Mineralogical composition of Late Permian coal seams in the Songzao Coalfield, southwestern China

International Journal of Coal Geology 116–117 (2013) 208–226

Contents lists available at ScienceDirect

International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo

Mineralogical composition of Late Permian coal seams in the Songzao Coalfield, southwestern China Lei Zhao a, b,⁎, Colin R. Ward b, David French c, Ian T. Graham b a b c

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, PR China School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney 2052, Australia CSIRO Energy Technology, PO Box 52, North Ryde 1670, Australia

a r t i c l e

i n f o

Article history: Received 18 December 2012 Received in revised form 17 January 2013 Accepted 18 January 2013 Available online 25 January 2013 Keywords: Mineral matter Coal Tonstein K-bentonite Late Permian SW China

a b s t r a c t Coals from three seam sections in the Songzao Coalfield, SW China, are mainly high-ash, high-sulphur semianthracites. Minerals within the Songzao coals are mainly kaolinite, pyrite (or marcasite in some cases), and quartz, with various proportions of non-kaolinite clay minerals, carbonates, feldspars, and anatase. The illite and mixed-layer illite/smectite (I/S) are Na-rich in some of the Datong coal samples. The I/S in the lower coals of the Datong section is most likely an alteration product of dispersed volcanic ash, due to the availability of necessary ions (e.g. K, Na, and Mg) in the marine-influenced coal swamp. Organically-bound Na, which was expelled from the organic matter with coal rank advance, especially with anthracitization, may have supplied additional Na for the formation of Na-rich illite. Authigenic I/S also occurs in a Tonghua coal ply that is overlain by a mafic bentonite and underlain by an alkali tonstein. Potassium, Na, and Mg for the formation of such I/S were probably derived from the leaching of the adjacent alkali tonstein and mafic bentonite. Although the marine water was also a possible supplier of the alkali elements, authigenic I/S is rare in coal plies that occur further away from the altered volcanic layer. Leaching of the volcanic claystones in the Tonghua coal seam probably led to the formation of relatively abundant anatase and rhabdophane in the underlying coal ply. Fracture-filling REE minerals (probably REE-hydroxides or oxyhydroxides) also occurring in that coal ply crystallized from ascending REE-rich hydrothermal fluids, probably associated with contemporaneous volcanic activity. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Knowledge of the mineral matter in coal is of great significance in solving industrial problems, including difficulties associated with materials handling, boiler erosion, ash formation, and slagging, in coal processing or utilisation (e.g. Gupta et al., 1999; Ward, 1984). Knowledge of the mineral matter is also important in understanding the inorganic processes associated with coal formation (e.g. Finkelman, 1994; Ward, 2002), and thus provides important information about the depositional conditions and the geological history of coal-bearing sequences, along with the regional sedimentary and tectonic history (e.g. Ren, 1996; Ward, 2002). Intra-seam clayrock layers with a volcanic ash origin are common in coal-bearing sequences, and have been described as tonsteins or bentonites in the literature, such as the many publications referred to herein. Leaching of tonsteins or the precursor volcanic ash by ground waters and organic acids in the peat-forming environment would be expected to result in enrichment of some elements that were released from the ⁎ Corresponding author at: State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, PR China. Tel.: +86 1062341868. E-mail address: [email protected] (L. Zhao). 0166-5162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2013.01.008

ash and accumulated as minerals in the coal (e.g. Crowley et al., 1989; Hower et al., 1999). The occurrence of such minerals may therefore be indicative of volcanic influence during peat accumulation and coal formation. Although the actual definition of tonsteins is still regarded in some cases as controversial, Spears (2012), among others, define tonsteins as “thin, widespread clay-altered layers of volcanic ash, dominated by kaolinite, that are commonly found in coals and associated sediments”. Following Lyons et al. (1994) and Spears (2012), claystones of volcanic ash origin with a kaolinite content greater than 50% are regarded as tonsteins in this paper. Likewise, the claystones are referred to as bentonites and K-bentonites, respectively, when smectite or mixed-layer illite/smectite (I/S) exceeds 50% of the clay mineral assemblage. Altered volcanic claystones or tuffs are widespread in the Permian strata of SW China (e.g. Dai et al., 2011; Wang et al., 2012; Zhou and Ren, 1994; Zhou et al., 2000). Although the geochemistry of many Late Permian coal tonsteins throughout the world indicates an origin from silicic to intermediate volcanic ash fallout (e.g. Kramer et al., 2001; Spears and Kanaris-Sotiriou, 1979; Zhao et al., 2012; Zhou et al., 1982; Zielinski, 1985), alkali tonsteins that developed in the early part of the Late Permian in SW China have also been reported (Zhou, 1999; Zhou and Ren, 1994; Zhou et al., 2000). Dai et al. (2011) recently distinguished three types of tonstein bands (silicic, mafic, and alkali) in the

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

Songzao Coalfield, SW China, based on the distinctive chemical compositions of the materials. This study discusses the modes of occurrence of the mineral matter and trace elements in the coal and associated non-coal strata from three individual seam sections in the Songzao Coalfield. The main purpose of the study was to investigate more fully the mineral assemblages in the volcanic-influenced coal seams in the Songzao Coalfield. The study was also expected to provide an opportunity to evaluate the geological factors responsible for the mineralogical characteristics of the coal seams, especially the relationship between the coals and intra-seam volcanic claystone bands.

2. Geological setting The Songzao Coalfield is located in Qijiang County, SW Chongqing (Fig. 1). It is 39.5-km long (S–N) and 1.1-11-km (E–W) wide, with a total area of 140.8 km2. It includes eight mines, the Datong, Yuyang, Shihao, Songzao, Tonghua, Fengchun, Zhangshiba, and Liyuanba mines (Fig. 1). The coalfield is located on the northwestern flanks of the Jiudianya, Jiulongshan, and Sangmuchang anticlines. The main structures within the Songzao Coalfield are three folds (Fig. 1). Faults are minor, and only those associated with the main folds affect mining conditions. The coal-bearing sequence in the Songzao Coalfield is the Late Permian Longtan Formation, consisting (from base to top) of limestone, sandstone, silty mudstone, mudstone, coal seams, and tuffaceous sediments (Fig. 2). This unit was deposited in a tidal flat system along the western margin of an epicontinental sea basin (e.g. Dai et al., 2010). The Kangdian Oldland in the west was the major sediment source for the coalfield (China Coal Geology Bureau, 1996). The coal-bearing sequence has an average thickness of 71.8 m. The strata contain 6–11 coal seams, among which the No. 8 coal is workable through the entire coalfield and the

209

Nos. 6, 7, 8, 11, and 12 are locally workable. The Longtan Formation is disconformably underlain by the Maokou Formation, which is a limestone unit of Early Permian age. The Longtan Formation is conformably overlain by the Late Permian Changxing Formation, which is composed mainly of thick layers of limestones intercalated with thin layers of mudstones and rich in marine fossils.

3. Sampling and analytical techniques A total of 24 coal and non-coal samples (channel samples) were collected from three seam sections taken at the underground working faces of three operating coal mines in the Songzao Coalfield, namely, the Datong (No. 7 coal), Tonghua (No. k2b coal), and Yuyang (No. 11 coal) mines (Fig. 1). The individual samples from these sections were differentiated from each other on the basis of their megascopic lithology (Fig. 3). Epoxy-mounted block samples were made from chips representing each coal and non-coal sample for petrographic and/or electron microscope/microprobe analyses. Each sample was also ground to fine powder (about 200 mesh) using a zirconia mill, and split into representative portions for further analyses. Proximate analysis was carried out for the Songzao coals at SGS Australia Pty Ltd. Forms of sulphur were analysed for selected coal samples. This was partly to investigate the forms of sulphur in the coals, and partly to cross check the mineralogical data, especially the percentages of pyrite and sulphate minerals. Low-temperature oxygen-plasma ashing was carried out on the powdered coal and non-coal samples. The resultant low-temperature ashes (LTAs) were subjected to X-ray diffraction (XRD) analysis, using a Philips PW 1830 diffractometer system with Cu-Kα radiation and a graphite monochrometer, and with a tube voltage of 40 kV and current

Fig. 1. Locality map of the Songzao Coalfield, China, with indication of mining areas and main structures (modified from unpublished Songzao Coalfield data).

210

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

non-coal sample was analysed using the Rietveld-based Siroquant™ software package (Taylor, 1991), to obtain the quantitative mineral proportions. Oriented aggregates of the clay (b2 μm) fraction of the coal LTAs and the non-coal rocks were also analysed by XRD, as described by Ruan and Ward (2002), to obtain a better understanding of the clay mineralogy, especially the nature of the expandable clay minerals. Mean maximum vitrinite reflectance was measured on the polished surfaces of epoxy-impregnated block samples in oil immersion, according to Standards Australia (2000). Polished thin-sections of the intra-seam claystone and other non-coal rock samples were subjected to petrographic analysis using a Leica DM 2500P polarizing microscope equipped with Leica LAS digital imaging software. Selected coal and rock polished sections were also studied using Hitachi 3400-X and Hitachi 3400-I scanning electron microscopes. The accelerating voltage was 15 kV, and the beam current was within 40–60 mA during SEM operation. Electron microprobe analysis was applied to a polished block sample of a Songzao coal to obtain precise quantitative chemical analysis (particularly for rare earth elements, REE) of REE-bearing mineral veins occurring in that coal. A JEOL JXA-8600 Super-probe, fitted with wavelength-dispersive spectrometry (WDS), was used at the School of Natural Sciences, University of Western Sydney. The microprobe was operated at 15 kV and 15 nA. Standards employed were CaWO4 for Ca, LaB6 and CeO2 for La and Ce, and pure metals for the other REEs. A Renishaw Raman spectrophotometer was used at the University of Sydney, to obtain laser Raman spectra from the REE-bearing minerals. Major element concentrations in the coal and rock samples were determined by X-ray fluorescence techniques. All the coal samples were ashed at 815 °C. Along with the powdered associated non-coal rocks, the ashes were then fused into borosilicate glass discs (Norrish and Hutton, 1969). The borosilicate discs were analysed using a PANalytical (formerly Philips) PW2400 XRF spectrometer, equipped with a wavelength dispersive (WD) detection system. SuperQ software was used to determine the concentrations of major elements present.

Fig. 2. Sedimentary sequences of the Songzao Coalfield, showing the location of the coal seams (modified from unpublished Songzao Coalfield data). Numbers in brackets are equivalent nomenclature among different mines.

4. Results and discussion

of 30 mA. Each XRD pattern was recorded over a 2θ interval of 2–60°, with a step size of 0.04° and a count time of 2 s per step. The XRD pattern obtained from each low-temperature ash (LTA) and each

Proximate analysis and total sulphur data for the individual coal plies, and forms of sulphur of selected coals from the Tonghua, Datong, and Yuyang sections, are listed in Table 1.

4.1. Coal quality and chemistry

Fig. 3. Lithologic columns of coal seams in the Songzao Coalfield: No. 7 coal at the Datong Coal Mine (left), No. K2b coal at the Tonghua Coal Mine (centre) and No. 11 coal at the Yuyang Coal Mine (right).

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

carbonate minerals, and the high volatile yields may be associated with the loss of carbon dioxide from the carbonates during the analysis process. The sulphur in the Songzao coals is mainly pyritic, consistent with results for other coals from the Songzao Coalfield (Dai et al., 2007, 2010). The total sulphur contents in the Datong section increase upwards. Sulphur varies considerably, from 1.64% to 13.39%, and is generally higher in the Yuyang section. With the exception of one coal ply (th-k2b-4), total sulphur in the Tonghua section is relatively low. Based on the volatile matter and fixed carbon percentages, the Songzao coals are mainly classified as semianthracite under the ASTM classification (ASTM, 2012).

Table 1 Proximate analysis, forms of sulphur (selected samples), and mean maximum vitrinite reflectance value of the coal samples from three Songzao seam sections (%, unless indicated). Sample No. 7 coal dt-7-1 dt-7-2 dt-7-3 dt-7-4 dt-7-5

Thickness Mad Ashd VMdaf FCdaf TSd (cm) at Datong mine 15 2.7 22 2.1 12 1.9 24 2.1 20 1.9

SSd

PSd

OSd

Rv, max

20.6 18.2 19.4 23.2 35.4

10.0 10.3 9.4 8.0 9.5

90.0 89.7 90.6 92.0 90.5

6.73 4.89 4.62 3.73 3.21

0.78 – 0.16 – –

5.0 – 4.1 – –

0.95 – 0.36 – –

2.22 2.17 2.36 2.33 2.29

No. K2b coal at Tonghua mine th-k2b-1 30 2.0 32.3 th-k2b-2 7 2.1 43.7 th-k2b-4 11 1.6 36.6 th-k2b-6 6 1.7 36.6

8.1 6.9 11.1 10.7

91.9 93.1 88.1 89.3

2.97 0.78 10.1 1.22

– – – –

– – – –

– – – –

2.40 2.42 2.30 2.42

No. 11 coal at Yuyang yy-11-1 10 yy-11-2 7 yy-11-3 7 yy-11-4 14 yy-11-5 8 yy-11-7 11

7.0 8.9 10.0 9.5 7.7 8.4

93.0 91.2 90.0 90.5 92.3 91.6

8.11 8.13 13.39 10.08 3.03 1.64

– – 0.51 0.61 0.21 –

– – 11.0 8.5 2.4 –

– – 1.88 0.98 0.42 –

2.09 2.25 2.31 2.40 2.28 2.25

mine 1.4 1.4 1.4 1.4 2.0 1.6

35.9 24.1 24.2 23.6 41.3 31.5

211

4.2. Mineralogical and ash chemical data The proportion of minerals in each coal LTA and each non-coal sample from each seam section, as well as the LTA percentages of the coals, are given in Table 2. Data on clay mineralogy was obtained from the b 2 μm fractions of all the coal LTA and non-coal samples (Table 3), and the results are expressed as graphic profiles in Fig. 4. The relation between ash chemistry and mineralogy was studied to check on the reliability of the quantitative XRD data, following the calculation procedure described by Ward et al. (1999). The inferred chemical composition of the mineral assemblage in the LTAs and rock samples determined by Siroquant was calculated and compared with the actual ash composition of the 815 °C coal ash and rocks as determined by XRF (Table 4). The chemical composition inferred from the XRD data was modified for each sample by deducting the CO2 and H2O + to derive an equivalent to an ash analysis. The results, and also the actual chemical composition determined by XRF, were normalized to an SO3-free basis, to allow for any differences in sulphur retention in the two different types of materials.

ad, air-dried basis; d, dry basis; daf, dry, ash-free basis; M, inherent moisture; FC, fixed carbon; TS, total sulphur; SS, sulphate sulphur; PS, pyritic sulphur; OS, organic sulphur; Rv,max, mean maximum vitrinite reflectance; –, no data.

The percentage of volatile matter is similar in the three sections, in the range of 6.9 to 11.1% (dry, ash-free basis). High volatile percentages, however, occur in some of the Tonghua samples (th-k2b-4, th-k2b-6). As shown below, these samples contain relatively high proportions of

Table 2 Mineralogy of the Songzao LTA and associated non-coal samples from three Songzao seam sections by XRD and Siroquant (wt.%). Sample

Kao

I/S

I

Alb

No. 7 coal at Datong mine dt-7-0 95.7 4.5 dt-7-1 28.5 6.4 dt-7-2 24.2 1.9 dt-7-3 25.4 2.1 dt-7-4 31.6 5.3 dt-7-5 47.6 19.9 dt-7-6 98.4 1.0

LTA

Qtz

34.0 37.2 54.9 46.9 50.8 35.1 22.5

19.9 10.2

16.7

3.2

No. K2b coal th-k2b-0 th-k2b-1 th-k2b-2 th-k2b-3 th-k2b-4 th-k2b-5 th-k2b-6 th-k2b-7

2.1 24.2 48.5 8.0 16.9 45.1 31.7 14.9

5.4 8.4 9.6 22.7 23.7 62.3 78.4 48.1 18.0

at Tonghua mine 91.8 18.3 45.0 15.8 54.6 8.2 95.0 0.5 47.3 5.7 95.7 2.7 47.6 14.8 98.0

No. 11 coal at Yuyang yy-11-0 95.0 yy-11-1 43.6 yy-11-2 31.5 yy-11-3 35.3 yy-11-4 32.8 yy-11-5 49.8 yy-11-6 90.4 yy-11-7 37.7 yy-11-8 96.5

mine 24.9 56.9 37.2 15.0 11.4 10.7 1.2 22.3 5.7

13.8

7.8 24.2 16.2 31.7

0.8

25.8 33.9 20.4 62.7 21.9 32.1 14.9 56.7

27.3

2.5

4.5 33.1

Cha

Py

Mar

18.3 40.4 21.3 20.9 14.4 8.4 6.4

Cal

0.7 10.3 4.2 1.8 10.4

6.0

2.1 1.8

2.8

23.0 5.0 2.9 2.4 3.0

24.9

0.5 0.3

4.6

12.0

2.4

0.7

2.1

1.1 30.1 39.3 53.0 51.5 4.3

2.6

12.0 3.3

5.0 11.9 17.7 3.0 13.0

2.1 4.8

20.0

4.2

39.4 1.6 5.7

22.0

5.7

1.9 12.5 15.6 6.9 40.8

2.2

4.4

1.1

Dol

Ank

1.8 0.6 1.0

3.5

17.4 14.2

1.9 1.9 1.3 4.4

0.3 1.6 1.1

3.2 26.2

4.2

3.4 0.4

Sid

Ana

0.8

2.6 1.7 1.4 0.6 1.6 1.2 5.4

Rut

1,0 2.0 3.3 4.0 0.9 2.5 0.5 4.2

0.8 0.5 0.7 0.7 0.4 0.5 1.5 0.3 2.8

Jar

Bass

Gp

2.9 2.9 0.4 0.1

0.7 5.4 2.1 1.8 1.8

1.4

Sz

2.4 0.8 0.9

1.2

1.0

0.7

0.6

3.7

0.8 2.0 1.6 0.9

1.9 2.9 2.9 1.1

0.6

1.9 1.9 1.7 1.5

2.1

Qtz, quartz; Kao, kaolinite; I/S, mixed-layer illite/smectite; I, illite; Alb, albite; Cha, chamosite; Py, pyrite; Mar, marcasite; Cal, calcite; Dol, dolomite; Ank, ankerite; Sid, siderite; Ana, anatase; Rut, rutile; Jar, jarosite; Bass, bassanite; Gp, gypsum; Sz, szomolnokite.

212

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

Table 3 Mineralogy of b2 μm fraction of coal LTAs and non-coal strata from three Songzao seam sections using oriented-aggregate XRD techniques (wt.%). No. 7 coal at Datong mine dt-7-0 Kaolinite Illite Expandable clays

71 7 23

dt-7-1 86 0 14

dt-7-2 100 0 0

dt-7-3 86 0 14

dt-7-4 78 4 18

dt-7-5

dt-7-6

80 7 12

69 26 5

th-k2b-5

th-k2b-6

th-k2b-7

66 10 25

90 0 10

40 35 26

No. K2b coal at Tonghua mine th-k2b-0 Kaolinite(+ Chlorite) Illite Expandable clays

11 25 64

th-k2b-1 76 1 22

th-k2b-2 89 0 10

th-k2b-3 22 28 50

th-k2b-4 78 0 22

No. 11 coal at Yuyang mine

Kaolinite Illite Expandable clays

yy-11-0

yy-11-1

yy-11-2

yy-11-3

yy-11-4

yy-11-5

yy-11-6

yy-11-7

yy-11-8

27 45 28

100 0 0

100 0 0

100 0 0

100 0 0

93 0 7

88 0 12

93 0 7

41 26 33

The percentages of each major oxide indicated by both sets of data were plotted against each other (Fig. 5), to provide a basis for comparing the XRD results to the chemical analysis data for the same coal or non-coal samples. As discussed for other materials by Ward et al. (1999), the respective data sets are presented as X–Y plots, with a diagonal line on each plot indicating where the points would fall if the estimates from the two different techniques were equal. 4.2.1. Al2O3 and SiO2 The plot for Al2O3 shows a high degree of correlation between the Al2O3 proportions from the Siroquant data and those indicated by the ash chemistry (Fig. 5). The SiO2 plot, however, has the majority of points falling above the diagonal equality line, indicating a slight overestimation of the SiO2 inferred from Siroquant. This appears to be accompanied by an underestimation of Fe2O3, which is also a dominant oxide in most of the Songzao coal samples. 4.2.2. Fe2O3, CaO and MgO A relatively strong correlation is observed for the Fe2O3 plot, but with most points plotting slightly below the equality line. This indicates an underestimation of Fe2O3 inferred from Siroquant, relative to the observed values. MgO values in the majority of samples, all of which have observed MgO values lower than 1.5%, also appear to be underestimated by Siroquant. A fair degree of scatter is observed in

the CaO plot, especially where high percentages of CaO are indicated. As indicated by EDS data, calcite, dolomite, and ankerite, in most cases, show chemical variation due to element substitution (e.g. Fe and/or Mn for Mg in the dolomite), which was not allowed for in the stoichiometric calculations used in this part of the study. 4.2.3. TiO2 Comparison of TiO2 data from the two sources shows a relatively good level of agreement, but with all the points falling below the equality line to a certain degree. It appears, therefore, that the proportions of anatase in most samples were slightly underestimated. As mentioned below, fine-grained Ti-phases (mostly b 0.5 μm in diameter), were observed in the Songzao claystones. With possibly poor crystallinity, such fine phases may not have been detected by the XRD and Siroquant techniques. Ti was also detected by EDS in the kaolinite in some of the coal samples, and this was also not allowed for in the stoichiometric calculations. 4.2.4. K2O and Na2O The plots for K2O and Na2O show relatively scattered correlations. However, with the exception of a few points (circled) that are farthest away from their respective equality lines, both plots are generally parallel to the equality lines. It appears an overestimation of K2O in most samples is coupled with an underestimation of Na2O by Siroquant. As

Fig. 4. Column section showing vertical variations in clay mineralogy for the three seam sections, Datong (left), Tonghua (centre) and Yuyang (right), in the Songzao Coalfield.

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

213

Table 4 Major element analyses of Songzao coal ash and non-coal samples from three Songzao seam sections (%), as determined by XRF analysis. SiO2

Al2O3

TiO2

Fe2O3

MgO

CaO

Na2O

K2O

MnO

P2O5

45.27 33.17 27.17 35.26 43.43 45.68 48.19

30.47 22.75 20.71 24.97 29.12 20.49 36.03

4.68 1.45 1.00 1.27 2.89 1.58 6.86

11.08 35.11 28.92 25.33 17.00 11.70 1.62

0.80 0.30 0.44 0.57 0.55 0.86 0.41

0.61 2.09 9.09 5.48 2.22 9.06 0.24

1.07 0.38 0.21 0.26 0.49 0.29 1.94

1.28 0.35 0.29 0.58 1.09 0.94 1.38

0.181 0.028 0.056 0.038 0.018 0.040 0.011

0.277 0.230 0.147 0.054 0.063 0.073 0.077

0.48 2.37 9.91 5.04 2.12 5.66 0.22

3.12 1.84 3.08 2.34 1.75 2.68 3.54

No. K2b coal at Tonghua mine th-k2b-0 77.59 53.27 th-k2b-1 33.35 46.72 th-k2b-2 44.35 51.97 th-k2b-3 84.26 46.94 th-k2b-4 38.42 21.62 th-k2b-5 82.3 51.60 th-k2b-6 38.32 35.65 th-k2b-7 88.8 47.50

20.79 22.93 34.26 33.89 12.62 37.65 18.77 35.39

2.31 1.53 4.03 4.86 0.84 1.75 1.14 5.95

14.43 12.58 2.25 4.95 35.93 1.07 18.91 2.70

0.92 2.15 0.57 0.63 3.81 0.66 4.47 0.74

0.83 5.42 1.69 0.30 8.75 0.26 9.66 0.27

1.17 0.54 0.65 2.39 0.38 1.46 0.21 2.46

2.53 1.03 0.89 1.54 0.46 1.67 0.45 1.53

0.064 0.061 0.014 0.030 0.085 0.008 0.105 0.016

0.147 0.111 0.127 0.061 0.103 0.038 0.176 0.056

0.71 4.25 1.35 0.30 10.66 0.06 6.60 0.22

2.19 1.9 1.64 4.19 3.18 4.43 1.93 3.17

No. 11 coal at Yuyang mine yy-11-0 85.14 yy-11-1 35.83 yy-11-2 24.21 yy-11-3 23.14 yy-11-4 28.63 yy-11-5 41.77 yy-11-6 78.12 yy-11-7 32.14 yy-11-8 85.27

23.84 6.43 8.64 8.86 13.69 32.45 40.72 24.92 30.01

2.16 1.04 1.17 0.73 0.95 1.42 1.82 0.75 3.44

6.18 29.47 37.89 58.28 48.58 8.46 0.47 7.05 5.64

1.04 0.07 0.24 0.39 0.53 0.71 0.46 1.31 1.12

0.33 0.35 2.53 3.96 3.36 2.28 0.25 6.75 0.37

1.48 0.04 0.09 0.08 0.05 0.45 0.74 0.29 0.87

2.89 0.11 0.17 0.10 0.21 0.80 1.04 0.49 3.46

0.034 0.030 0.019 0.027 0.033 0.025 0.010 0.064 0.028

0.132 0.034 0.028 0.041 0.065 0.064 0.023 0.156 0.086

0.20 0.36 2.67 4.24 3.59 1.20 0.04 3.02 0.21

2.29 0.55 1.01 nd nd 2.04 2.77 2.49 4.26

Sample

HTA

No. 7 coal at Datong mine dt-7-0 81.59 dt-7-1 20.93 dt-7-2 19.45 dt-7-3 20.18 dt-7-4 23.49 dt-7-5 37.23 dt-7-6 87.3

59.16 60.27 45.27 21.81 26.91 50.31 53.28 52.21 50.47

noted below, EDS data show the common presence of Na-rich illite and I/S, rather than regular K-illite and I/S. This indicates an overall low saturation of K and higher saturation of Na than allowed for in the stoichiometric calculations. The calculated K2O and Na2O values from the Siroquant data, however, are based only on regular K-illite and I/S. Lesser degrees of K saturation in naturally-occurring I/S was also suggested to explain similar differences in the studies of Ruan and Ward (2002) and Permana et al. (2010). Generally, with the exception of some variation in the chemical compositions of non-kaolinite clay and carbonate minerals, the XRD and Siroquant results are comparable with the observed ash chemistry determined by XRF. 4.3. Minerals in roof and floor strata The roof and floor strata of the seam sections are mainly carbonaceous shale. The dominant minerals in the roof and floor strata are generally I/S, illite, and kaolinite, with quartz, albite, and anatase as minor components. Pyrite is present as a minor mineral in most of these clastic materials, but is relatively abundant in the roof samples of the Datong and Tonghua seam sections (Table 2). The clay mineralogy of the roof and floor samples in the Tonghua and Yuyang sections is dominated by illite and expandable clay minerals (Table 3; Fig. 4). The roof and floor samples in the Datong section contain slightly lesser proportions of kaolinite than the LTA of the coal samples. XRD patterns indicate that the kaolinite in all the roof and floor samples has a poorly-ordered structure. The expandable clays in the roof and floor samples are mainly I/S, which appears to be regularly interstratified. This is indicated by peaks at around 28 Å in the powder XRD traces, and at around 30 Å in the glycol saturated material. 4.4. Minerals in volcanic claystones XRD analysis indicates that the mineralogy of the three non-coal (claystone) partings of the Tonghua and Yuyang sections is dominated by clay minerals. The non-clay minerals pyrite, quartz, albite, and anatase are only present in trace proportions (Table 2). Under the optical

SO3

LOI

microscope, the thick claystone band, th-k2b-3, shows elongated pellets consisting mainly of cryptocrystalline to microcrystalline clay minerals (Fig. 6A). However, the thin claystones, th-k2b-5 and yy-11-6, show a more dense texture, with the clay minerals largely being fine-grained (Fig. 6B). Accessory minerals include angular-shaped quartz (Fig. 6C) and chloritised biotite (Fig. 6D). The occurrence of these minerals, along with the lateral continuity of the claystones within the coalfield, are thought to indicate a volcanic origin. The difference between the microscopic appearance of the thick (th-k2b-3) and thin claystones (th-k2b-5 and yy-11-6) may be related to the original volcanic materials and the leaching efficiency of the claystone-forming environment. Dense tonsteins (or bentonites) may result from more complete in situ decomposition (Diessel, 1965). The thin claystones, which may have been more efficiently leached, would therefore tend to have a relatively dense appearance. 4.4.1. Kaolinite Kaolinite makes up 22% to 89% of the clay mineral assemblage in the claystones (Table 3). The mineral also appears to be more abundant in the thin claystones than in the thicker claystones. X-ray diffraction patterns show that the kaolinite in all the claystones has a poorly-ordered structure. Kaolinite of vermicular (Fig. 7A) and tabular (Fig. 7B) forms is rarely present. This contrasts to tonsteins associated with bituminous coal from the Great Northern seam in the Sydney Basin, Australia, which contain well-ordered kaolinite (Zhao et al., 2012). The reason for the contrast is unclear at this stage. The presence of poorly-ordered kaolinite in the claystones also contrasts with the presence of well-ordered kaolinite in most of the coal samples, as indicated below. This may indicate different processes of kaolinite formation in the coals and the associated clay partings, with a greater potential for pore-filling precipitation in the coal beds than in the parting materials. This may also indicate that kaolinite tends to have decreased crystallinity in claystones associated with higher-rank coals. 4.4.2. I/S and illite XRD analysis of the b 2 μm fractions of the claystone samples shows relatively more abundant I/S and illite in the clay mineral assemblage

80 70 R² = 0.914 60 50 40 30 20 10 0 0 20

40

60

80

Al 2O3 from Siroquant (%)

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

SiO 2 from Siroquant (%)

214

50 40

R² = 0.9413

30 20 10 0 0

10

70 60

R² = 0.9466

50 40 30 20 10

0 0

10

20

30

40

50

60

70

8 7 R² = 0.834 6 5 4 3 2 1 0 0 2

R² = 0.8465

8 6 4 2 2

4

6

8

10

12

MgO from Siroquant (%)

CaO from Siroquant (%)

12

0 0

3 2 1 1

2

3

4

5

K2O by XRF (%)

Na2 O from Siroquant (%)

K2 O From Siroquant (%)

R² = 0.6954

0

50

4

6

8

5

R² = 0.9497

4 3 2 1 0 0

1

2

3

4

5

6

MgO by XRF (%)

5

0

40

6

CaO by XRF (%)

4

30

TiO 2 by XRF (%)

Fe 2O3 by XRF (%)

10

20

Al 2 O 3 by XRF (%) TiO 2 from Siroquant (%)

Fe2O3 from Siroquant (%)

SiO 2 by XRF (%)

3

R² = 0.8088 2

1

0 0

1

2

3

Na 2O by XRF (%)

Fig. 5. Comparison between proportions of major element oxides in coal ashes and non-coal strata from three seam sections in the Songzao Coalfield, inferred from Siroquant and determined by XRF. The diagonal line represents equality in each plot. Circled points are discussed in the text. Relevant trendlines and squared correlation coefficients (R2), obtained from linear regression analysis, are also shown in each case.

than in that of the adjacent coal LTA samples (Table 3; Fig. 4). The thick claystone, th-k2b-3, appears to contain more I/S and less kaolinite than the thin claystones. According to Spears (2012), volcanic ash layers that have I/S exceeding 50% of the clay mineral assemblage are more appropriately referred to as K-bentonites rather than tonsteins. The th-k2b-3 claystone is therefore referred to as a K-bentonite in the present study, while the other two claystones, in which kaolinite makes up >50% of the clay mineral assemblage, are regarded as tonsteins. The XRD patterns also show that the I/S in these claystones is regularly interstratified, indicated by the peaks at around 30, 13 and 9.2 Å in the glycol saturated oriented-aggregate XRD patterns. EDS data show that the I/S in the Songzao claystones typically has Na, rather than K, as the predominant cation (Fig. 8H). Bentonites, smectite-rich altered volcanic ashes, are frequently present in Mesozoic marine sediments, and were probably formed when the relevant ions were sufficiently available. K-bentonites and

metatonsteins (illite-rich claystones) have also been reported in coal seams (e.g. Burger et al., 1990). Altaner et al. (1984) suggested that the I/S in K-bentonite was produced by the interaction of smectite in the original bentonite and K+ ions in the pore fluids, with the K+ being derived from breakdown of K-bearing minerals (e.g. micas and K-feldspar) in the host rock. In such a case, the I/S in the K-bentonites could be an intermediate product of the conversion of smectite to illite during burial diagenesis (e.g. Spears, 2012). The I/S in K-bentonite is typically regularly interstratified, with various percentages of illite and smectite layers (e.g. Altaner et al., 1984; Pevear et al., 1980; Spears, 1971), although randomly interstratified I/S has also been reported (e.g. Huff et al., 1998). The I/S in typical K-bentonite that formed in a marine environment, however, is mainly K-I/S. This is probably dependant on the availability of cations for the I/S formation. Relatively high contents of K in K-bentonite probably reflect both seawater and parent material composition at the time of formation (Huff and

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

215

200 µm

A

200 µm

B

50 µm

C

50 µm

D

Fig. 6. Thin-section photomicrographs of claystone samples. (A) Elongated pellets in sample th-k2b-3, plane polarised light (PPL); (B) A homogeneous texture shown in sample yy-11-6, PPL; (C) Angular volcanic quartz with in lower part of image, yy-11-6, PPL; (D) Chloritised biotite in sample th-k2b-3, cross polarised light (XPL).

Tuerkmenoglu, 1981). The Na in the Na-rich I/S of the Songzao claystones may have been derived from a relevant source during burial diagenesis. Sodium was probably released as a non-mineral inorganic component from the organic matter during the coal rank advance, especially during anthracitization. Another possible mechanism for the formation of illite involves the illitisation of a kaolinite precursor, which may take place in high-rank coal seams. Burger et al. (1990) described a relationship between the clay mineralogy of metatonsteins (which they described as illite tonsteins) and the rank of the adjacent coals. They found that the proportion of illite increases in tonsteins or metatonsteins that are associated with coals having lower volatile matter (VM) percentages (i.e. higher rank), accompanied by chloritization when VM is less than 8%. Susilawati and Ward (2006) described rectorite-like minerals in non-coal rocks associated with heated coals (≥1.3% R) at Bukit Asam,

A

200 µm

Indonesia. They also suggested that the rectorite-like clay may have been altered from a kaolinite precursor. Although the Songzao coals are high in rank, illitisation of kaolinite does not appear to have taken place. This is especially notable in the thin claystones, in which kaolinite is the dominant clay mineral. Given the presence of significant proportions of smectite in the associated coal samples, it is most likely that the volcanic ash was originally converted to smectite in the marine-influenced coal-forming environment. The smectite was in turn converted to I/S and illite during diagenesis, assuming that the necessary ions (e.g. K, Na, and Mg) were available from the marine water percolating through the peat deposit. 4.4.3. Quartz Quartz is a minor constituent in all the claystones, making up 0.5% to 2.7% of the mineral assemblage (Table 2). The proportions of quartz are

K

B

K

Fig. 7. Kaolinite in claystone sample th-k2b-5. (A) Thin-section photomicrograph of vermicular kaolinite (K), PPL; (B) SEM image of kaolinite (K) of tabular structure in an I/S matrix. Also shown are Ti-rich material (bright).

216

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

A

B

C

D

E

F

Pt 1

Pt 2

cps/eV

cps/eV

G

12

H

12

Pt 1

10

Pt 2 10

O

8

Ti

8

Na Al 6

6

4

4

2

2

0

0 1

2

3

4

5

6

7

8

9

10

KeV

1

Si

K

2

3

4

5

KeV

Fig. 8. SEM images showing the modes of occurrence of anatase in claystone samples. (A), (B), (C) and (D): Anatase appears to be a replacement of pumice or other volcanogenic components, sample th-k2b-3; (E) and (F): Anatase appears to be a replacement of glass shards and other volcanogenic components, sample yy-11-6; (G) EDS spectrum of point 1; (H) EDS spectrum of point 2.

also much lower than in the LTA of the adjacent coals. The difference is even greater if allowance is made for dilution by the abundant pyrite in the coals. Under the optical and electron microscopes, the quartz is mostly angular, with some elongated particles. Some quartz has fluid inclusions (Fig. 6C), which indicate that the quartz crystallized from a fluid source. The low proportions of quartz in the claystones indicate that the input of epiclastic sediment into the peat swamp during the accumulation of the volcanic ash layers was rare. 4.4.4. Chlorite XRD patterns of the clay fractions of the tonstein/K-bentonites show the presence of a small proportion of chlorite. As noted above, biotite pseudomorphs with chlorite laminae have also been observed (Fig. 6D). Chlorite may form diagenetically from kaolinite in metatonsteins, which have been described as illite tonsteins, in high rank coal seams (e.g. Burger et al., 1990). 4.4.5. Anatase Small proportions of anatase (1.5% to 4%) are present in all the claystones, at concentrations that are generally higher than those in the LTA residues of the adjacent coal samples. Under the SEM, different modes of anatase occurrence were observed in the claystones (Fig. 8).

The mineral largely occurs as discrete particles (with a grain size commonly less than 1 μm) disseminated in the I/S matrix (Triplehorn and Bohor, 1983) (Fig. 8A), and probably as a replacement of glass shards (Fig. 8B). Anatase also appears to be a replacement of pumice or other volcanogenic components (Fig. 8A, C–F); however, this is not yet clearly understood and needs further investigation. Coal tonsteins throughout the world frequently contain anatase (Price and Duff, 1969). Ruppert and Moore (1993) described anatase occurring as a replacement of glass shards and probably glass gas bubbles in an Indonesian tonstein, where it may have been derived from the breakdown of Ti-rich volcanic glass, ilmenite, magnetite, or rutile.

4.4.6. Phosphates REE-Ba-phosphates, probably gorceixite, with a typical grain size of b2 μm, were observed under the SEM in claystone sample th-k2b-5 (Fig. 9), although the gorceixite is below the detection limit for XRD and Siroquant analysis. Such fine-grained material was probably precipitated from the REE-rich leachate of the original volcanic components. Triplehorn and Bohor (1983) reported Ce-bearing goyazite in a kaolinised tuff from Colorado, and suggested that the goyazite was probably precipitated from solution during early diagenesis. REE-rich

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

REE-, BaPhosphates

Fig. 9. SEM image showing REE- and Ba- phosphates, probably gorceixite, in claystone sample th-k2b-5.

Ca phosphate, which may be replacing pumice, was also reported in an Indonesian tonstein by Ruppert and Moore (1993). 4.5. Minerals in coal samples The overall percentage of LTA for the Tonghua coals, which is in the range of 45 to 54.6% (Table 2), is the highest among the three Songzao seam sections. The LTA percentage of the Datong coals is relatively low, and is in the range of 24.2 to 47.6%. Minerals in the LTA residues of the coal samples are mainly kaolinite; pyrite (and marcasite in a few cases); I/S; quartz; and minor proportions of carbonates (calcite, dolomite, and ankerite) ; feldspar (albite); anatase; and secondary sulphate minerals. 4.5.1. Kaolinite In contrast to most of the roof and floor samples, the clay mineralogy of all the coals is dominated by kaolinite (Table 3, Fig. 4), especially in the Yuyang section. With the exception of coal th-k2b-1, the XRD patterns of the kaolinite show a well-ordered structure in all the LTAs, including those from coals near the top and bottom of each seam section. Under the SEM, both detrital and authigenic kaolinite can be recognised. The former occurs as laminae and bands (Fig. 10A), and the latter as infills of cells or crack cavities, and as cleat/ fracture-infillings (Fig. 10A). Multiple stages of kaolinite veining are evident (Fig. 10A), suggesting a number of epigenetic stages. Vermicular kaolinite also occurs (Fig. 10B), a feature which indicates in situ precipitation. As discussed by Ward (1989), the well-ordered kaolinite in the coal appears to be the result of in situ leaching and reprecipitation processes. Although not entirely diagenetic, an increase in the proportion of diagenetic kaolinite would lead to an increase in the overall degree of order in the kaolinite as a whole (e.g. Spears, 2012). The presence of poorly-ordered kaolinite in an Australian bituminous coal affected by igneous intrusions was described by Ward (1989). Poorly-ordered kaolinite also occurs in heat-affected coal from the Bukit Asam deposit, Indonesia, with Rvmax > 1.0% (Susilawati and Ward, 2006). However, among the coal samples of the present study, coal th-k2b-1 is the only coal that has poorly-ordered kaolinite. This may indicate that the well-ordered structure of the kaolinite is more persistent in coals that have been subjected to burial metamorphism than in those subjected to thermal metamorphism, where higher temperatures may have acted for a shorter period of time. 4.5.2. Expandable clay minerals The Songzao coals have significant proportions of expandable clays in the Tonghua and Datong sections, and the lower few samples in the Yuyang section (Table 2). However, oriented-aggregate XRD analysis indicates that the proportion of I/S does not exceed that of kaolinite in

217

any coal sample (Table 3, Fig. 4). The upper few coals in the Yuyang section have kaolinite as the only clay mineral in the clay fractions of the LTA residues. Glycol-saturated oriented-aggregate XRD patterns of the Tonghua coals show a strong reflection at 16.97 to 17.26 Å, which indicates the presence of smectite. However, under the SEM, I/S is more frequently observed. The I/S commonly occurs in cell cavities or pore spaces and in cracks within the macerals (Fig. 10B), as observed in a Tonghua coal (th-k2b-4). This suggests that at least some of the I/S in the Tonghua coal is of authigenic origin. As discussed by Dai et al. (2011), the bentonite (which they also described as tonstein) of sample th-k2b-3 and the tonstein of sample th-k2b-5 are mafic and alkali in composition, respectively. The original alkali volcanic ash was enriched in K and Na, and the mafic volcanic ash was relatively enriched in Mg. Such ions may have been leached from the ash during diagenesis, which may have then led to the precipitation of I/S in the cracks, cell cavities, and pore spaces of the maceral components. Although marine water was also a possible supplier of the alkali elements, authigenic I/S is rare in other coal samples taken further away from the altered volcanic layers. Under the SEM, the expandable clay in the Datong coals occurs as irregular flakes, with the presence of K, Na, Mg, and Fe indicated by the EDS spectrum. Such material resembles the I/S in the claystone partings, and was probably diagenetically altered from the original volcanic ash that was incorporated in the original peat swamp. The ordering of the I/S in the Datong coals, however, cannot be accurately determined, due to the small proportion of I/S present and the poor resolution of the XRD patterns. An increase in the proportion of I/S, accompanied by a decreasing proportion of kaolinite, has been reported in sediments due to thermal metamorphism or burial metamorphism. For example, a significant decrease in kaolinite was noted in the thermally metamorphosed coal from the Bukit Asam deposit, Indonesia, with a vitrinite reflectance >1.41%, and kaolinite disappeared in coals with vitrinite reflectance >2.2%, associated with the appearance of I/S (Susilawati and Ward, 2006). No significant change in the proportions of kaolinite, however, is observed in most of the Songzao coals, although all the coals are of high-rank levels. The most likely precursor for the formation of I/S in coals, especially the Datong coals, is smectite, which is usually pyroclastic in origin. The I/S is probably an intermediate product of the conversion of smectite to illite during burial diagenesis, with the necessary ions (e.g. K, Na, Mg) being available in the marine-influenced coal swamp. The formation of I/S from a kaolinite precursor may be less important. 4.5.3. Illite Oriented-aggregate XRD analysis indicates that small but significant proportions of illite occur in the lower two coal plies of the Datong section (Table 3, Fig. 4). Other coals, including those from the two other sections, have no or trace amounts of illite. SEM data indicate the common presence of Na-rich illite or paragonite, within regular flakes of I/S matrix in the Datong coals (Fig. 10C, D). Paragonite, if present as a separate phase, however, is below the detection limit of the XRD analysis for both the LTA residues and the clay fractions. Daniels and Altaner (1990) suggested smectite as a precursor for Na-bearing illite and paragonite formation in anthracites of eastern Pennsylvania, with the Na being provided from metasomatic hydrothermal fluids. Paragonite is also present in the heat-affected coals at Bukit Asam, Indonesia, where it was suggested to have formed from the reaction of kaolinite with inorganic Na released from the coal's organic matter by igneous intrusions (Susilawati and Ward, 2006). As indicated in the clay mineral profiles (Fig. 4), illite generally coexists with expandable clay minerals when present in the Songzao coals. This may suggest the formation of Na-rich illite in the coals from the alteration of smectite. The extra Na for the formation of Na-rich illite was probably also derived from organically-bound Na which was expelled from the organic matter with coal rank advance.

218

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

B

A K K

Q

K

Py

K

I/S

Py

Q

D

C Na-rich illite

P Pt 2 Pt 1

cps/eV

cps/eV

6

Pt 1

Si

6

Al

5

4

Na 3

3

2

2

1

1

0

1

Al

5

4

2

3

4

5

6

7

8

9

10

keV

0

Pt 2

Si

Na

1

P

2

3

4

5

6

7

8

9

10

keV

Fig. 10. SEM images of clay minerals in coal samples. (A) Detrital kaolinite (K) laminae and cleat- and fracture-filling kaolinite (K). Also shown are syngenetic pyrite (Py) and detrital quartz (Q), sample yy-11-1; (B) Smectite or I/S in pore spaces. Also shown are vermicular kaolinite (K), pyrite (py) and detrital quartz (Q), sample th-k2b-4; (C) Na-rich I/S, or paragonite (see EDS data for pt 1), and phosphates (P) in an I/S matrix. The bright area is also probably phosphates (see EDS data for pt 2); (D) Na-rich I/S, or paragonite, showing regular flakes.

4.5.4. Chamosite XRD analysis shows small proportions of chlorite (b5%) present in some of the Tonghua coal LTA residues. The EDS spectrum of the chlorite shows peaks of Fe and Mg, as well as Al and Si. EDS data indicate that all the chlorite in the Tonghua coals is Fe-rich, which suggests that it is chamosite (e.g. Dai and Chou, 2007). Under the SEM, the chamosite was commonly observed as cell- and cleat/fracture-infillings, in most cases coexisting with kaolinite (Fig. 11A, B, C, D). Dai and Chou (2007) reported chamosite replacement of kaolinite in cell cavities in a semianthracite from the Zhaotong Coalfield, SW China, and suggested that the chamosite was derived from the reaction between kaolinite and Fe-Mg-rich fluids during early diagenesis. Chamosite with a similar mode of occurrence in the No. 12 coal of the Songzao Coalfield was also described by Dai et al. (2010). Chlorite in the thermally metamorphosed Bukit Asam coal was also suggested to have formed by reactions between kaolinite and Fe and Mg ions, which were probably driven from the organic matter with rank advance (Susilawati and Ward, 2006).

The intergrowth texture of chamosite and kaolinite (Fig. 11B, D) in the present study indicates that kaolinite was probably precipitated earlier in the fractures, and is thus the precursor of the chamosite. The formation of the chamosite in the Songzao coal may be the result of reactions between the earlier-precipitated kaolinite and Fe–Mg-bearing fluids during late diagenesis. Coexistence of chamosite, ankerite, and quartz in fractures was also observed in the Songzao coals (Fig. 12C, D). Well-defined contacts among these minerals indicate that they probably formed from different fluid-injection events. Both the chamosite and the ankerite contain fragments of quartz, indicating that quartz was the earliest-formed mineral, followed by chamosite and ankerite. Such chamosite, therefore, has a different origin from that in the intergrowths with kaolinite, and was probably formed epigenetically from fluid reactions at a late diagenetic stage. Chlorites formed from epigenetic processes have been reported in several coals, most of which are of high-rank levels (Dai et al., 2008, 2012a; Faraj et al., 1996).

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

219

B

A 1 K

C

2

D

C C

K

F

E C

C Q Q

A

Fig. 11. SEM images showing modes of occurrence of chamosite (C) in coal sample th-k2b-4. (A) Kaolinite and chamosite occurring in fractures probably formed in different stages. The earlier-formed fractures are parallel to each other, and are confined to a vitrinite band. Note the displacement of the vitrinite band later formed during tectonic deformation; (B) enlargement of rectangle 1 in (A), showing chamosite (C) intergrown with kaolinite (K); (C) enlargement of rectangle 2 in (A), showing fracture-filling chamosite and kaolinite; (D) enlargement of the rectangle in (C), showing chamosite (C) intergrown with kaolinite (K); (E) chamosite (C), ankerite (A) and quartz (Q) in fracture; (F) enlargement of the rectangle in (E) showing chamosite (C) intergrown with quartz (Q).

A

200 µm

50 µm

B

Q

Fig. 12. Photomicrographs of quartz in Songzao coal samples. In air, reflected light. (A) Abundant detrital quartz (Q) grains in sample dt-7-1; (B) euhedral quartz in sample th-k2b-4.

220

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

4.5.5. Quartz Quartz is abundant in most of the coal samples, making up to 57% of the LTA residues (Table 2). The quartz occurs largely as detrital grains (Figs. 10A, 12A). High proportions of detrital quartz in the coals throughout all the seam sections indicate that detrital material introduced by epiclastic processes was important during peat accumulation. On the other hand, poorly-ordered kaolinite, which also represents detrital material introduced by epiclastic processes, is almost absent in most of the coals. Ward (2002) suggested that there may be greater opportunities for alteration of the detrital input, if any, in dominant and widespread peat-forming conditions. This may be the case for the Songzao coals from the main part of each seam, and detrital material other than quartz was not preserved. Authigenic quartz is relatively minor, occurring as euhedral crystals (Fig. 12B), and as cell cavity- and cleat-infillings. Such authigenic quartz was syngenetically precipitated from silica-bearing solutions. Epigenetic quartz also occurs, appearing to be enclosed by ankerite and chamosite in fractures (Fig. 11E, F).

4.5.6. Pyrite/marcasite Pyrite occurs in all the Songzao coal samples in various proportions (3% to 53% of the LTA residues) but is more abundant in the coals from the upper part of each section. Minor marcasite occurs in some samples, with one coal ply (th-k2b-4) having a particularly higher concentration (13.9%). Pyrite has a variety of modes of occurrence in the coals, including isolated or clustered framboids (Fig. 13A), subhedral to euhedral crystals (Fig. 13B), cell cavity-infillings (Fig. 13C), and, to a lesser extent, fracture/cleat-infillings (Fig. 13D). Pyrite also tends to be disseminated in vitrinite as aggregates in bands parallel to stratification. Under the SEM, pyrite framboids appear to have a different form, and can be seen to consist of pyrite microcrystals, generally b0.5 μm in diameter, under high magnification (Fig. 14C). Pyrite, especially the microcrystals, also has a tendency to be embedded in a clay matrix (Fig. 13E). Later-formed pyrite includes overgrowths on earlier-formed subhedral pyrite and massive pyrite on framboids (Fig. 13 F).

B

A

C

E

50 µm

D

F

Fig. 13. Modes of occurrence of pyrite in Songzao coal samples. (A) Clustered and isolated framboidal pyrite, sample yy-11-4, in air, reflected light; (B) Euhedral pyrite crystals, sample yy-11-1, in air, reflected light; (C) Cell-filling pyrite, sample th-k2b-4. In air, reflected light; (D) Cleat-filling pyrite, sample dt-7-1, oil immersion, reflected light; (E) SEM image of framboidal pyrite and isolated pyrite crystals in a clay matrix, sample dt-7-2; (F) SEM image showing framboidal pyrite cemented by later-formed massive pyrite, sample dt-7-2.

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

A

221

50µm

B Py Ma Py

Py

C

20µm

D

XPL Fig. 14. Photomicrographs showing modes of occurrence of pyrite and marcasite in coal samples. (A) Different associations of pyrite and marcasite, sample dt-7-3, in air, plane PPL; (B) association of pyrite (Py) and marcasite (Ma), sample dt-7-3, in air, PPL; (C) SEM image showing pyrite framboids with different density; (D) marcasite with bladed morphology, sample th-k2b-4, in air, XPL.

Marcasite has tabular and bladed crystal habits and also occurs as massive bodies and as a replacement of pyrite (Fig. 14D). Although below the detection limit for XRD and Siroquant analysis, marcasite was observed in coal dt-7-3 under the optical microscope and the SEM. The marcasite occurs as radiating crystals growing on pyrite framboids and coated with a layer of massive pyrite (Fig. 14A, B). Three stages of early diagenesis are shown: pyrite framboids formed in the early syngenetic stage; marcasite precipitated in the middle syngenetic stage; and massive pyrite precipitated in the late syngenetic stage. Spherical grains composed of marcasite without any pyrite nuclei were also observed (Fig. 14A). Similar occurrences and associations between pyrite and marcasite have been reported in other coals (Querol et

al., 1989; Wiese and Fyfe, 1986). Epigenetic pyrite, however, is uncommon in the Songzao coals. 4.5.7. Carbonates XRD analysis indicates that minor proportions of calcite, dolomite, and ankerite occur in the Songzao coal LTAs. Carbonates in coals from the Datong section mainly occur as cleat- and fracture-infillings, but the carbonates in coals of the Tonghua section are more commonly present as cell-infillings (Fig. 15A). The mineral matter of the Tonghua seam contains more abundant Mg-bearing carbonates, dolomite, and ankerite than the other two seams. In some cases, the carbonates show evidence of formation postdating that of euhedral pyrite in

A

B

Dol Py

I/S A

Ca

Fig. 15. SEM images showing modes of occurrence of carbonates in coal sample th-k2b-2. (A) I/S and dolomite (Dol) coexisting in cell cavities. Cell-filling calcite (Ca) is also indicated; (B) ankerite (A) enclosing earlier-formed pyrite (Py).

222

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

A

B

I/S K G

G

Fig. 16. SEM images showing gorceixite in coal sample th-k2b-4. (A) Gorceixite (G) in the matrix of band rich in I/S; (B) gorceixite (G) in a matrix of kaolinite (K) within pores of the organic matter.

inertinite (Fig. 15B). Such carbonates are probably epigenetic, precipitated from hydrothermal fluids. 4.5.8. Phosphates Although occurring at concentrations below the detection limit for XRD analysis and Siroquant interpretation, gorceixite was frequently observed under the SEM in coal sample th-k2b-4 from the Tonghua section. The gorceixite particles are up to 2 μm in diameter, and tend to occur particularly in the matrix of I/S-rich bands or pore-filling kaolinite (Fig. 16A, B). The EDS spectrum shows peaks of Ba and P, as well as Al and Si. As noted above, Sr-, Ba-, Ca-, and REE-aluminophosphate minerals have been reported in tonsteins and/or altered tuff layers (e.g. Triplehorn and Bohor, 1983). The presence of such phosphates has also been frequently recognized in coals, most of which are associated with tonsteins, may have been affected by volcanic ash (Crowley et al., 1989, 1993; Hower et al., 1999), or were derived from an oxidized sediment-source region with bauxite in the weathered crust during peat accumulation (Dai et al., 2012b). In some cases, a P-bearing material is seen to occur between illite flakes in the Datong coal (Fig. 10C, pt 2). The P-bearing material in that coal also occurs as small particles, generally b0.5 μm in diameter, randomly disseminated in the I/S matrix (Fig. 10C). Fine-grained (particle size b2 μm) REE-rich phosphates, tentatively identified as rhabdophane (Seredin and Dai, 2012), were also identified under the SEM in coal sample th-k2b-4. The phosphate occurs in both Na-rich I/S in cracks of the organic matter (Fig. 17A) and in thicker I/S bands (Fig. 17B). Apart from the REE, some of these phosphate particles contain EDS-detectable Ba (Fig. 17A). The authigenic rhabdophane

particles in clay bands or clay-filled cracks in the Tonghua coals may be crystallisation products of REE-rich leachates (probably also Ba-bearing) derived from the overlying tonstein layers. Gorceixite in the Datong coals that does not contain REE was probably crystallised from Ba-rich fluids. The Sr, Ba, and REE in the phosphates found in coal are generally thought to have been leached from overlying tonstein bands or from volcanic components incorporated into the peat (Crowley et al., 1989; Hower et al., 1999; Wang, 2009). The P in the phosphates may have multiple possible sources, for example leaching of the volcanic components or decomposition of plant material (Rao and Walsh, 1999; Ward et al., 1996). 4.5.9. Anatase XRD analysis indicates that small proportions of anatase (up to 3.3% of the LTAs) occur in coals throughout the three sections. Multiple modes of anatase occurrence were observed under the SEM, particularly in coal sample th-k2b-4. Euhedral anatase crystals are associated with kaolinite, which also contains Ti and appears to be parallel to the bedding planes (Fig. 18A). The Ti-bearing kaolinite is not chemically homogeneous, as indicated by the back scattered electron SEM image. The textural relationship between anatase and kaolinite indicates that the euhedral anatase postdates the kaolinite. As suggested by Ward et al. (1999), Ti was possibly precipitated in conjunction with kaolinite as a separate phase or incorporated within the kaolinite structure. There were probably multiple stages of Ti-mineral formation, as the euhedral anatase appears to be later-formed. Anhedral anatase containing small proportions of Al and Si also occurs (Fig. 18B). Coal sample th-k2b-4 contains anatase with

A Ba-bearing REE-phosphate

B

REE-phosphate I/S

I/S

Fig. 17. SEM images showing REE-phosphates in coal sample th-k2b-4. (A) Fine-grained REE-phosphates, some of which probably contain Ba, in the matrix of crack-filling I/S; (B) fine-grained REE-phosphates in the matrix of I/S bands.

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

223

B

A Ti-bearing kaolinite

Py

Anatase

D

C

I/S

F

E

Zr

Fig. 18. SEM images showing modes of occurrence of anatase in coal sample th-k2b-4. (A) Euhedral anatase crystals associated with Ti-bearing kaolinite; (B) Probably an intimate mixture of anatase and kaolinite; (C) flattened circular bodies of anatase possibly replacing glass spherules; (D) enlargement of the rectangle in (C) showing detail of the possible replacement texture; the cavities are filled with I/S; (E) anatase possibly replacing shell or wood fragments; (F) enlargement of the rectangle in (E). The bright, fine-grained particles are probably Nb-bearing zircon (Zr).

unusual morphologies. Although the origin is not very clear, some of the anatase appears to be replacing glass spherules (Fig. 18C, D), and other anatase probably replaces shell or wood fragments (Fig. 18E, F). The bulk of the fragments are also parallel to the bedding planes. Anatase has been generally found in coals that are adjacent to tonstein layers (Dewison, 1989; Ruppert and Moore, 1993), and has been observed replacing glass shard material in an Indonesian tonstein (Ruppert and Moore, 1993). Anatase replacement of maceral components has also been reported in coals (Querol et al., 1989). Dai et al. (2007) noted that some fine-grained anatase is distributed in I/S, and also occurs as a cementing material for pyrite particles in coals from the Songzao Coalfield. 4.5.10. REE-bearing minerals Veins or fracture fillings comprising REE-bearing minerals were identified under the SEM in coal sample th-k2b-4 (Fig. 19). EDS data indicate the presence of two distinct rare earth phases in these veins.

Phase I in Fig. 19B is bright in the backscattered electron image, and shows a pattern of lamellar crystallisation; the EDS spectrum of this phase shows only the peaks for Ce. Phase II, the grey area in the centre of Phase I, contains both EDS-detectable Ce and Ca. A similar association between these two phases is common in all of the REE mineral-filled fractures in this particular coal sample. The concentrations of lanthanide oxides in the veins of these samples were also determined by an electron microprobe equipped with wavelength-dispersive X-ray spectrometry (WDS), due to the lower detection limit and more accurate quantitative analyses than available using SEM and EDS techniques. The results showed that, except for different calcium contents, both phases consist mainly of Nd and Ce with small amounts of La and Y (Table 5). Based on the chemical compositions, two REE-carbonates were considered possible: lanthanite-(Nd) [(Nd,Ce,La)2(CO3)3•8H2O] and kimuraite [Ca(Y,Nd)2(CO3)4•6H2O]. Laser Raman spectroscopy was further undertaken to evaluate the anionic and water molecules in these minerals, as a guide to better

224

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

A

B

Phase II

Phase I

Fig. 19. SEM images of REE-bearing minerals in coal sample th-k2b-4. (A) Fracture-filling REE minerals; (B) Enlargement of the rectangle in (A) showing two different REE minerals in fracture (see EDS data). Grey and bright areas are Ca-rich and Ca-poor, respectively.

identification. No stretching carbonate ion vibration at around 1080 cm−1, however, was observed in the Raman spectra obtained from the minerals, and, thus, the presence of carbonate ions was not confirmed. Although the actual mineral species have yet to be determined, this result ruled out identification of the materials as REE-carbonates, at least for Phase I. The same conclusions probably also apply to Phase II, although this could not be exactly located in the laser Raman analysis due to the small grain size. Based on the above chemical and laser Raman analyses, Phase I is thus thought to represent a REE-hydroxide or oxyhydroxide. Naturally-occurring REE-hydroxides or oxyhydroxides, however, have not been reported in the literature. REE-carbonates, such as kimuraite, are also rare in coal. Only one reference has been found (Seredin, 1998), which mentions lanthanite in Russian coals, but the identification was based solely on the morphology and EDS data, without confirmation from any other analyses. The REE-minerals in the Tonghua sample were probably crystallized from ascending hydrothermal fluids carrying high REE concentrations. The REE-minerals appear to have formed earlier than the fracturefilling kaolinite, although both were precipitated from epigenetic fluids during a late stage of diagenesis. The REE-bearing fluid in the Tonghua coals may have been associated with contemporaneous volcanic activity.

4.5.11. Sulphates The iron sulphate minerals jarosite and szomolnokite, found in the samples, are secondary minerals derived from oxidation of pyrite

Table 5 Averages and ranges of concentrations of REE and Ca determined by electron microprobe analyses for REE mineral veins in coal sample th-k2b-4 (wt.%).

Ca-poor REE mineral (18 points) Ca-rich REE mineral (10 points)

La2O3

Ce2O3

Nd2O3

Y2O3

CaO

Total

6.4 0–8.8 5.1 4.4–7.6

24.9 0–29.5 19.4 16.2–23.9

27.1 0–34.3 23.4 19.7–28

0.3 0–1.2 2.4 0–15.9

0.7 0–2.4 13.5 9.7–16

59.4 30.5–66.7 63.8 56.1–71.6

during storage of the coal. Bassanite and gypsum are also secondary, and were most likely formed from the reaction of sulphuric acid released by pyrite oxidation with Ca-bearing carbonates (Rao and Gluskoter, 1973). However, gypsum or bassanite is not always associated with jarosite and other products of pyrite oxidation (Table 2). An alternative mechanism for the formation of gypsum and/or bassanite involves interaction of organic sulphur with calcium released from organic matter during the low-temperature ashing process. However, this mechanism is less likely in the present case because non-mineral inorganic matter (e.g. Ca, Mg) is usually more prominent in lower-rank coals (e.g. Li et al., 2010; Ward, 2002).

5. Conclusions The coals from the Songzao Coalfield are mainly high-ash, highsulphur semianthracites and their mineralogy is dominated by kaolinite, pyrite (or marcasite in some cases), and quartz, with varying proportions of non-kaolinite clay minerals, carbonates (calcite, dolomite, and ankerite), feldspar (albite), and anatase. Separate orientedaggregate XRD study indicates that significant proportions of illite and expandable clays are also present, especially in some of the Datong and Tonghua coal plies. The lower two plies of the Datong section contain significant proportions of illite and I/S, and at least some of the illite and I/S is Na-rich. The precursor for the formation of such I/S and illite in coals is probably smectite, which was mainly pyroclastic, assuming the necessary ions (e.g. K, Na, Mg) were available in the marine-influenced coal swamp. Organically-bound Na, which was expelled from the organic matter with coal rank advance, especially with anthracitization, probably supplied additional Na for the formation of Na-rich illite. Cell or pore-filling I/S is common in a Tonghua coal ply that is adjacent to an alkali tonstein and a mafic bentonite. Potassium, Na, and Mg for the formation of such authigenic I/S were probably derived from leaching of the adjacent volcanic ashes during diagenesis. The minerals in the two thin intra-seam claystones (tonsteins) are essentially poorly-ordered kaolinite, regularly interstratified I/S, and

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

illite, with pyrite, quartz, albite, and anatase present in minor proportions. The relatively thick claystone (K-bentonite) contains 50% I/S in the clay mineral assemblage. The volcanic ash layers in the peat swamp may have been originally converted to smectite under the marine-influenced coal-forming environment. The smectite was, in turn, altered to I/S and illite during diagenesis and/or rank advance, assuming that necessary ions (e.g. K, Na and Mg) were available from the marine water percolating in the peat deposit. Na-rich I/S have also been formed in the claystones, with the additional Na probably being released from the organic matter during the coal's rank advance. The formation of relatively abundant anatase and rhabdophane in one of the Tonghua coal plies is tentatively attributed to leaching of the overlying volcanic claystones. REE minerals (probably REE-hydroxides or oxyhydroxides), which occur as fracture-infillings in that coal sample, were probably crystallized from ascending hydrothermal fluids carrying high REE concentrations, which may, in turn, have been associated with contemporaneous volcanic activity. Acknowledgments Thanks are expressed to Prof. Shifeng Dai of China University of Mining and Technology (Beijing), for providing the samples and other relevant data on which to base the investigation. Xibo Wang, Yanfeng Lu, and Xingwei Zhu are thanked for collecting samples from the Songzao underground coal mines. Dr. Zhongsheng Li of CSIRO is gratefully acknowledged for his advice on low-temperature ashing and clay slide preparation. Thanks are also expressed to Irene Wainwright of the UNSW Analytical Centre for provision of the XRF analyses, to Eugene White of the UNSW Electron Microscopy Unit for technical assistance, and to Rad Flossman and Joanne Wilde of the UNSW School of Biological, Earth and Environmental Sciences, for polished-section and thin-section preparation. Thanks are also expressed to Dr. Vladimir Seredin for his suggestions on REE mineral identification, and to Dr. James Hower and two anonymous reviewers for their constructive comments on the manuscript. This research was in part supported by the National Key Basic Research and Development Program (No. 2014CB238904). References Altaner, S.P., Hower, J., Whitney, G., Aronson, J.L., 1984. Model for K-bentonite formation: evidence from zoned K-bentonites in the disturbed belt, Montana. Geology 12, 412–415. ASTM, 2012. ASTM Standard D388, 2012. Classification of Coals by Rank. ASTM International, West Conshohocken, PA http://dx.doi.org/10.1520/D0388-12 (7 pp. www. astm.org). Burger, K., Zhou, Y., Tang, D., 1990. Synsedimentary volcanic-ash-derived illite tonsteins in Late Permian coal-bearing formations of southwestern China. International Journal of Coal Geology 15, 341–356. China Coal Geology Bureau, 1996. Sedimentary Environments and Coal Accumulation of Late Permian Coal Formation in Western Guizhou, Southern Sichuan, and Eastern Yunnan, China. Chongqing University Press, Chongqing, pp. 156–216 (in Chinese with English abstract). Crowley, S.S., Stanton, R.W., Ryer, T.A., 1989. The effects of volcanic ash on the maceral and chemical composition of the C coal bed, Emery Coal Field, Utah. Organic Geochemistry 14, 315–331. Crowley, S.S., Ruppert, L.F., Belkin, H.E., Stanton, R.W., Moore, T.A., 1993. Factors affecting the geochemistry of a thick, subbituminous coal bed in the Powder River Basin: volcanic, detrital, and peat-forming processes. Organic Geochemistry 20, 843–853. Dai, S., Chou, C.-L., 2007. Occurrence and origin of minerals in a chamosite-bearing coal of Late Permian age, Zhaotong, Yunnan, China. American Mineralogist 92, 1253–1261. Dai, S., Zhou, Y., Ren, D., Wang, X., Li, D., Zhao, L., 2007. Geochemistry and mineralogy of the Late Permian coals from the Songzao Coalfield, Chongqing, southwestern China. Science in China Series D: Earth Sciences 50, 678–688. Dai, S., Tian, L., Chou, C.-L., Zhou, Y., Zhang, M., Zhao, L., Wang, J., Yang, Z., Cao, H., Ren, D., 2008. Mineralogical and compositional characteristics of Late Permian coals from an area of high lung cancer rate in Xuan Wei, Yunnan, China: occurrence and origin of quartz and chamosite. International Journal of Coal Geology 76, 318–327. Dai, S., Wang, X., Chen, W., Li, D., Chou, C.-L., Zhou, Y., Zhu, C., Li, H., Zhu, X., Xing, Y., Zhang, W., Zou, J., 2010. A high-pyrite semianthracite of Late Permian age in the Songzao Coalfield, southwestern China: mineralogical and geochemical relations with underlying mafic tuffs. International Journal of Coal Geology 83, 430–445. Dai, S., Wang, X., Zhou, Y., Hower, J.C., Li, D., Chen, W., Zhu, X., Zou, J., 2011. Chemical and mineralogical compositions of silicic, mafic, and alkali tonsteins in the late

225

Permian coals from the Songzao Coalfield, Chongqing, Southwest China. Chemical Geology 282, 29–44. Dai, S., Jiang, Y., Ward, C.R., Gu, L., Seredin, V.V., Liu, H., Zhou, D., Wang, X., Sun, Y., Zou, J., Ren, D., 2012a. Mineralogical and geochemical compositions of the coal in the Guanbanwusu Mine, Inner Mongolia, China: further evidence for the existence of an Al (Ga and REE) ore deposit in the Jungar Coalfield. International Journal of Coal Geology 98, 10–40. Dai, S., Zou, J., Jiang, Y., Ward, C.R., Wang, X., Li, T., Xue, W., Liu, S., Tian, H., Sun, X., Zhou, D., 2012b. Mineralogical and geochemical compositions of the Pennsylvanian coal in the Adaohai Mine, Daqingshan Coalfield, Inner Mongolia, China: modes of occurrence and origin of diaspore, gorceixite, and ammonian illite. International Journal of Coal Geology 94, 250–270. Daniels, E.J., Altaner, S.P., 1990. Clay mineral authigenesis in coal and shale from the Anthracite region, Pennsylvania. American Mineralogist 75, 825–839. Dewison, M.G., 1989. Dispersed kaolinite in the Barnsley Seam coal (U.K.): evidence for a volcanic origin. International Journal of Coal Geology 11, 291–304. Diessel, C.F.K., 1965. On the Petrography of Some Australian Tonsteins. In: SchmidtThomé, P., Schönenberg, R. (Eds.), Max Richter-Festschrift. University of ClausthalZellerfeld, pp. 149–166. Faraj, B.S.M., Fielding, C.R., Mackinnon, D.R., 1996. Cleat Mineralisation of Upper Permian Baralaba/Rangal Coal Measures, Bowen Basin, Australia. In: Gayer, R., Harris, I. (Eds.), Coalbed Methane and Coal Geology: Geological Society, London, Special Publication, 109, pp. 151–164. Finkelman, R.B., 1994. Modes of occurrence of potentially hazardous elements in coal: levels of confidence. Fuel Processing Technology 39, 21–34. Gupta, R., Wall, T.F., Baxter, L.A. (Eds.), 1999. The impact of mineral impurities in solid fuel combustion. Plenum, New York (768 pp.). Hower, J.C., Ruppert, L.F., Eble, C.F., 1999. Lanthanide, yttrium, and zirconium anomalies in the Fire Clay coal bed, Eastern Kentucky. International Journal of Coal Geology 39, 141–153. Huff, W.D., Tuerkmenoglu, A.G., 1981. Chemical characteristics and origin of Ordovician K-bentonites along the Cincinnati Arch. Clays and Clay Minerals 29, 113–123. Huff, W.D., Bergstrom, S.M., Kolata, D.R., Sun, H., 1998. The Lower Silurian Osmundsberg K-bentonite. Part II: mineralogy, geochemistry, chemostratigraphy and tectonomagmatic significance. Geological Magazine 135, 15–26. Kramer, W., Weatherall, G., Offler, R., 2001. Origin and correlation of tuffs in the Permian Newcastle and Wollombi Coal Measures, NSW, Australia, using chemical fingerprinting. International Journal of Coal Geology 47, 115–135. Li, Z., Ward, C.R., Gurba, L.W., 2010. Occurrence of non-mineral inorganic elements in macerals of low-rank coals. International Journal of Coal Geology 81, 242–250. Lyons, P.C., Spears, D.A., Outerbridge, W.F., Congdon, R.D., Evans, H.T., 1994. Euramerican tonsteins: overview, magmatic origin, and depositional-tectonic implications. Palaeogeography, Palaeoclimatology, Palaeoecology 106, 113–134. Norrish, K., Hutton, J.T., 1969. An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochimica et Cosmochimica Acta 33, 431–453. Permana, A.K., Ward, C.R., Li, Z., Gurba, L.W., Davison, S., 2010. Mineral matter in the high rank coals of the South Walker Creek area, Northern Bowen Basin. In: Beeston, J.W. (Ed.), Proceedings of Bowen Basin Symposium 2010 — Back in (the) Black, pp. 27–34 (Mackay, Queensland). Pevear, D.R., Williams, V.E., Mustoe, G.E., 1980. Kaolinite, smectite, and K-rectorite in bentonites: relation to coal rank at Tulameen, British Columbia. Clays and Clay Minerals 28, 241–254. Price, N.B., Duff, P.M.D., 1969. Mineralogy and chemistry of tonsteins from Carboniferous sequences in Great Britain. Sedimentology 13, 45–69. Querol, X., Chinchon, S., Lopez-Soler, A., 1989. Iron sulfide precipitation sequence in Albian coals from the Maestrazgo Basin, southeastern Iberian Range, northeastern Spain. International Journal of Coal Geology 11, 171–189. Rao, C.P., Gluskoter, H.J., 1973. Occurrence and Distribution of Minerals in Illinois Coals. Illinois State Geological Survey Circular, p. 56. Rao, P.D., Walsh, D.E., 1999. Influence of environments of coal deposition on phosphorous accumulation in a high latitude, northern Alaska, coal seam. International Journal of Coal Geology 38, 261–284. Ren, D., 1996. Mineral matters in coal. In: Han, D. (Ed.), Coal Petrology of China. Publishing House of China University of Mining and Technology, Xuzhou, pp. 67–77 (in Chinese with English abstract). Ruan, C.-D., Ward, C.R., 2002. Quantitative X-ray powder diffraction analysis of clay minerals in Australian coals using Rietveld methods. Applied Clay Science 21, 227–240. Ruppert, L.F., Moore, T.A., 1993. Differentiation of volcanic ash-fall and water-borne detrital layers in the Eocene Senakin coal bed, Tanjung Formation, Indonesia. Organic Geochemistry 20, 233–247. Seredin, V.V., 1998. Rare earth mineralization in Late Cenozoic explosion structures (Khankai Massif, Primorskii Krai, Russia). Geology of Ore Deposits 40, 357–371. Seredin, V.V., Dai, S., 2012. Coal deposits as potential alternative sources for lanthanides and yttrium. International Journal of Coal Geology 94, 67–93. Spears, D.A., 1971. The mineralogy of the Stafford tonstein. Proceedings of the Yorkshire Geological Society, pp. 497–516. Spears, D.A., 2012. The origin of tonsteins, an overview, and links with seatearths, fireclays and fragmental clay rocks. International Journal of Coal Geology 94, 22–31. Spears, D.A., Kanaris-Sotiriou, R., 1979. A geochemical and mineralogical investigation of some British and other European tonsteins. Sedimentology 26, 407–425. Standards Australia, 2000. Coal petrography, Part 3: methods for microscopical determinations of the reflectance of coal macerals. Australian Standard 2856–3. (16 pp.). Susilawati, R., Ward, C.R., 2006. Metamorphism of mineral matter in coal from the Bukit Asam deposit, south Sumatra, Indonesia. International Journal of Coal Geology 68, 171–195.

226

L. Zhao et al. / International Journal of Coal Geology 116–117 (2013) 208–226

Taylor, J.C., 1991. Computer programs for standardless quantitative analysis of minerals using the full powder diffraction profile. Powder Metallurgy 6, 2–9. Triplehorn, D.M., Bohor, B.F., 1983. Goyazite in kaolinitic altered tuff beds of Cretaceous age near Denver, Colorado. Clays and Clay Minerals 31, 299–304. Wang, X., 2009. Geochemistry of Late Triassic coals in the Changhe Mine, Sichuan Basin, southwestern China: evidence for authigenic lanthanide enrichment. International Journal of Coal Geology 80, 167–174. Wang, X., Dai, S., Chou, C.-L., Zhang, M., Wang, J., Song, X., Wang, W., Jiang, Y., Zhou, Y., Ren, D., 2012. Mineralogy and geochemistry of Late Permian coals from the Taoshuping Mine, Yunnan Province, China: evidences for the sources of minerals. International Journal of Coal Geology 96–97, 49–59. Ward, C.R. (Ed.), 1984. Coal Geology and Coal Technology. Blackwell, Melbourne (345 pp.). Ward, C.R., 1989. Minerals in bituminous coals of the Sydney Basin (Australia) and the Illinois Basin (U.S.A.). International Journal of Coal Geology 13, 455–479. Ward, C.R., 2002. Analysis and significance of mineral matter in coal seams. International Journal of Coal Geology 50, 135–168. Ward, C.R., Corcoran, J.F., Saxby, J.D., Read, H.W., 1996. Occurrence of phosphorus minerals in Australian coal seams. International Journal of Coal Geology 30, 185–210. Ward, C.R., Spears, D.A., Booth, C.A., Staton, I., Gurba, L.W., 1999. Mineral matter and trace elements in coals of the Gunnedah Basin, New South Wales, Australia. International Journal of Coal Geology 40, 281–308.

Wiese, R.G., Fyfe, W.S., 1986. Occurrences of iron sulfides in Ohio coals. International Journal of Coal Geology 6, 251–276. Zhao, L., Ward, C.R., French, D., Graham, I.T., 2012. Mineralogy of the volcanicinfluenced Great Northern coal seam in the Sydney Basin, Australia. International Journal of Coal Geology 94, 94–110. Zhou, Y.P., 1999. The alkali pyroclastic tonsteins of early stage of late Permian age in southwestern China. Coal Geology and Exploration 27, 5–9 (in Chinese with English abstract). Zhou, Y., Ren, Y., 1994. Element geochemistry of volcanic ash derived tonsteins in Late Permian Coal-bearing formation of eastern Yunnan and western Guizhou, China. Acta Sedimentologica Sinica 12, 123–132 (In Chinese with English abstract). Zhou, Y., Ren, Y., Bohor, B.F., 1982. Origin and distribution of tonsteins in Late Permian coal seams of Southwestern China. International Journal of Coal Geology 2, 49–77. Zhou, Y., Bohor, B.F., Ren, Y., 2000. Trace element geochemistry of altered volcanic ash layers (tonsteins) in Late Permian coal-bearing formations of eastern Yunnan and western Guizhou Provinces, China. International Journal of Coal Geology 44, 305–324. Zielinski, R.A., 1985. Element mobility during alteration of silicic ash to kaolinite—a study of tonstein. Sedimentology 32, 567–579.