Chemical composition of macerals in bituminous coals of the Gunnedah Basin, Australia, using electron microprobe analysis techniques

International Journal of Coal Geology 39 Ž1999. 279–300

Chemical composition of macerals in bituminous coals of the Gunnedah Basin, Australia, using electron microprobe analysis techniques Colin R. Ward ) , Lila W. Gurba School of Geology, UniÕersity of New South Wales, Sydney NSW 2052 Australia Received 6 March 1998; accepted 24 July 1998

Abstract The chemical composition of the organic matter in the principal macerals of high-volatile bituminous coals from the Gunnedah Basin, New South Wales ŽRvmax of telocollinite between 0.6 and 1.1%. has been evaluated from polished section specimens using an electron microprobe technique. Highest proportions of carbon occur in the inertinite macerals, especially fusinite and secretinite Žformerly resino-sclerotinite., as well as in sporinite; lowest proportions of carbon occur in the different macerals of the vitrinite group. Oxygen shows the reverse trend, being most abundant in vitrinite and least abundant in the inertinite components, whereas sulphur is lowest in the inertinites and highest in the liptinite Žmainly sporinite. present. Evaluations of maceral composition, using the carbon content of telocollinite as a rank indicator, show that carbon is more abundant in both sporinite and semifusinite, relative to vitrinite, in low-rank high-volatile bituminous coals. The difference decreases with increasing rank, and the proportion of carbon in telocollinite becomes essentially the same as that in sporinite and semifusinite at carbon contents of about 89 and 91%, respectively. The carbon content of fusinite and secretinite, on the other hand, does not seem to vary appreciably with rank advance. No significant difference in composition occurs in the rank range studied between the three vitrinite varieties present, desmocollinite, telocollinite and a more highly reflecting telocollinite resembling pseudovitrinite. No evidence was found to indicate a higher hydrogen content, relative to telocollinite, for the vitrinite matrix of desmocollinite. q 1999 Elsevier Science B.V. All rights reserved. Keywords: macerals; electron microprobe; Australia; carbon; oxygen; sulphur

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Corresponding author. Tel.: q61-2-9385-4285; Fax: q61-2-9385-5935; E-mail: [email protected]

0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 Ž 9 8 . 0 0 0 4 9 - 4

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1. Introduction Knowledge of the chemical composition of individual coal macerals is essential in understanding the petrography of the different materials ŽInternational Committee for Coal and Organic Petrology, 1963, 1971., and in determining the contribution of the various macerals to whole-coal chemistry and utilization behaviour. The chemical composition of the vitrinite maceral group also provides a better indication of coal rank ŽStach et al., 1982. than indices based on the more diverse mixture of components inherently incorporated in whole-coal analytical results Že.g., American Society for Testing and Materials, 1981.. Much of the information in the literature on the composition of individual macerals has been drawn from analysis of concentrates, isolated from bulk coal samples by hand picking or similar processes. Such materials are, however, always likely to incorporate small amounts of other components, for example sporinite particles in vitrinite or mineral inclusions in semifusinite, which may affect the analysis process. Some macerals, such as sporinite and inertodetrinite, are also individually too small to be reliably isolated for conventional analysis techniques. The electron microprobe analyser provides a means of determining in-situ the elemental composition at an individual point within a larger mass of material. It has been widely used in other branches of geology to study the composition of mineral particles in a range of different rock types, and as a result done much to increase the understanding of many geological processes. Because of operational difficulties, however, its application to coal has mostly been limited to the study of the relatively heavy elements, such as sulphur and chlorine, that make up minor but nevertheless important inorganic components Že.g., Sutherland, 1975; Raymond and Gooley, 1978; Straszheim et al., 1983; Harrison, 1991.. The development of improved techniques to determine the relative proportions of light elements, namely the carbon and oxygen that constitute the bulk of the different macerals ŽBustin et al., 1993, 1996., has widened the applicability of the method in coal analysis. It has enabled in-situ study of the composition of the individual coal components in the same polished sections as are used for conventional coal petrology, increasing the ability to discriminate between macerals and avoiding the problems associated with mechanical separation methods. The present paper discusses the results of electron microprobe analysis of a range of high-volatile bituminous coals from the Permian strata of the Gunnedah Basin, New South Wales, Australia. This study has led to a better understanding of the distribution of carbon, oxygen and organic sulphur among coal macerals generally, including some of the variations associated with rank advance. It also provides an opportunity to extend the work of Bustin et al. Ž1993, 1996. to coal deposits outside the Euramerican sphere. Petrographic terminology used within the paper mainly follows that of Standards Australia Ž1998.. 2. Previous research on maceral composition The literature contains relatively few data on the composition and properties of individual coal macerals, especially macerals other than vitrinite. Relevant works

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include Dormans et al. Ž1957., Millais and Murchison Ž1969., Ghosh Ž1971., Dyrkacz et al. Ž1984, 1991., van Krevelan Ž1993., and Mastalerz and Bustin Ž1993a,b, 1997.. This largely reflects the difficulty of isolating the macerals, especially fine-grained or intimately mixed materials, for individual analysis. As pointed out by Bustin et al. Ž1993., mechanical separation of macerals may give rise to chemical changes resulting from the crushing and grinding process. Apart from the possibility of contamination, concentration of macerals by dense liquid separation from crushed coals may also result in such a fine particle size that the components are no longer distinguishable under the optical microscope, and thus the degree of separation cannot be properly evaluated. Dormans et al. Ž1957. investigated the composition and properties of maceral fractions separated from European bituminous coals. Similar investigations were carried out on high-volatile bituminous coals by Dyrkacz and Horowitz Ž1982.. These and other studies show the carbon content of liptinite Žexinite. to be slightly less and that of the corresponding inertinite somewhat higher than that of vitrinite in coal of the same rank. At higher rank these differences disappear. Inertinite is reported from these studies to be much poorer and liptinite much richer in hydrogen than vitrinite, whereas oxygen is highest in vitrinite and lower in the inertinite and liptinite components. Ghosh Ž1971. described the chemistry of maceral concentrates Žvitrinite, liptinite and inertinite. from Indian coals Žcarbon content 76–91% dry, ash free., concluding that the rates of chemical change of the individual macerals with rank are distinctly different. According to Ghosh Ž1971. significant and rapid changes in chemical composition and structure take place at ranks corresponding to 82–83%, 89–90% and 92% carbon content. The electron microprobe has been applied to the direct determination of organic sulphur in coal macerals over many years ŽSutherland, 1975; Harris et al., 1977; Solomon and Manzione, 1977; Raymond and Gooley, 1978; Raymond, 1982; Maijgren et al., 1983; Straszheim et al., 1983; Harrison, 1991., as well as to other elements such as Cl, Na, Mg, Si, Ca, Fe and Sr ŽDutcher et al., 1964; Karner et al., 1986; Harrison, 1991.. Younkin et al. Ž1987. also used the technique to analyse oxygen in coal, providing a potentially better basis than determination by difference in the ultimate analysis process. Bustin et al. Ž1993., Mastalerz and Bustin Ž1993a,b., Mastalerz and Bustin Ž1997., Mastalerz et al. Ž1993. and Bustin et al. Ž1996. modified the electron microprobe technique to study the light elements ŽC, O, N. in a range of mainly vitrinite-rich coals from the Northern Hemisphere. The present paper, based on this approach, represents the first attempt to study the chemical composition of individual macerals in Australian coals, including a number of more inertinite-rich materials, using the electron microprobe technique. The study aims to investigate the chemistry of the individual macerals in iso-rank coals, including several different vitrinite types, and also variations in the chemistry of these macerals associated with rank advance. It deals mainly with high-volatile bituminous coals ŽRvmax of telocollinite between 0.6 and 1.1%., although a few higher rank coals have also been included in the sample suite. The work has provided an insight into the relationship between the chemical properties derived from ultimate analyses of whole-coal samples and the elemental composition of the individual macerals represented at different rank levels.

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3. Geological setting The Gunnedah Basin is part of the Permo-Triassic Sydney-Bowen foreland basin system of eastern Australia ŽFig. 1.. It contains up to 1200 m of marine and non-marine Permian and Triassic sediments, resting unconformably upon Permian Žand possibly Late Carboniferous. silicic and mafic volcanics ŽTadros, 1993, 1995.. Coals occur in two separate stratigraphic intervals, the Early Permian Leard and Maules Creek Formations near the base of the sequence and the Black Jack Group at the top of the Permian succession ŽTadros, 1993, 1995.. A thick marine succession, the Porcupine and Watermark Formations, occurs in between. The Permian strata are unconformably overlain by an upward fining Triassic fluvial sequence up to 200 m thick ŽJian and Ward, 1993., and in the western and northern parts of the basin also by rocks of the overlying Jurassic to Cretaceous Surat Basin. Several igneous intrusions, thought to be mainly of Jurassic age ŽMartin, 1993., also occur within the Permian coal measures and other Gunnedah Basin strata, especially in the south-eastern portion of the basin. Numerous fully-cored boreholes, drilled to support basin-wide exploration programs, have penetrated the coal-bearing sequence. Samples from several of these holes were

Fig. 1. Map of the Gunnedah Basin showing location of boreholes with electron microprobe analyses.

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used for the present study ŽFig. 1., which is part of a wider project ŽGurba, 1998; Gurba and Ward, 1998. aimed at evaluating regional coal-rank trends.

4. Methodology 4.1. Sample selection and preparation The coals used in the electron microprobe study ŽTable 1. represent the various coal-forming periods and depositional environments and cover most of the rank range in the Gunnedah Basin succession. Samples of heat-affected coal are also included, taken from parts of the sequence in which the rank of the coal has been locally raised by proximity to igneous intrusive bodies. The specimens were prepared in accordance with Australian Standards as either polished blocks or as grain mounts in the same way as for optical petrographic examination. The polished surfaces were coated with carbon for examination under the electron microprobe. Samples and standards were coated at the same time to ensure a similar coat thickness ŽBustin et al., 1993.. For the first experiments with the electron microprobe photo-mosaics of the coal sections were prepared from examination under the optical microscope using oil

Table 1 Location of samples studied Borehole name

Depth Žm.

Host sequence

Rvma x Ž%. of telocollinite

DM Bando DDH1

359.64 408.25 421.70 591.00 887.70 975.70 895.83 938.87 312.80 380.00 441.80 187.50 197.70 203.60 258.50 272.90 266.50 296.60 301.10 468.30

Black Jack Group Black Jack Group Black Jack Group Black Jack Group Maules Creek Formation Maules Creek Formation Black Jack Group Black Jack Group Black Jack Group Black Jack Group Black Jack Group Maules Creek Formation Maules Creek Formation Maules Creek Formation Maules Creek Formation Maules Creek Formation Black Jack Group Black Jack Group Black Jack Group Black Jack Group

0.76 2.20 a 0.86 0.68 0.72 0.99 0.77 0.67 0.80 0.69 0.73 0.70 0.80 1.05a 0.82 0.75 0.86 0.81 0.83 0.80

ACM Yannergee 1 DM Brigalow DDH 1 DM Doona Point DDH 3 DM Texas DDH 1

DM Morven DDH 1

DM Nombi DDH 1 a

Heat affected. Coals influenced by marine conditions are listed in italics.

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immersion, as described by Mastalerz et al. Ž1993. and Bustin et al. Ž1996., to help identify the macerals at each point in the subsequent microprobe study. This step was omitted in the later sessions, however, to avoid contamination of both the sample and the microprobe with immersion oil residues. The light optics of the microprobe were found to be adequate for recognition of the maceral represented at each analysis point. The carbon coat was removed after the electron microprobe analysis, and the macerals studied further by conventional petrographic methods, including vitrinite reflectance measurement. 4.2. Analytical conditions The analyses were performed on a Cameca SX-50 electron microprobe equipped with four wavelength-dispersive spectrometers and using TAP, PET, PC1 and PC2 analysing crystals. The analytical routine followed was essentially that described by Bustin et al. Ž1993, 1996. for analysing major and minor elements in coal by an electron microprobe with the PAP matrix correction routine ŽPouchou and Pichoir, 1991.. Analyses were made using an accelerating voltage of 10 kV, a beam current of 10 nA and a beam diameter of between 5 and 10 mm. The elements determined in each measurement were carbon, oxygen, nitrogen, sulphur, silicon and aluminium. Iron and calcium were also measured in some samples, and full spectra frequently checked to ascertain if any other elements were present. The elements other than C, O, N and S were monitored to check for mineral contamination in each analysed field; if the intensities of these elements were significantly higher than background the data were discarded as representing mineralised rather than pure maceral components. Oxygen was measured at the beginning of each analysis, with a counting time of 20 s. The counting times chosen for carbon and nitrogen were 10 and 20 s, respectively. Operating conditions were checked frequently to ensure that they remained stable. 4.3. Analytical standards A pre-analysed anthracite Žprovided by Dr. M. Mastalerz. was used as the standard for carbon analysis. Quartz ŽSiO 2 . or dolomite was used as an oxygen standard, boron

Table 2 Microprobe analysis of different points within the one layer of Ža. semifusinite and Žb. pseudovitrinite, DM Texas DDH 1, 187.50 m Žfrom Gurba and Ward, 1997. C, wt.%

O, wt.%

S, wt.%

Si, wt.%

Al, wt.%

C, wt.%

0.30 0.26 0.31 0.35 0.31 0.36 0.32

0.06 0.22 0.27 0.04 0.07 0.08 0.04

0.03 0.13 0.31 0.03 0.03 0.02 0.01

81.51 81.43 81.59 81.16 82.22 81.71 81.99

Ža. Semifusinite 86.64 85.59 85.14 86.63 86.46 86.58 86.94

9.66 10.42 10.81 9.72 8.55 8.48 8.49

O, wt.%

S, wt.%

Si, wt.%

Al, wt.%

0.40 0.39 0.44 0.39 0.48 0.32 0.54

0.07 0.07 0.11 0.07 0.04 0.08 0.11

0.09 0.05 0.11 0.09 0.10 0.15 0.09

Žb. Pseudovitrinite 13.40 14.04 13.76 13.44 13.22 13.13 13.67

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Fig. 2. Reflected light photomicrographs showing the reaction spots caused by the microprobe on carbon-coated polished sections. Left: spots on sporinite and desmocollinite, DM Bando DDH 1, 591.0 m. Right: spot on telocollinite, ACM Yannergee DDH 1, 895.93 m. Spots are approximately 5 mm in diameter.

nitride ŽBN. as a nitrogen standard and anhydrite ŽCaSO4 . as a sulphur standard. Quartz was used as a standard for silicon, and marcasite and sanidine as standards for iron and aluminium, respectively. No significant peak shifts were observed between the standards and the coals analysed. In the initial part of the study diamond was used as the standard for carbon analysis. This gave rise to several problems in carbon determination, possibly due to the different crystal structure involved. The problems were overcome however, following adoption of the anthracite standard. Several standards were also checked for oxygen determination, but quartz and dolomite were found to yield the most reliable data. No suitable standard was available that more closely approximates the proportion of oxygen in coal. Multiple analyses were carried out on individual maceral bands and particles in the coal, together with repeated analyses of identical macerals at other locations in the individual specimens. Table 2 provides an example of data derived from the electron microprobe for individual semifusinite and pseudovitrinite layers in the same coal sample, showing the relative homogeneity of the organic matter in the materials concerned. In some cases, particularly with sporinite ŽFig. 2., the size of the electron microprobe beam Ž5 mm. exceeded that of the target maceral, and the area analysed by the microprobe Žindicated by a spot on the sample after analysis. embraced some of the surrounding material Že.g., vitrinite. as well. Data from such points were not taken into account in evaluating the composition of either maceral component.

5. Difficulties in the analysis of light elements The problems encountered in the analysis of light elements are generally regarded as being of two varieties ŽNash, 1992.: those resulting from the physics of X-ray generation

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and detection and those associated with preparation of the sample and standards concerned. Care was taken to ensure that the standards and samples were clean and well polished, had the same thickness of carbon coating, and had their surfaces oriented perpendicular to the electron beam. These procedures are particularly important for oxygen because the signal is derived from very near the surface, and C has a high mass absorption coefficient for oxygen K a radiation ŽNash, 1992.. According to Nash Ž1992. errors of up to 1% per degree of inclination are produced by samples that are inclined to the primary electron beam. One of the problems encountered during microprobe analysis was beam deflection, in which the analysed spot was displaced a short distance away from the main instrument axis. Samples containing pseudovitrinite ŽBenedict et al., 1968; Gurba and Ward, 1998. were found to be especially prone to this problem. This material contains a characteristic pattern of cracks and slits, often with pyrite inclusions. Pyrite inclusions just below the surface, or possibly oil in a crack left over from oil-immersion microscopy, are thought in such cases to have deflected the electron beam. Special care was therefore taken to

Table 3 Microprobe analyses of maceral types, repeated at different times on the same coal samples Sample location

Maceral Average, Min, wt.% wt.%

Max, wt.%

No. of Standard values deviation

Carbon DM Morven DDH 1, Depth 266.5 m 97r07r07 97r07r08 97r07r07 97r07r08

TC TC SF SF

81.56 82.66 87.99 88.69

80.28 81.98 87.11 87.81

83.00 84.00 88.77 89.45

10 15 3 6

0.76 0.57 0.83 0.63

Oxygen DM Morven DDH 1, Depth 266.5 m 97r07r07 97r07r08 97r07r07 97r07r08

TC TC SF SF

12.74 13.37 6.78 8.63

12.21 13.11 12.60 13.89 6.58 7.08 8.14 9.02

10 15 3 6

0.32 0.33 0.27 0.41

21 6 3 19 8 17 5 5 5 13 6 11 3

0.07 0.11 0.03 0.09 0.09 0.04 0.12 0.09 0.14 0.11 0.05 0.08 0.06

Sulphur DM Nombi DDH 1, Depth 468.3 m

Date, yyr mmrdd

95r11r12 96r06r26 96r08r21 96r08r26 96r06r26 96r08r08 96r08r26 96r08r08 96r08r26 95r11r12 96r06r26 96r08r08 96r08r21

PSV PSV PSV PSV TC TC TC DSC DSC SF SF SF SF

0.97 0.96 0.93 0.94 0.89 0.85 0.86 0.83 0.88 0.45 0.46 0.49 0.46

0.83 0.85 0.90 0.72 0.73 0.77 0.73 0.72 0.71 0.18 0.37 0.35 0.39

1.09 1.16 0.95 1.054 1.04 0.94 1.06 0.95 1.09 0.66 0.53 0.61 0.49

TC s telocollinite; DSC sdesmocollinite; PSVs pseudovitrinite; SF ssemifusinite.

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minimise potential oxidation or contamination of the samples. Such deflection Žor sliding. of the beam was noticed only on macerals with a complex system of cracks. Other materials analysed under the same conditions did not show significant deflection effects. 5.1. Repeatability Repeatability of the method was checked by comparing results obtained from the same samples in different analysis sessions ŽTable 3.. The samples were re-polished and separately carbon coated prior to each analysis run. Very good results were obtained for organic sulphur, with analytical conditions and standards being kept constant for all microprobe sessions. Although the same analytical conditions were not consistently applied for oxygen determination, moderately good repeatability Žvalues within 10% on a relative basis. were obtained for oxygen. A similar degree of repeatability was obtained for carbon in the coal samples.

6. Distribution of elements in Gunnedah Basin macerals A number of individual points were analysed under the electron microprobe on the different macerals in each coal sample. Average values for each individual maceral are summarised in Table 4. These represent averages of a number of individual determinations, with mineralised materials Ži.e., points with significant Si, Al, Ca or Fe. excluded from the averaging process. 6.1. Carbon A significant range of carbon content was noted within the different macerals of each sample studied ŽTable 4.. An indication of the variation among individual data points in selected samples is given in Fig. 3. The percentage of carbon in the different macerals of the vitrinite group within the same coal sample may vary by about 1 to 2% ŽTable 4; Fig. 3.. More significant differences are noted between the carbon content of the vitrinite group as a whole and the carbon content of the inertinite and liptinite components. Higher carbon values occur in the inertinites Žmainly semifusinite, fusinite and secretinite. and liptinites Žmainly sporinite., relative to the different vitrinites in the same coal samples. The carbon content of telocollinite varies from 77% to 86% over most of the rank range studied. Values up to 89% occur in some heat-affected coal samples. Semifusinite varies from 82% to 90% carbon over the same rank range. The composition of the vitrinite macerals, especially telocollinite and pseudovitrinite, is relatively uniform within each individual coal sample ŽTable 2.. The only exception is the sample from DM Doona Point DDH3, where significant proportions Žup to 1%. of Si and in some cases Al occur at some points within visibly-homogeneous vitrinite layers. These probably represent sub-microscopic particles of quartz andror kaolinite within the vitrinite components Žcf. Ward and Swaine, 1995..

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Table 4 Microprobe analysis of carbon, sulphur and oxygen in macerals of Gunnedah Basin coals. Sample location

DM Brigalow DDH 1, 312.80 m DM Brigalow DDH 1, 380.00 m

ACM Yannergee 1, 895.83 m

ACM Yannergee 1, 938.87 m

DM Doona Point DDH 3, 441.80 mb

DM Morven DDH1, 266.50 m

DM Morven DDH1, 296.60 m

DM Morven DDH 1, 301.10 m

DM Texas DDH 1, 187.50 m

DM Texas DDH 1, 272.90 m

DM Bando DDH 1, 359.64 m

Maceral

TC SF TC DSC SF SP TC DSC SF F SP TC DSC SF F SP b TC DSC SF SC TC PSV SF F TC DSC SF F SP TC DSC PSV SF SP TC DSC PSV SF F TC DSC PSV SF F TC DSC

N

13 3 15 8 5 10 29 16 11 1 8 9 19 8 2 9 30 18 15 8 37 10 9 2 16 10 16 2 3 10 4 15 5 1 17 25 17 15 2 9 11 24 19 3 15 1

Carbon

Oxygen

Sulfur

Avg. Žwt.%.

SD

Avg. Žwt.%.

SD

Avg. Žwt.%.

SD

80.19 85.89 81.53 81.92 85.83 88.08 84.45 84.18 87.89 92.58 85.88 85.18 84.06 86.61 90.87 85.56 80.37 81.40 83.28 89.91 81.91 81.76 88.46 89.76 82.30 83.21 86.26 91.46 86.06 79.17 80.37 77.68 85.37 85.23 80.55 81.76 81.34 86.15

0.71

15.62 10.59 13.55 13.16 11.73 7.61 10.21 10.27 8.25 4.64 9.28 9.19 10.14 9.21 7.15 9.25 11.04 10.03 9.67 4.33 13.13 12.25 8.01 6.86 13.50 12.31 10.93 6.27 10.34 12.94 10.83 14.32 8.06 9.95 15.48 13.52 13.57 10.55

0.96

0.67 0.26 0.50 0.55 0.33 0.79 0.47 0.47 0.26 0.20 0.55 0.45 0.52 0.27 0.23 0.64 0.50 0.50 0.38 0.17 0.84 0.82 0.43 0.31 0.50 0.57 0.26 0.14 0.65 0.44 0.41 0.45 0.31 0.82 0.40 0.41 0.41 0.27 0.20 0.45 0.44 0.45 0.27 0.14 0.66 0.66

0.06

81.93 82.13 82.42 84.80 88.13 81.73 81.15

0.91 0.38 0.97 0.41 1.00 0.59 1.53 0.87 0.72 1.03 1.36 1.14 1.29 1.21 2.85 0.89 0.90 0.43 0.74 0.64 1.09 1.18

0.71 0.71 0.67 2.41 0.79 0.77 0.53 1.64 0.51 0.67 0.85 1.81 1.13

13.84 13.48 12.78 10.72 5.75 13.94 14.61

0.79 0.59 0.99 0.28 0.60 0.48 0.91 0.51 0.36 0.48 1.08 0.60 0.67 0.46 1.51 0.55 0.78 0.32 0.99 0.36 0.63 0.82

1.76 0.85 0.57 1.96 0.39 0.62 0.51 1.35 0.41 0.60 0.83 1.30 1.18

0.05 0.06 0.03 0.08 0.07 0.07 0.07 0.09 0.05 0.08 0.05 0.10 0.05 0.06 0.11 0.04 0.09 0.09 0.10 0.06 0.06 0.07

0.03 0.08 0.03 0.05 0.07 0.05 0.07 0.08 0.10 0.07 0.07 0.07 0.10

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Table 4 Žcontinued. Sample location

DM Bando DDH 1, 408.25 ma

DM Bando DDH 1, 421.70 m DM Bando DDH 1, 591.00 m

DM Bando DDH 1, 887.70 m

DM Bando DDH 1, 975.70 m

Maceral

PSV SF TC PSV SF TC SF TC DSC SP SF TC DSC SP SF SC TC SF

N

23 10 33 9 4 37 25 20 14 12 13 17 10 4 19 2 28 11

Carbon

Oxygen

Sulfur

Avg. Žwt.%.

SD

Avg. Žwt.%.

SD

Avg. Žwt.%.

SD

82.55 81.13 89.06 89.06 89.22 79.75 84.11 84.45 84.07 86.02 87.98 86.16 86.58 89.73 89.47 92.47 85.89 86.53

0.68 1.16 0.82 0.41 0.56 0.63 2.78 0.91 0.43 0.80 1.58 0.67 1.14 0.41 0.84

13.63 8.98 6.72 7.03 6.24 15.55 11.73 10.62 11.0 9.56 8.44 8.12 7.88 5.94 6.57 4.42 7.79 7.40

0.51 0.84 0.70 0.19 0.29 0.53 2.23 0.80 0.50 0.48 0.92 0.40 0.36 0.68 0.60

0.63 0.30 0.74 0.81 0.69 0.66 0.41 0.63 0.66 0.72 0.35 0.46 0.48 0.54 0.28 0.18 0.56 0.49

0.08 0.09 0.08 0.04 0.05 0.07 0.10 0.12 0.06 0.10 0.17 0.10 0.07 0.04 0.06

0.63 1.39

0.30 0.76

0.06 0.11

TC s telocollinite; DSC sdesmocollinite; PSVs pseudovitrinite; SF ssemifusinite; F s fusinite; SC s secretinite; SPssporinite; Av saverage; Mins minimum; Max s maximum; SDsstandard deviation; N s number of analyses. a Heat-affected; bAbundant mineral matter.

The carbon content of semifusinite within individual samples varies more widely than that of vitrinite ŽFig. 3.. Within individual layers, however ŽTable 2., it is generally more consistent. The carbon content of sporinite, the principal liptinite maceral in the samples analysed, varies from 83 to 89% ŽTable 4.. This is higher than that of the vitrinite and close to or higher than that of the semifusinite in the same coal sample. 6.2. Oxygen The oxygen content of the different macerals follows essentially the reverse trend to that of the carbon content. It is highest in vitrinite and lowest in fusinite and secretinite Žformerly known as resino-sclerotinite, International Committee for Coal and Organic Petrology, 1997., the more carbon-rich members of the inertinite group. The oxygen content of telocollinite in the Gunnedah Basin coals ranges from around 14–15% to below 7% ŽTable 4; Fig. 4., depending on the rank of the coal concerned. For semifusinite the oxygen content is consistently lower, ranging from 11–12% to around 6% over an equivalent rank range. The oxygen content of sporinite is intermediate in value, but may approach or be even lower than that of semifusinite in the same coal sample.

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Fig. 3. Distribution of carbon content in macerals in a typical Gunnedah Basin coal. Each point represents an individual electron microprobe determination.

Fig. 5 shows the relationships between the average carbon and oxygen contents for the individual macerals in the Gunnedah Basin coal samples. A clear linear trend can be seen between the carbon and oxygen content of the different macerals when plotted on

Fig. 4. Distribution of oxygen content among macerals in a typical Gunnedah Basin coal. Each point represents an individual electron microprobe determination.

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Fig. 5. Relationships between carbon Žwt.%. and oxygen Žwt.%. in individual macerals of the Gunnedah Basin determined by electron microprobe analysis. Each point represents the average of electron microprobe determinations for the relevant maceral in a single coal sample. Carbon and oxygen Ždry, ash-free. for other Gunnedah Basin coals, determined by conventional ultimate analysis, are also shown.

this graph, with an overall correlation coefficient Ž r . of y0.89. The relationship between carbon and oxygen is relatively well-defined for telocollinite, desmocollinite, sporinite and semifusinite, but is less distinct for fusinite and secretinite. Vitrinite, liptinite Žsporinite. and inertinite Žsemifusinite and fusinite. plot in different parts of the overall graph. There is no significant difference between telocollinite and desmocollinite within the vitrinite group, but the data points are slightly more scattered for the pseudovitrinite components. Also plotted in Fig. 5 are carbon and oxygen Ždry, ash-free. data from ultimate analysis of whole-coal samples of Gunnedah Basin materials Žnot necessarily the same seams or seam subsections., taken from the records of the New South Wales Department of Mineral Resources. These data are generally consistent with carbon and oxygen trends from ultimate analysis of other Sydney-Bowen Basin coals ŽJoint Coal Board and Queensland Coal Board, 1974. and for data published in more general reference works Že.g., van Krevelan, 1993.. The main cluster of Žwhole-coal. ultimate analysis data points lies between the areas covered by the individual vitrinite and inertinite macerals based on the electron microprobe results. Such results may be anticipated, since the individual seams analysed by conventional techniques represent a mixture of both vitrinite and inertinite components. The electron microprobe data in Fig. 5 appear to indicate, at least at the low carbon end of the C–O plot, up to 2% more oxygen for a given carbon content than results obtained from similar coals by conventional ultimate analysis methods. This particularly

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arises with the vitrinite macerals in relatively low-rank samples. The difference could indicate an inherent problem with the microprobe analysis process ŽNash, 1992; see discussion above. or possibly oxygen uptake by the coal samples during storage before the electron microprobe study. Younkin et al. Ž1987. obtained significantly lower oxygen values by electron microprobe compared with ultimate analysis techniques. This is the reverse of the inconsistency encountered with the Gunnedah Basin materials. Potential causes for the lower microprobe results were suggested by Younkin et al. Ž1987. to include sample devolatilisation, oxygen standards, matrix correction routines Že.g., for carbon absorption of oxygen radiation. and other analytical factors. The oxygen values obtained in the present study, when considered in relation to other parameters such as carbon content, vary in a consistent manner among the different components. Notwithstanding the difficulties in correlation with ultimate analysis results, meaningful comparisons regarding oxygen content can therefore still be drawn from the electron microprobe data. 6.3. Sulphur In contrast to more commonly-used techniques Že.g., Standards Australia, 1993., in which organic sulphur is determined by subtraction of pyritic and sulphate sulphur from the total sulphur content, the spot size used in electron microprobe analysis allows direct measurement of the sulphur content over very small areas in the coal sample. In the absence of significant Fe at the points studied, this is interpreted to represent the organic sulphur of the macerals concerned.

Fig. 6. Distribution of organic sulphur among macerals of a typical Gunnedah Basin coals. Each point represents an individual electron microprobe determination.

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The liptinites in the Gunnedah Basin coals Žmainly sporinite. generally have the highest proportion of organic sulphur ŽTable 4; Fig. 6.; the inertinites, which are dominated by fusinite and semifusinite, usually have the lowest. The vitrinite macerals have an intermediate sulphur content, typically around twice that of the inertinites in the same coal sample. These relationships are similar to those reported in the literature for US and other Northern Hemisphere coals. However, inertinite is more abundant in the Gunnedah Basin materials than in most Carboniferous coals of the Northern Hemisphere ŽTadros, 1993., and hence makes an equal if not greater contribution than vitrinite to the total organic sulphur content. Inertinite-rich coals in the Gunnedah Basin and adjoining areas tend to have lower total organic sulphur contents than vitrinite-rich coals in the same geologic succession ŽWard and Gurba, 1997., simply because of the greater abundance and lower sulphur content of the inertinite components. No significant difference has been identified between the Žorganic. sulphur of the different vitrinite components Ždesmocollinite, telocollinite and pseudovitrinite. in the individual coal samples. Based on its sulphur content, as well as the carbon and oxygen data discussed above, the material identified as pseudovitrinite is also clearly a member of the vitrinite group. Despite its higher reflectance relative to other vitrinites, it is distinctly different in composition from the inertinite components in the same coal samples. 7. Variations in maceral chemistry with coal rank An increase in carbon content of vitrinite Žespecially telocollinite. with coalification provides one of the principal indices of coal rank ŽStach et al., 1982.. Comparison of the carbon content of the other macerals to the carbon content of telocollinite in the same coal samples, as determined by microprobe, therefore provides a useful basis to evaluate some of the changes in maceral chemistry associated with rank advance. Although mean maximum vitrinite reflectance, measured on telocollinite, can also be used as a rank indicator for the Gunnedah Basin deposits, anomalously low reflectance values occur at several horizons within the sequence, due to the influence of marine depositional conditions ŽGurba and Ward, 1998.. These render the relation of vitrinite reflectance to other rank indicators somewhat complex, and beyond the scope of the present study. 7.1. Sporinite The carbon content of sporinite increases slightly relative to that of the telocollinite in the same sample over the range of coals analysed ŽFig. 7a.. Given that the carbon content of telocollinite may be taken as a rank indicator, a poorly-defined trend of increasing carbon content for sporinite is therefore apparent with the telocollinite-defined rank level. The difference in carbon content diminishes from about 5% at the lower end of the rank range Ž80% carbon in telocollinite., and projects effectively to zero at the upper end of the range studied. The meeting point corresponds to about 89% carbon for both maceral groups.

294 C.R. Ward, L.W. Gurbar International Journal of Coal Geology 39 (1999) 279–300 Fig. 7. Carbon content of different macerals in Gunnedah Basin coals, determined by electron microprobe, plotted against carbon content of telocollinite in the same coal sample. Values represent averages for each individual maceral in a single coal sample: A: sporinite; B: desmocollinite and pseudovitrinite; C: semifusinite; D: fusinite and secretinite.

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Ultimate analysis data by workers such as Ghosh Ž1971. indicate that the chemical coalification track of liptinite remains essentially linear up to 82–83% carbon content, and thereafter dips sharply to join the track of vitrinite at 89–90% carbon. Using the electron microprobe to study the chemistry of individual macerals, Mastalerz and Bustin Ž1993a. reported that vitrinite and liptinite converge at a maturation level corresponding to a reflectance of 1.25% and 88.5% carbon. These trends are consistent with the results of the present study. Fig. 8 presents the carbon and oxygen content distribution in a single desmocollinite band, with separate spots analysed on sporinite particles and the clean vitrinite matrix. The diagram shows that the sporinite in this sample typically contains 3–4% more carbon than the desmocollinite, but only 2–3% less oxygen. There is only a small increase Ž0.1% in this instance. in sulphur in the sporinite relative to the desmocollinite ŽTable 4.. Assuming equivalent nitrogen contents, the bulk of the aggregate difference of 1% in C and O is therefore probably due to a higher hydrogen content in the sporinite material. 7.2. Vitrinite macerals The results of the electron microprobe analyses ŽTable 4; Figs. 3, 4 and 6. indicate that there is little contrast in carbon, oxygen and organic sulphur content between the different vitrinite types in iso-rank coals of the Gunnedah Basin. As an example, the average carbon content of telocollinite Ž84.5%. in the sample referred to in Fig. 8 is

Fig. 8. Carbon and oxygen content as determined by electron microprobe for different points within a single desmocollinite band, showing differences between sporinite and the clean vitrinite matrix. DM Bando DDH 1, Lower Black Jack Group, depth 591.0 m. This sample represents marine influenced coal with a maximum telocollinite reflectance of 0.68%.

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almost identical to that of desmocollinite Ž84.1%. in the same coal sample; the average oxygen and sulphur contents of these two vitrinite components are similarly almost equal. Given that there is no significant difference in the abundance of these elements, and assuming similar proportions of nitrogen, the relative abundance of hydrogen is also probably similar for all three vitrinite types. Suggestions of a significantly higher hydrogen content for desmocollinite relative to telocollinite Že.g., Brown et al., 1964. are therefore not substantiated by the electron microprobe data. This may be because the microprobe results were obtained from spots consisting of pure vitrinite material, whereas natural desmocollinite bands typically contain additional intimately admixed liptinite particles. Analysis of material isolated by processes such as hand picking may therefore also embrace a certain amount of hydrogen-rich liptinite as well as the desmocollinite matrix component. The carbon content of desmocollinite and pseudovitrinite in the Gunnedah Basin coals is essentially equal to that of the telocollinite in the same coal samples ŽFig. 7b.. Data points in both cases plot close to the diagonal line of the graph, indicating equality of carbon content within the group at all rank levels. The chemical changes that take place with coalification are therefore interpreted to be similar for all three vitrinite components ŽGurba and Ward, 1997., at least over the rank range studied. The electron microprobe data also show little difference in carbon, oxygen or sulphur content between the material identified as pseudovitrinite in the Gunnedah Basin and the other vitrinite components. The similarity in oxygen value is contrary to the suggestion of Thompson and Benedict Ž1974. that pseudovitrinite originates from in-situ oxidation at the peat stage and is therefore more oxygen-rich. Johnson et al. Ž1985. found that, up to a rank equivalent to 92% carbon Žcorresponding to Rvmax of approximately 2%., pseudovitrinite has a significantly different photoacoustic response to other vitrinites. This was thought to reflect a different chemical composition and molecular structure in the respective maceral types. The electron microprobe data in the present study, however, suggest that these changes may reflect variation only in structure; the elemental chemistry of the three vitrinite components in the Gunnedah Basin materials Žincluding pseudovitrinite. seems to be identical across this particular rank range. 7.3. Semifusinite The difference in carbon content between iso-rank telocollinite and semifusinite ŽFig. 7c. decreases from around 6% in low-rank coals to less than 3% at the higher end of the rank range. It is typically around 5% in most of the samples studied. These differences apparently disappear at rank levels above those studied; the plot in Fig. 7c approaches the diagonal line indicating equality at around 90–91% carbon for both maceral groups. 7.4. Fusinite and secretinite The carbon content of fusinite and secretinite is consistently around or above 90% ŽFig. 7d., while the carbon content of telocollinite increases steadily with rank from 80

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to about 86%. No significant change in elemental composition with rank is therefore apparent in either of these macerals for the range of coals studied. van Krevelan Ž1993. also suggests that pure fusinites from seams of different rank show only slight differences in carbon and hydrogen content compared with the variations shown by other macerals. Although based on earlier techniques, Ghosh Ž1971. indicates that inertinite joins the vitrinite coalification path at about 92% carbon Ždaf.. This is consistent with the compositional relationships exhibited by semifusinite ŽFig. 7c., but is less apparent for the fusinite or secretinite components.

8. Conclusions Although some minor problems appear to exist with precise oxygen determination, the electron microprobe can be used to obtain direct data for comparative purposes on the chemical composition of coal macerals in polished-section specimens. The process provides a number of advantages over conventional analysis of concentrates and isolated particles, including the capacity to discriminate between individual components in closely admixed materials. The results obtained from electron microprobe study of Gunnedah Basin coals are consistent with trends in maceral chemistry identified from other sources. With some exceptions Že.g., DM Doona Point DDH 3. data from the present study indicate an essentially uniform distribution of carbon, oxygen and sulphur within the organic part of individual vitrinite and semifusinite masses. The data also show significant differences in carbon, oxygen and sulphur between members of the vitrinite, liptinite and inertinite groups. The contrasts between maceral groups, especially in carbon content, vary with the rank of the coal concerned. The three varieties of vitrinite recognised in the present study, desmocollinite, telocollinite and pseudovitrinite Žwhich is itself a variety of telocollinite—International Committee for Coal and Organic Petrology, 1995., each have essentially the same elemental composition at equivalent rank levels. There is no indication from the present study of either higher or lower oxygen in pseudovitrinite, relative to other vitrinite components. There is also no indication that pseudovitrinite, despite its higher reflectance, is anything but another vitrinite type. The actual vitrinite matrix of desmocollinite is indistinguishable in composition from telocollinite or pseudovitrinite, and hence it is suggested that indications of a higher hydrogen content from other sources may reflect incorporation of intimately admixed liptinite in the analysed desmocollinite material.

Acknowledgements Thanks are expressed to the New South Wales Department of Mineral Resources and to Pacific Power for access to the samples used in this study. Rad Flossman and Harold

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Read are thanked for assistance with sample preparation. Appreciation is also expressed to Maria Mastalerz, of Indiana University, for provision of anthracite reference standards and advice on microprobe analysis techniques, and to Marc Bustin and Paul Robert for constructive comment on the manuscript. Sincere thanks are due to Fred Scott and Barry Searle, of the Electron Microscope Unit at the University of New South Wales, for assistance with operational aspects of the microprobe facility. The project was funded in part by the Small Grants scheme of the Australian Research Council.

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