in separate paper.  
 
2. GEOLOGICAL SETTING
The Bukit Asam coalfield is part of the South Sumatra Basin. This basin is one of several Tertiary coal basins that form a major coal and hydrocarbon producing area in Indonesia. The geology of this Basin has been discussed in some detail by several authors, such as: Adiwidjaja and de Coster (1973), de Coster (1974), Haan (1976), Matasak and Kendarsi (1980), Koesoemadinata (1978) and Hutchison (1996).  Authors such as Stalder (1976), Walujo (1993) and Pujobroto (1997) have discussed the geology of Bukit Asam, with particular emphasis on the occurrence of coal in the area.De Coster (1974) indicates that the widespread areas of swamp-land and marshes throughout the whole basinal area, and the enormous amount of organic matter that accumulated and was later altered into the coals of the South Sumatra basin, occurred in the regressive phase of the Neogene depositional cycle. Coals deposited in this area tend to be wide-spread in distribution but still relatively young in age. According to Stalder (1976), the delicate balance between plant growth, subsidence and the occurrence of a high water table that prevented oxidation of the organic material was the likely cause of the maximum coal deposition on the basin axes. The coals at Bukit Asam are part of the Miocene Muara Enim Formation. During deposition of this Formation, the tectonic setting of the South Sumatra Basin produced a quite regular pattern of coal deposition in respect of thickness, ash and split pattern, caused by exceptionally regular subsidence coupled with the right type of climate (Stalder, 1976).According to Stalder (1976) coalification in Muara Enim Formation was controlled much more by burial depth variations, with a geothermal gradient of constant value throughout the basin, than by variations in the value of geothermal gradient. On a regional scale therefore, the coal rank distribution was controlled by the burial variations during the basinal stage.  The large gradient encountered in some areas is due to the effect of the igneous intrusions.  

 
Figure 1. Location of the Bukit Asam coalfield 

3. Coal At Bukit Asam Deposit
Three economic coal bearing intervals present in the study area; the Mangus (A) seam, the Suban (B) seam and the Petai (C) seam. The Mangus seam in the Air Laya pit is split into two coal seams, called the A1 and A2 seams. The Suban seam is also split into the B1 and B2 seams, while the Petai seam is not split in this area.Petrographic data indicates that the coals are vitrinite-rich (Table 1), with relatively low proportions of visible mineral components.  Vitrinite reflectance measurements on coals from the principal seam sections studied (Table 2) show mean maximum reflectance values ranging from 0.45 to 4.17%.  The sample suite thus embraces some coals of higher rank than those included in previous studies.  Daulay (1985), for example, reported reflectance values at Bukit Asam ranging from 0.3% to 2.6%, and Pujobroto (1997) reported values ranging from 0.35% to 2.7%. Except for one location (Suban), the actual intrusions were not exposed at the time of the present study. Nevertheless, in line with a suggestion by Pujobroto (1997), vitrinite reflectance values of more than 0.5%, where encountered in the study, were assumed to be a result of igneous intrusion effects.
Table 1. Maceral Analysis of some Bukit Asam coal plies
Sam
Code
Vitrinite

Liptinite

Inertodetrinite
Min
Matter
Tv
Dtv
Glv
Total
Cut
Sbr
Other
Total
Fs
Sf
Sc
Int
Total
C15
31.8
47
5
83.8
1.8
1
1.8
4.6
2.2
4.4
0.8
0.4
7.8
8.2
C1
70
21.8
0.4
93.2
1.4
nd
0.8
2.2
1.4
1.8
0.2
0.2
3.6
2
C25
61.9
29.6
nd
91.5
nd
nd
2.3
2.3
nd
0.2
0.2
Nd
0.4
5.8
C30
60.8
27.8
nd
88.6
2
nd
1.4
3.4
nd
nd
0.1
0.3
0.4
9.6
B1
39.2
44.4
1.2
84.8
1.2
nd
2.2
3.4
3.2
2.6
1.4
0.8
8
3.8
B13
51.8
40
1
92.8
0.6
0.8
1.4
2.8
1.4
0.8
0.2
0.4
2.8
1.6
Frid-4
44.4
47
0.8
92.2
1.8
0.3
2.3
4.4
0.6
0.4
0.8
0.2
2
1.4
Frid-5
74.4
22.2
nd
96.6
nd
nd
1.6
1.6
nd
nd
0.2
0.8
1
0.8
A17
48.6
40.4
2.2
91.2
2
nd
0.8
2.8
3.4
0.6
0.6
0.4
5
1
A27
54.6
32.2
1.8
88.6
1.2
nd
2.4
3.6
2
2.6
1.4
0.8
6.8
1
A32
45.6
31.4
0.4
77.4
2
0.4
1.4
3.8
10.6
5
0.4
1.2
17.2
1.6
A54
35.6
37.8
4
77.4
2.8
nd
2.6
5.4
4.2
8.6
1.8
0.8
15.4
1.8
MTB
32
49.4
5.4
86.8
2
4
3
9
0.2
0.6
0.8
1
2.6
1.6
BK
43.4
42.6
5
91
0.4
nd
2.4
2.8
1
0.6
1
0.4
3
3.2





















Tabel 2.  Mean maximum vitrinite reflectance of Bukit Asam coals measured
during the present study
Sample No
Seam
Location in Air Laya pit
Rvmax
Standard deviation
Number of measurements
A5
A9
A17
A27
A30
A34
A54
Mangus
1
1
2
2
2
2
3
0.52
0.49
0.50
0.57
0.56
0.02
0.02
0.03
0.03
0.02
50
30
25
30
37
B1
B13
B19
B25
Ant-1
Ant-2
Frid-4
Frid-5
Suban
4
6
7
8
Suban
Suban
7
8
0.45
0.67
0.51
1,40
4.17
2.09
0.54
2.45
0.02
0.02
0.02
0.03
0.16
0.04
0.05
0.17
30
30
30
30
50
25
30
50
C1
C15
C25
C30
Frid-1
Frid-2
Frid-3
MTB
BK
Petai
9
10
11
12
10
11
9
Muara Tiga
Banko
1.71
0.49
1.37
1.50
0.49
1.64
2.31
0.48
0.48
0.03
0.03
0.03
0.02
0.03
0.04
0.12
0.02
0.04
25
30
25
30
25
25
50
30
30
 
4. Sampling And Methodhology
Approximately 100 samples of coal and non-coal lithologies from a total of 15 localities within the Bukit Asam coal mining area were collected for the present project. The majority of the coal samples were taken from exposures of the open cut mine workings in the Air Laya pit, and collected on a ply-by-ply basis. The main suite of samples from the Air Laya provide an opportunity to study the properties of coals of different rank within a single succession, as the coal samples represent different seams and different rank levels, including variation in both vertical and lateral directions.  In order to examine the lateral variations more fully, several additional coal samples were also taken from the Muara Tiga, Banko and Suban pits within the Bukit Asam deposit.Low temperature ashing (LTA), X-ray diffraction (XRD) and optical microscopy were used to make a systematic assessment of the predominant mineral types present in the coal and non-coal lithologies, with quantification of the bulk mineralogy conducted using SIROQUANT analysis. Scanning electron microscope analysis was carried out to make qualitative judgements on the morphology, shape or grain size of the minerals present. Quantification of bulk chemistry for both coal and non-coal samples was accomplished using X-ray fluorescence (XRF) spectrometry. A series of selective leaching processes was also conducted to remove the elements occurring in different ways within the non-mineral inorganic fraction of coals.  The rank of the coals was determined by vitrinite reflectance analysis, and the petrographic properties of the coal were characterised using reflected light microscopy. Although some clay minerals (eg. kaolinite and illite) are clearly recognized in whole rock diffractograms, others, particularly those that are very fine grained (<2μm) and poorly crystalline, are unlikely to give recognizable diffraction patterns in a whole rock scan.  In order to identify more fully the clay minerals present in the coal and associated non-coal strata, special clay separation and analysis techniques were also used in this study, along with XRD analysis of the whole-rock (or whole-LTA) samples. The <2μm clay fraction was separated from 6 LTA and 6 non-coal rock samples by gravitational settling, using the pipette-on-glass-slide technique, following the method described by Moore and Reynolds (1987).
 
5.   Mineralogy of Coal and Non-Coal Rocks
The overall oxygen plasma ash or mineral percentage for the A, B and C seams, shows that typically Bukit Asam coal has a relatively low ash yield and low mineral matter content.  Generally, the low temperature ash from the coals, contains material which comes from two different sources: (a) the crystalline minerals originally present in the coal (eg; kaolinite, quartz, pyrite etc), and (b) product derived from the non mineral inorganics, formed as mineral artifacts by reactions during oxidation of the organic matter (eg; bassanite, hexahydrite, coquimbite, jarosite etc), which may also include non-crystalline or poorly crystalline material that is not detected by XRD analysis.Table 3 provides a summary of mineral matter present in the coal LTAs in Bukit Asam area. The major crystalline minerals in the coals are kaolinite and quartz, but in some samples, particularly those influenced by igneous intrusions, significant proportions of illite/smectite (including rectorite) and paragonite are also present. Illite, chlorite, boehmite, calcite, siderite, pyrite and muscovite are present in some samples as minor phases. In addition to these minerals, the LTA of almost all of the coal samples contains minor to significant proportions of a range of sulphate species, including anhydrite, bassanite, coquimbite, gypsum, jarosite, hexahydrite and alunogen. On the other hand the mineralogy of the non-coal rocks in the Bukit Asam area is generally similar to the mineralogy of the associated coals.  Clay minerals in the form of kaolinite and mixed-layer clay, along with quartz, are thus also the major minerals in the non-coal beds. Indeed,  these  minerals make up almost the entire mineral suite in the non-coal rock samples studied. The minor minerals in the non-coal samples include pyrite, calcite, siderite, gypsum, and muscovite.
 
5.1. Clay Mineral
Clay minerals, in the form of kaolinite and mixed-layer I/S, are the most widespread and abundant mineral components in the coal and associated non-coal rocks of the study area. 

Table 3. A summary of mineral matter in the coal LTAs in  Bukit Asam area, as determined by XRD and Siroquant analysis (% in LTA)
5.1.1. Kaolinite
Kaolinite is present in all of the Bukit Asam samples. It dominates most of the lower rank coals, and makes up almost the entire clay fraction in the majority of the low rank coals in the A, B and C seams.  The XRD pattern indicates that the kaolinite in the low rank coals is largely well ordered, as demonstrated by prominent (001), (020), (-110) (-1-11) and (-111) reflections at 12.3o 2q and between 19o and 22o 2q (Figure 2) while the kaolinite in the higher rank coals mostly found in the form of poor ordered kaolinite (Figure 3).  
Figure 2. The XRD pattern of sample B1 (low rank coal), showing
 the typical well ordered kaolinite pattern
  
Figure 3. The XRD pattern of sample C1 (high rank coal), showing the typical poorly ordered kaolinite pattern.
SEM-EDAX analysis (Figure 4) indicates that the kaolinite in Bukit Asam coal is generally found as broken plates and aggregates, and is present as layers, lenticles and lenses, and encrustations on the sulphides and organic matter, or as finely dispersed particles in different maceral components. These occurrences suggest that the kaolinite formed in situ, in the original peat deposit, by authigenic processes (Ward, 1986). Kaolinite in the study area is also found infilling cleats and other fractures, and in some instances is also present as a matrix associated with crystalline pyrite and quartz.  Well-ordered kaolinite in coals is probably authigenic in origin. Authigenic kaolinite in the coals of the Bukit Asam area may have been precipitated from the waters of the peat swamp or in the pores of the original peat deposit. The authigenic kaolinite may also have been formed from the interaction between silica and alumina in the waters of the peat, in association with pH changes and in the presence of organic matter  (Ward, 1978; Renton and Cecil, 1978; Spears, 1987).
 
5.1.2.  Interstratified Illite/smectite
Interstratified illite/smectite (I/S) is present in the LTA isolated from most of the higher rank coal samples.  It is not, however present in significant proportions in the LTA of the lower-rank coal samples of the Bukit Asam deposit.Mixed layer clay minerals in the form of interstratified illite/smectite (I/S), are present in most of the mineral matter isolated from the higher rank coals of the study area.  The XRD pattern of the I/S, shown in figure 5, has distinct peaks at around 8.8o 2q (10Å) and 17o 2q (5Å) and a broad reflection on the low-angle (high d-spacing) side of the 001 illite diffraction peak. The occurrence of this material is further confirmed by the XRD pattern of oriented clay fractions (<2mm) of some coal LTA samples.  

 
Figure 4. Images from SEM-EDAX analysis, showing the typical mode of kaolinite occurrence in Bukit Asam coals: (a)  Finely dispersed kaolinite in vitrinite  maceral  (sample Frid-5). Figure 1 and 2 are enlargements of 6.2a, showing the morphology of kaolinite crystal flakes. (b) Kaolinite infilling cell cavities in inertinite macerals (sample Frid-3); note also the occurrence of the trace element Mo 
 
The I/S in the coals is usually an irregularly interstratified material, with diffraction peaks at 11.7, 9.36 and 5.04 Å in oriented-aggregate samples after ethylene glycol treatment (Figure 6) that collapse to a 10 Å series after heating to 400ºC.  In some LTAs and non-coal samples, however, the glycol-treated oriented-aggregate XRD pattern has peaks at around 29 Å, 12.2 Å and 5.1 Å (Figure 7), which, as indicated by Moore and Reynolds (1997) and other authors, suggests a 1:1 stacking regularity and development of a super-lattice structure similar to that of rectorite.
    
Figure 5. The XRD pattern of sample C1 (high rank coal), showing the typical poorly ordered kaolinite pattern
 
Figure 6. Oriented aggregate X-ray diffractograms of the clay fraction of the LTA from a typical coal sample in the Bukit Asam area (sample C1) showing the occurrence of interstratified I/S. Three reflections appear at 11.7, 9.36 and 5.04 Å on the ethylene glycol pattern , which then resembles a pure illite (10 Å structure) after heating to 400oC  for one hour. Ad= Air dried, Gly=glycolated, H4= Heated 400oC
 
Figure 7. Oriented aggregate diffract-tograms of clay fraction of coal sample C9,  showing two superstructure peaks of rectorite mineral. For clarity only the scan interval from 2o to 30o 2q is shown. Two reflections occur at 27.09 Å and 12.2 Å in air-dried conditions, which move to 29.78 Å and 11.96Å in the glycol-saturated traces  and collapse to around 10Å on heating to 400o C for one hour. Note the occurrence of a paragonite peak at around 9.7 Å that remains fixed during all treatments. Ad= Air dried, Gly=glycolated, H4= Heated 400oC
 
The mixed layer clay minerals typically have an irregular flaky shape under the SEM (Figure 8), with EDX data indicating the presence of Ca, Fe, K and Na as well as Al and Si in the individual particles.  Velde (1995) has pointed out that, compared to more common I/S of equivalent smectite content, the interlayer composition of rectorite tends to be rich in Na and Ca.  Chemical analysis of rocks in the present study consisting almost entirely of rectorite (Table 5) shows that sodium is slightly higher than potassium, especially as the proportion of rectorite increases.
 
Table 5.    XRF analyses of clay partings that consist almost solely of rectorite, in high rank coals of the C seam at Bukit Asam.
Sample NoNa2OK2OPercent rectorite
C2C4C70.81.21.70.60.91.075.699.594.2
 
Figure 8. SEM-EDAX images and spectra of Bukit Asam coal LTA, illustrating morphological features of I/S:  (a) Irregular flake shaped crystals of I/S (sample C8); note the presence of Na and Ca along with K.  (b) Pyrite encrustation in flake and lath shaped of I/S matrix; note the occurrence of Na and Pb (sample C3). (c) Lath shaped crystals of I/S; Ca is more dominant than Na and K  (samples C1).

5.1.3.  Illite, montmorilonite (?) and Chlorite
Illite in the Bukit Asam samples is mostly found as part of the mixed layer illite/smectite minerals. A significant proportion of separate illite, characterised by a d(001) spacing of 10 Å, especially after ethylene glycol treatment, has been identified in a silicified coal sample from a low-rank coal area (sample B22, figure 10) and in one of the anthracite samples  collected near an intrusive contact.  Illite is also present in trace proportions in a few other coal and intra-seam non-coal sample
. 
Figure. 10. The XRD pattern of LTA of sample B22, showing illite 001, 002 and
003 reflections at 9.98A, 5.08A and 3.3A
  
Figure. 11. The X-ray diffractograms of oriented aggregrate clay fraction of the LTA from
sample Ant-1, showing the peaks of chlorite and paragonite
 
The XRD analysis shows traces of discrete montmorillonite to be present in one tuffaceous claystone in an intra-seam band of the Bukit Asam area. This mineral is thought to have formed as a product of the volcanic activity in the area during peat accumulation.  Later processes, such as leaching, and dehydration or layer collapse, may have transformed this mineral into kaolinite or illite (Demchuk, 1992a).  These processes may have removed montmorillonite from the materials in which it initially occurred, such as coal or associated non-coal strata, causing the virtual absence of this mineral in the same rocks when sampled.Chlorite has been identified in a few samples, mainly from the distinctive 004 peak at 3.54Å in the XRD traces (Figure 11).  As the 001 peak (14 Å) is not present, it is likely that the chlorite material in these samples represents an iron-rich chlorite (cf. Moore and Reynolds, 1997).

5.2. Quartz

Quartz is identified in X-ray diffraction (Figure 12) by distinct (101) and (100) peaks at the respective angles of 26.7 o 2q (3.35 Å) and 20.9 o 2q (4.26Å).  It is present almost in all of the samples studied, occurring in both coal and non-coal rocks.  In general, however, quartz is the second most abundant mineral in Bukit Asam coal after the clay minerals.  Renton (1982) and Ward (1989,1996) have noted that quartz in coal is mostly considered to be detrital in origin, although a portion may be authigenic or biogenic. In Bukit Asam coal, the quartz appears to have both a detrital and an authigenic origin. SEM-EDAX data from some coal polished blocks shows that quartz also appears to be secondary in origin.  
Figure 12. X-ray difractogram of random powder of sample B24, containing 100 % quartz
 
Authigenic quartz in the Bukit Asam area occurs as microcrystalline minerals infilling cell lumens of the coal macerals or penetrating the cracks of the macerals (Figure 6.11a). The silica in such cases may have come from transformation of clay minerals or been derived from the alteration of volcanic ash. The silica polymorph trydimite, distinguished by XRD peaks at 4.11 Å, 2.99 Å and 2.32 Å, has also been identified in the LTA of some coals from the A and B seams.  The tridymite is mostly found in coal samples taken from close to pyroclastic clay partings, and it is possible that silica released by alteration of glass, feldspar and other components in the pyroclastic sediment may have been re-precipitated in the pores of the peat to form the trydimite mineral.
 
5.3.     Mica
5.3.1.       Paragonite
Paragonite (sodium mica) is an unusual mineral in coal, but is commonly present in low-grade metamorphic rocks (Chatterjee, 1962, 1971; Frey, 1987; Weaver and Broekstra, 1984). Paragonite, as well as regularly interstratified mica/smectite in the form of mixed layer paragonite/smectite with 50% paragonite layers, has nevertheless been reported in the high rank anthracites of eastern Pennsylvania (Daniels, 1992). 

 











Figure 13. SEM photomicrographs and EDAX spectra of coal polished block of sample C3. (a) Quartz crystals in finely dispersed layers infilling cells in vitrinite maceral that seem to be authigenic in origin. (b) and (c) Enlargement of this layer revealing that the quartz has particles angular to sub-rounded in shape, which may additionally imply the presence of detrital quartz. Note also the poikilotopic texture in figure b that appears in one of the quartz crystal  (see arrow) suggesting a detrital origin.

Paragonite has been identified by the present study in a number of higher-rank Bukit Asam coals, on the basis of strong XRD peaks at 9.67-9.7 Å, 4.8-4.85 and 3.20-3.25 Å (Figure 11). The 9.7 Å peak is clearly recognized in most XRD patterns of paragonite-bearing rocks, except where the paragonite is only a minor mineral component.  The mineral’s identity is further confirmed by oriented XRD patterns of the clay fractions (figure 14) and by SEM-EDX observations (figure 15). As paragonite is a sodium-bearing mineral, and in addition to the distinctive d spacing in the XRD results, its occurrence is confirmed by the Na2O percentages indicated by the XRF data. The linear relationship appears when the normalised Na2O percentages indicated by the XRF data from relevant samples are plotted against the paragonite content of the mineral matter (Figure 16).

 

5.3.2. Muscovite

XRD analysis has identified the occurrence of muscovite in the LTA of some coal samples from the Bukit Asam area, and also in some intra-seam sediments.  Muscovite is identified by the two characteristically intense peaks at 9.9 Å (001) and 3.3Å (003), and by a relatively weak peak at 5 Å (002). However, the XRD pattern of many untreated samples shows only a poorly defined peak at 10 Å and 5 Å, due to a superimposed shoulder overlapping with the mixed layer clay and paragonite peaks.    
Figure 14. X-ray diffractrograms of sample C9, showing distinct peaks of
 paragonite both in random powder mounts and the oriented-aggregate
clay fractions. Ad= Air dried, Gly=glycolated, H4= Heated 400oC

 
Figure 15. (a) SEM photomicrograph and EDAX data from sample C5, showing paragonite like mineral.  The material has a mica-like structure, which is composed mainly of Al, Si and Na. Note the very low intensity of the K peak on the EDAX spectrum. The mineral is surrounded by illite lath structures of illite rich I/S material. (b) and (c) SEM photomicrograph and EDAX data of paragonite like mineral associated with vitrinite maceral in sample Ant-1. 
Figure 16. Correlation between Na2O concentration as indicated by XRF analysis
and paragonite concentration as determined by XRD and Siroquant analysis.

 

7.1       Pyrite

Pyrite and siderite are present as minor minerals in most Bukit Asam coal samples. Some plies however have pyrite as a major mineral. The mineral is identified by an XRD pattern that shows d spacings at 1.63, 2.71, 3.12 and 2.43 Å. Petrographic analysis and SEM-EDAX observations of the coal reveal that most of the pyrite in the Bukit Asam coal, is syngenetic in origin. Syngenetic pyrite may occur as a cell and pore infilling, a replacement of the maceral components, or as an individual and clustered euhedral crystals, isolated anhedra and massive but internally crystalline accumulations (Ward, 2002). Similarly, in Bukit Asam coal, syngenetic pyrite is present as individual crystals dispersed in organic matter (figure 17), infilling cavities and cell lumens in coal macerals (figure 18), and also as framboidal forms (Figure 19).  Pyrite in the Bukit Asam coals is also found as epigenetic veinlets and nodules or as incrustations on vitrain (figure 20). 
 
Figure 17. SEM-EDAX images showing pyrite occurring as finely dispersed crystals in vitrinite
 macerals (Sample C5).
Figure.18. Photomicrograph of pyrite (a) Infilling cell lumens in gelovitrinite.  (b) Infilling cell
lumens in fusinite. Polish section of sample C3, field of width 0.2mm. 
 
Figure 19. SEM-EDAX analysis of framboidal pyrite (Sample C5)
 
Figure 20. Epigenetic pyrite in the C seam.

7.2       Siderite and Calcite

Siderite is identified in XRD studies by prominent d spacings of 2.79 Å and 3.59 Å.  The mineral appears to be randomly distributed both in the coal and associated non-coal lithologies.  The siderite in Bukit Asam coal is mainly syngenetic, occurring as fine-grained and massive accumulations or forming lenses and nodules. Calcite, represented in XRD patterns by a distinct (104) reflection at 3.04 Å (29.4o 2q), is present in some samples of coal and associated non-coal lithologies in the Bukit Asam area. This mineral has been also identified in scanning electron microscope studies (Figure 21).  

 
Figure 21. SEM-EDAX image and spectrum of calcite-like mineral, sample A-17. Note Fe and
 Mg possibly also present in the calcite in this sample

5.6. Boehmite
 Boehmite was identified in the LTA of some coal samples from its (001) XRD peak at 6.1Ã…. Its occurrence was also confirmed by SEM-EDAX observations that show an aluminium-bearing mineral present in the coal LTA (Figure 22). The concentration of this mineral in the LTA of Bukit Asam coal is very low, mostly representing less than 3% of the coal mineral matter.
The boehmite may have been formed from the precipitation of Al released by leaching from the detrital minerals, including volcanic ash, through processes such as those indicated by Ward (2002).  These suggest that oxidation of the organic matter may give rise to low pH in the peat, which may in turn lead to an increase in Al solubility.  Under such low pH conditions, Al would be easily leached from any detrital mineral material, including volcanic ash, and transferred with the acidified swamp water to other parts of the peat deposit. The Al mobilization process also be supported by the development of organometallic complexes, formed from the interaction of partly degraded aluminosilicates with the swamp environment.  The Al in this leachate may be precipitated to form boehmite when the leachates move to a higher pH area.

Figure 22. SEM-EDAX image and spectrum of boehmite like mineral in sample Frid-3
 
 5.7.  Mineral Artefacts
Sulphate minerals such as gypsum (CaSO4.2H2O), bassanite (CaSO4.1/2 H2O), hexahydrite (MgSO4.6H2O), coquimbite (Fe2(SO4)3.9H2O), jarosite (Fe3(SO4)2(OH)6  and alunogen (Al2(SO4)3·17(H2O)  are present almost in all plasma ashes of Bukit Asam coals from the Air Laya field. In addition, potash alum is present in the plasma ash of the C seam from the Muara Tiga field.Miller and Given (1978) and Ward (1991,1994, 2002) have indicated that the bassanite and other sulphates in coal LTA residues may represent artifacts produced in the plasma ashing process, formed by interaction between the organic sulphur in the coal and Ca, Mg or other elements occurring as inorganic components of the organic matter. The occurrence of bassanite in bituminous coal samples may also reflect the dehydration during ashing of gypsum, with the gypsum being produced by reactions between calcite and sulphuric acid derived from pyrite oxidation during storage of the coal sample (Rao and Glukoster, 1973; Ward and Christie, 1994).In the present study, abundant jarosite was also found on the cleat at the top of the weathered C seam. Its occurrence is thought to be an oxidation product of pyrite framboids, or possibly as sulphates precipitated from evaporation of organically associated Fe in the coal’s pore water.
 
6.       Conclusions
The low temperature ash from the Bukit Asam coals contains material that comes from two different sources: (a) the crystalline minerals originally present in the coal including kaolinite, I/S, paragonite, quartz, pyrite, calcite and siderite,  and (b) products derived from the non mineral inorganics, formed as mineral artifacts by reactions during oxidation of the organic matter, such as bassanite, hexahydrite, coquimbite, jarosite and alunogen.  The latter group may also include non-crystalline or poorly crystalline material that is not detected by XRD analysis.Well crystallized kaolinite, along with some poorly crystalline kaolinite and quartz, are the major minerals in almost all of the Bukit Asam unheated or slightly heated coals. The mineralogy of the equivalent Bukit Asam high rank coals in contrast, dominated by the occurrence of I/S mixed layer clay (including rectorite) and paragonite. Other minerals are mostly present as minor components, including calcite, siderite, pyrite, boehmite, gypsum and muscovite. A great proportion of artifact minerals, including bassanite, coquimbite, jarosite, hexahydrite, and alunogen, are present in all Bukit Asam low rank coals. These minerals still persist, however, in high rank coals of Bukit Asam, represented mainly by coquimbite gypsum and boehmite.
 
7. Acknowledgements
I gratefully acknowledge, my super-visor, Prof. Colin. R. Ward, for his exceptional interest,  patient guidance, understanding and encouragement, to the completion of this study. Thanks are expressed the Australian Development Scholarship Program for financial support to pursue this study, and to Perusahaan Tambang Batubara Bukit Asam for their generous assistance with the field work.  Thanks are also expressed to Rad Flossman, Irene Wainwright, Ervin Slansky, Zhongseng Li, Dorothy Yu and Vera Piegerova, for their assistance with sample preparation, instrument operation, XRD interpretation and electron microscope analysis.

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