International Journal of Coal Geology 131 (2014) 344–362

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International Journal of Coal Geology

j ourna l homepage: www.e lsev ie r .com/ locate / i j coa lgeo

Petrological and organic geochemical properties of lignite from theKolubara and Kostolac basins, Serbia: Implication on Grindability Index

Dragana Životić a,⁎, Achim Bechtel b, Rainhard Sachsenhofer b, Rainhard Gratzer b, Dejan Radić c,Marko Obradović c, Ksenija Stojanović d

a University of Belgrade, Faculty of Mining and Geology, Djusina 7, 11000 Belgrade, Serbiab Department Angewandte Geowissenschaften und Geophysik, Montanuniversität Leoben, Peter-Tunner-Str. 5, A-8700 Leoben, Austriac University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, 11000 Belgrade, Serbiad University of Belgrade, Faculty of Chemistry, Studentski trg 12-16, 11000 Belgrade, Serbia

⁎ Corresponding author. Tel.: +381 11 3219 251; fax: +E-mail address: draganar@rgf.bg.ac.rs (D. Životić).

http://dx.doi.org/10.1016/j.coal.2014.07.0040166-5162/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 January 2014Received in revised form 3 July 2014Accepted 4 July 2014Available online 11 July 2014

Keywords:Xylite-rich coalMatrix coalLigniteGrindabilityOrganic matterBiomarker

The influence of different coal lithotypes on grindability has been investigated using lignite from two of the mostimportantUpperMiocene lignite basins in Serbia (Kolubara andKostolac). Yellowxylite-rich types demonstratedthe most negative impact on Hardgrove Grindability Index (HGI). All different types of xylite-rich coal, as well astotal xylite-rich coal from the Kolubara basin have a negative influence on the grindability properties, while onlythe yellow type of xylite-rich coal from the Kostolac showed a negative impact on HGI.Matrix coal does not showa clear effect on HGI.A negative correlation between textinite content and HGI is observed in both basins, whereas contents of othermacerals do not show influence on grindability properties.Content of total organic carbon demonstrated the negative impact on HGI. Correlation analysis indicates that thenegative impact of the yellow type of xylite-rich coal and the sum of total xylite-rich coal on the grindabilityproperties partly can be related to content of total organic carbon and high amount of soluble organic matter.Matrix lithotype does not show any significant correlation with bulk geochemical parameters in both basins.The peat-forming vegetation of all samples from both basins were dominated by decay-resistant gymnosperm(coniferous) plants, belonging to one or several of the families Taxodiaceae, Podocarpaceae, Cupressaceae,Araucariaceae, Phyllocladaceae and Pinaceae. Lignite from the Kolubara basin is characterized by a highercontribution of angiosperm vegetation than coal from the Kostolac basin. Peatification of the Kolubara coaloccurred under more oxic conditions than the Kostolac one.Analysis of biomarkers indicated that the negative impact of all types of xylite-rich coal from the Kolubara onHGIcan be related to the higher proportion of angiosperms, abundance ofmid-chain n-alkanes and sesquiterpenoids,aromatization of non-hopanoid triterpenoids and hopanoids, and intense degradation of wood tissues in a moreoxic environment. The positive impact of matrix coal on HGI in the Kolubara samples can be attributed toelevated content of non-aromatic hopanoids and low amounts of aromatic non-hopanoid triterpenoids andsesquiterpenoids, which seems to hinder the grindability properties.

381 11 3235 537.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Production of energy in Serbia is based on coal sources (49%), follow-ed by oil (29%), natural gas (15%) and hydroenergy (7%) (http://www.megatrend-info.com/forum/index.php?action=dlattach;topic=11480.0;attach=21858). The largest resources of coals in Serbia represent lig-nite (92%, http://www.smeits.rs/include/data/docs0066.doc). The mainlignite deposits are located in the UpperMiocene Kolubara and Kostolacbasins, and in the Kovin deposit (Jelenković et al., 2008). The Kolubara

basin is located about 60 km SSW of Belgrade, and covers an area of al-most 600 km2, extending in the E–W direction up to 55 km, and in theS–N direction up to 15 km (Fig. 1). The Kostolac basin is located atabout 90 km east of Belgrade and covers an area of 145 km2 (Fig. 1). An-nually, the Kolubara basin produces about 30Mt of lignite (http://www.rbkolubara.rs/index.php?option=com_content&view=article&id=83&Itemid=189&lang=sr), while the Kostolac basin produces about7 Mt (http://www.te-ko.rs). Most of the excavated lignite (90%) isused for electricity generation in the thermal power plants (TPP)“Nikola Tesla” in Obrenovac town and “Kolubara” in Veliki Crljenivillage (Kolubara basin), and Kostolac A and Kostolac B (Kostolacbasin), with total capacities of 3160 MW and 1.070 MW, respectively(http://www.eps.rs).

Fig. 1. Locations of the Kolubara and Kostolac Basins.

345D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

Geological exploration was initiated at the eastern part of theKolubara basin and at the central part of the Kostolac basin sincethe late 19th century. The Upper Miocene (Pontian) age of the coal-bearing sediments from both basins was confirmed in studies carriedout by Stevanović (1951) and Pantić and Dulić (1993). Averagehuminite reflectance of coal from both basins is 0.30% (Ercegovacet al., 2006), thus placing the coal at lignite stage of coalification.

Efficient combustion of pulverized coal requires coal with a desiredparticle size range. Coal grindability is usually measured by theHardgrove Grindability Index (HGI), which is an important propertyfor coal handling and utilization aspects. Because of non-linearity, the

HGI should not be used as the sole criterion for judging the grindabilityof a coal. Coal grindability characteristics reflect the coal hardness, te-nacity and fracture that are influenced by coal lithotypes, maceral com-position, rank and the distribution and the types of minerals (Bagheriehet al., 2008; Chehreh Chelgani et al., 2008; Ganguli and Bandopadhyay,2008; Hower, 1998; Hower and Calder, 1997; Hower and Wild, 1988;Hower et al., 1987; Ural and Akyildiz, 2004). Different lithotypes oflow rank coals, especially the type and content of xylite have differenteffects on coal grindability.

Lignite lithotypes can be macroscopically determined based ondifferences of the macropetrographic composition, the structure

346 D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

and the texture of the sample. Lithotype varieties (sublithotypes)can be differentiated by their degree of gelification and color,reflecting their composition and the degree of transformation. Thedifferences in sublithotypes indicate the possibilities of coal utiliza-tion (Kwiecińska and Wagner, 1997). The lithotype classificationsystem for soft brown coal (lignite) proposed by ICCP (Taylor et al.,1998) distinguishes: xylite-rich coal, matrix coal, charcoal-rich coaland mineral-rich coal.

Different varieties of xylite-rich coal expressed through the differentdegrees of coalification and color have different grindability properties.High content of a fibrous xylite-rich type, of pale to dark yellow colorwith well-preserved wood structure (Ercegovac, 1989; Kwiecińskaand Wagner, 1997; Novaković, 1973) hinders the grinding process.However, there is no information available about the effect of matrixcoal on grindability.

Charcoal-rich coal, or fusain coal, is rarely found in larger amounts inlignite deposits. This lithotype mainly occurs in lenses and also inpersistent horizons. It is very brittle and has a negative impact on thegrindability properties (Kwiecińska and Wagner, 1997).

In terms of its influence on grindability, mineral matter wasclassified into four groups: (1) clays and sulfates; (2) quartz, oxides,and silicates; (3) pyrite and other sulfides; and (4) carbonates. Mineralsof Group 2 are the hardest, whereas minerals of Group 1 are the softest(Hower et al., 1987 and references therein). Previous investigations alsoshowed that content of quartz as well as content of water- andacid-soluble mineral matter has great influence on the grindability oflow-rank coal (Ural and Akyildiz, 2004). Therefore, it can be expectedthat mineral-rich coal, particularly that rich in quartz, oxides and non-clay silicates, generally has a positive influence on HGI.

Dopplerite is a black very brittle type of coalmade of humic gel iden-tified as eugelinite (Feller et al., 2010; Suárez-Ruiz et al., 2012; Tayloret al., 1998). This type mainly occurs in small lenses and it is very rarelyfound in larger amounts in lignite deposits. Generally speaking calcium-rich deposition environment leads to formation of a higher content ofgelinite–dopplerite (Taylor et al., 1998).

The study outlined in this paper focuses on the effects of petrologicaland organic geochemical properties of different lithotypes of low-rankcoal and types of xylite-rich coal from the Kolubara and Kostolacthermal power plants (TPP) on Hardgrove Grindability Index (HGI).

2. Geological setting

The Upper Miocene (Pontian–Messinian ICS) coal-bearing seriesfrom the Kolubara and Kostolac lignite basins are related to sandy–clay-ey sediments. Theywere formed in the Pannonian basin System in shal-low lacustrine, delta plain and fluvial environments.

The area of the Kolubara basin consists of Paleozoic, Mesozoic,Tertiary, and Quaternary rocks (Fig. 2) (Ercegovac and Pulejković,1991). The basement of this basin consists of Devonian and Carbon-iferous schists, gneisses, slates and sandstones, Mesozoic mica-richsandstones, shales, dolomitic limestones, limestones and flysch (al-ternation of limestones, marlstones, sandstones and siltstones),and Tertiary phenoandesites, phenodacites, quartz-latite, ignim-brites and quartz-latite tuffs. The Pontian (Upper Miocene) freshwater clastic sediments host three coal seams (Kezović, 2011):Seam III or Lower Coal Seam, Seam II or Main Coal Seam, and SeamI or Upper Coal Seam, having average thicknesses of 7 m, 25 m, and11 m, respectively. The total thickness of the Pontian series isbetween 250 and 320 m.

The tectonic features of Neogene sediments are relatively uniform inthemajor part of the basin; coal seams dip at low angles to the northernand central parts of the basin. Only along the southern border of the SEpart of the basin, coal seams are characterized by a synform, due tointense post-sedimentary faulting, causing occasional coal erosion inthe SE part of the basin.

The basement of the Kostolac basin consists of Devonian crystallinerocks overlain by Neogene sediments (Stojanović et al., 2012; Fig. 2).The fresh water clastic coal-bearing series of the Kostolac basin is alsoof Upper Miocene (Pontian) age and hosts five coal seams, namely theSeam III (the oldest and deepest) and the Seams II-a, II, I-a, and I. Onlycoal seams III, II and I are (or have been) explored in Drmno, ĆirikovacandKlenovnik open pits. The average thickness of coal Seam III through-out the whole basin is 19.38 m, while it is 1.43 m for IIa, 4.14 m for II,1.53 for Ia and 13.90 m for I coal seam.

All Neogene strata generally dips towards NW at low angles of5–15°; coal seams following the same dip, as well.

3. Samples and analytical methods

Eleven raw feed coal samples (Table 1) from the TPP Nikola TeslaB (Kolubara basin) weighing about 4–5 kg were collected from thepre-boiler mills from April 2009 to July 2010. Thirteen raw feedcoal samples (Table 1) from the TPP Kostolac (Kostolac basin) werecollected from the pre-boiler mills in January–February 2010. Allsamples were crushed, blended, and split to obtain 0.5–1 kg subsam-ples for analysis.

3.1. Petrographic analysis

For lithotype analyses, the lignite samples were crushed to a maxi-mum particle size of 3 mm and dried at the room temperature. Afterdrying, samples were separated into portions of 100 g of coal, whichwere manually separated into different lithotypes and mineral matter,under the stereomicroscope (Fig. 3).Weight of each separated lithotypefraction andmineralmatter weremeasured and correspondingwt. per-cent of different lithotypes was calculated (Table 1). The macroscopicdescription of the coal lithotypes followed the nomenclature adoptedby the ICCP (Taylor et al., 1998). The description of xylite-rich lithotypesand different types used in this study follows the terminologydeveloped by Jacob (1961) and modified by Ercegovac (1989).

For maceral analyses (Fig. 4; Table 2), the lignite samples werecrushed to a maximum particle size of 1 mm, mounted in epoxy resinand polished. The maceral analyses were performed on a Leitz DMLPmicroscope in monochromatic and UV light illumination on 500 points(ISO 7404-3, 2009) in oil immersion. The maceral description used inthis study follows the terminology developed by the International Com-mittee for Coal and Organic Petrology for huminite (Sykorova et al.,2005), liptinite (Taylor et al., 1998) and inertinite (InternationalCommittee for Coal Petrology (ICCP), 2001) nomenclature.

3.2. Hardgrove Grindability Index

Experimental procedure for determining the grindability index wasperformed in accordance with ISO 5074, i.e. equivalent national stan-dard SRPS ISO 5074, 1992. The method was based on the treatment ofpre-prepared coal samples, having particle sizes ranging from 0.6 mmto 1.18 mm. Experimental investigation was conducted using the stan-dard Hardgrove apparatus manufactured by EKO-LAB, type 02H/04.Hardgrove machine includes the stationary grinding bowl of steel,with a horizontal track in which eight steel balls each of a diameter25.4 mm are run. The balls are driven by an upper grinding ring rotatedat 20 ± 1min−1. The upper grinding ring is connected to a spindle andis driven by an electric motor trough reduction gears. A load is added tothe spindle so that the total vertical force on the balls due to the top ring,gear and spindle is 284 ± 2 N, i.e. closely equivalent to a total mass of29 ± 2 kg. The machine is equipped with a revolution counter and au-tomatic device for stopping the machine after 60 ± 0.25 revolutions.Hardgrove Grindability Index (HGI) (Table 1) was determined by siev-ing a 50 g of coal sample through a 0.075 mm sieve and subsequentlyutilizing the equation HGI = 13 + 6.93 ⋅ (50 − m1), where m1 is the

Fig. 2. Schematic lithostratigraphic column of the Neogene from the Kolubara and Kostolac.

347D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

mass (in grams) of coal sample residue remaining on the screen of0.075 mm sieve.

3.3. Organic geochemical analysis

Organic carbon content (Table 3) was determined after removal ofcarbonates with diluted hydrochloric acid (1:3, v/v) using a Vario ELIII CHNS/O Elemental Analyzer.

For the determination of the molecular composition of the organicmatter (OM) approximately 5 g of pulverized sample (b150 μm) waspowdered, mixed with a standard inert substance and homogenized.Extraction was performed using a Dionex ASR 200 apparatus with di-chloromethane for 1 h at 75 °C and a pressure of 50 bars. Solvent wasevaporated and extracts were concentrated by a Zymark Turbo Vap500 device. Extracts were dissolved in a mixture of n-hexane: dichloro-methane (80:1, v:v) and asphaltenes were subsequently separated bycentrifugation. The hexane-soluble organic compounds (maltenes)were separated into saturated hydrocarbons, aromatic hydrocarbonsand NSO fraction (polar fraction, which contains nitrogen, sulfur, andoxygen compounds) using a Kohnen–Willsch MPLC (medium pressureliquid chromatography) instrument (Radke et al., 1980).

The saturated and aromatic hydrocarbon fractionswere analyzed bygas chromatography–mass spectrometry (GC–MS) (Figs. 5–7). A gaschromatograph equippedwith a 30mDB-5MS fused silica capillary col-umn (i.d. 0.25 mm; 0.25 μm film thickness) coupled to a Thermo FisherISQ quadrupole mass spectrometer was used. The oven temperaturewas programmed from 70 °C to 300 °C at a rate of 4 °Cmin−1 followedby an isothermal period of 15 min. Helium was used as carrier gas. Thesamplewas injected in the split lessmodewith the injector temperatureat 270 °C. The mass spectrometer was operated in the EI (electronimpact) mode over a scan range from m/z 50 to m/z 650 (0.7 s totalscan time).

Data were processed with an XCalibur data system (Tables 4–7).Identification of individual compounds was accomplished based on re-tention time, and comparison of the mass spectra with literature data(Killops et al., 1995, 2003; Otto and Simoneit, 2002; Peters et al.,2005; Philp, 1985; Stout, 1992; Wakeham et al., 1980) and libraryNIST5a. Relative percentages and absolute concentrations of differentcompound groups in the saturated and aromatic hydrocarbon fractionswere calculated using peak areas from the gas chromatograms inrelation to those of internal standards (deuterated n-tetracosane and1,1′-binaphthyl, respectively).

Table 1Lithotype composition (wt.%) and Hardgrove Grindability Index (HGI) of studied samples.

Sample Matrix coal Xylite-rich coal Doppleritecoal

Charcoal

Mineral -richcoal

Mineralmatter

HGIc

Pale yellowtype

Dark yellowtype

Browntype

Blacktype

Yellowa Brown andblackb

Total

Kolubara 3 53.86 0.09 8.30 24.16 1.29 8.39 25.45 33.84 1.91 0.23 5.72 4.44 44.0Kolubara 7 47.23 0.001 6.48 31.68 0.03 6.48 31.71 38.19 0.03 0.07 8.32 6.19 56.9Kolubara 8 56.20 0.00 16.26 24.47 0.98 16.26 25.45 41.71 0.65 0.03 0.85 0.56 44.9Kolubara 9 50.13 0.00 14.74 32.83 1.23 14.74 34.06 48.80 0.12 0.02 0.44 0.49 40.3Kolubara 10 61.97 0.02 14.22 19.37 1.42 14.24 20.79 35.03 0.02 0.05 1.09 1.84 46.3Kolubara 13 41.91 0.00 17.44 37.75 0.86 17.44 38.61 56.05 0.06 0.03 1.18 0.77 39.6Kolubara 17 42.80 0.001 19.57 33.62 1.56 19.57 35.18 54.75 0.16 0.04 0.57 1.68 42.2Kolubara 23 17.20 0.00 45.75 33.70 0.93 45.75 34.63 80.38 0.16 0.08 1.40 0.78 30.7Kolubara 29 45.55 0.02 20.78 29.47 2.22 20.80 31.69 52.49 0.05 0.02 0.60 1.29 35.8Kolubara 34 57.85 0.00 11.68 28.55 1.00 11.68 29.55 41.23 0.04 0.02 0.59 0.27 42.0Kolubara 36 58.47 0.03 15.89 14.72 1.15 15.92 15.87 31.79 0.06 0.03 3.54 6.11 48.5Kostolac 1 50.96 0.00 10.12 24.28 3.42 10.12 27.70 37.82 0.01 0.01 1.11 0.09 45.6Kostolac 2 56.77 0.00 18.73 23.20 0.03 18.73 23.23 41.96 0.01 0.03 0.67 0.57 41.3Kostolac 4 65.64 0.00 12.02 13.05 6.09 12.02 19.14 31.16 2.99 0.05 0.06 0.10 40.8Kostolac 5 46.57 0.00 13.77 29.25 6.03 13.77 35.28 49.05 1.12 0.04 2.83 0.40 42.0Kostolac 7 54.30 0.00 9.13 24.39 5.56 9.13 29.95 39.08 1.35 0.01 4.05 1.21 47.0Kostolac 9 49.82 0.001 13.84 23.34 9.48 13.84 32.82 46.66 0.20 0.31 2.19 0.82 44.1Kostolac 11 54.93 0.00 11.52 25.54 6.72 11.52 32.26 43.78 0.49 0.02 0.40 0.38 44.8Kostolac 13 50.71 0.00 17.73 26.39 2.70 17.73 29.09 46.82 1.64 0.02 0.56 0.25 41.7Kostolac 15 45.24 0.001 16.44 26.31 6.39 16.44 32.70 49.14 0.11 0.05 5.03 0.43 43.4Kostolac 16 51.81 0.00 17.50 24.29 1.92 17.50 26.21 43.71 0.16 0.01 3.46 0.85 43.6Kostolac 17 47.59 0.001 16.30 24.07 6.89 16.30 30.96 47.26 0.38 0.02 3.94 0.82 44.6Kostolac 22 45.77 0.00 12.32 33.83 6.29 12.32 40.12 52.44 0.29 0.03 1.45 0.02 43.1Kostolac 23 47.32 0.001 17.90 27.42 4.87 17.90 32.29 50.19 0.08 0.04 2.34 0.04 42.7

Correlation coefficientKolubara 0.62 −0.77 −0.41 −0.53 −0.77 −0.46 −0.79 0.04 0.09 0.69 0.72Kostolac −0.22 −0.57 0.17 0.29 −0.57 0.28 −0.06 −0.34 −0.01 0.43 0.51

a Xylite yellow = xylite-rich coal − pale yellow type + Xylite-rich coal − dark yellow type.b Xylite brown and black = xylite-rich coal − brown type + xylite-rich coal − black type.c HGI — Hardgrove Grindability Index; correlations, r N |0.55| for Kolubara samples and r N |0.51| for Kostolac samples, significant at the 0.05 level (Pearson test) are given in bold.

Fig. 3. Photomicrographs of studied matrix coal and different xylite-rich coal types from the Kolubara and Kostolac. a) Matrix coal; b) xylite-rich coal-pale yellow type; c) xylite-richcoal-dark yellow type; d) xylite-rich coal-brown type; and e) xylite-rich coal-black type. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)

348 D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

Fig. 4. Photomicrographs of typical macerals from the Kolubara (a–d) and Kostolac (e–h). a) Textinite (Te); b) textinite (Te), ulminite (Ul), corpohuminite (Ch); c) attrinite (At), densinite(De), funginite (Fn); d) fusinite (Fu); e) textinite (Te); f) textinite (Te), ulminite (Ul), corpohuminite (Ch), suberinite (Su); g) densinite (De), sporinite (Sp), inertodetrinite (Id); h) attrinite(At), densinite (De), corpohuminite (Ch), pyrite (Py).

349D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

Table 2Maceral composition (vol.%) and Hardgrove Grindability Index (HGI) of studied samples.

Sample Te Ul At De Ge Ch HUM Sp Cu Re Su Ld LIP Fu Sf Ma Fn Id INER MM HGI

Kolubara 3 30.2 10.3 8.8 24.8 0.9 3.1 78.1 1.7 0.4 2.0 0.9 0.8 5.8 0.5 0.7 0.1 0.6 1.2 3.1 13.0 44.0Kolubara 7 13.3 9.0 10.1 21.4 0.9 2.5 57.2 3.0 0.2 1.1 2.3 1.4 8.0 0.5 0.5 1.4 1.8 4.2 30.6 56.9Kolubara 8 29.1 11.9 8.0 25.5 0.6 3.9 79.0 1.1 0.3 2.2 1.1 0.6 5.3 0.3 0.8 0.3 0.8 0.8 3.0 12.7 44.9Kolubara 9 37.9 10.3 6.4 15.6 1.1 1.6 72.9 1.1 1.9 1.8 0.5 5.3 1.9 0.8 0.8 0.5 4.0 17.8 40.3Kolubara 10 39.6 6.2 3.8 23.2 1.1 1.9 75.8 1.1 0.8 0.8 0.5 3.2 1.1 1.1 0.5 0.8 3.5 17.5 46.3Kolubara 13 46.9 3.8 4.1 16.3 0.4 5.8 77.3 3.2 1.2 0.9 1.7 7.0 0.3 0.3 0.6 2.3 3.5 12.2 39.6Kolubara 17 51.5 6.7 3.8 11.5 0.6 5.1 79.2 0.5 1.3 2.6 1.5 5.9 1.5 0.3 0.5 1.3 3.6 11.3 42.2Kolubara 23 53.6 5.6 1.8 12.5 0.6 4.5 78.6 2.1 0.3 1.2 3.2 1.5 8.3 0.3 0.3 2.7 0.6 3.9 9.2 30.7Kolubara 29 44.7 14.0 5.9 13.8 0.2 3.2 81.8 0.7 0.5 1.0 1.0 1.0 4.2 1.5 0.5 1.4 3.4 10.6 35.8Kolubara 34 45.4 11.7 0.9 20.9 1.7 5.1 85.7 0.9 0.3 2.2 0.9 4.3 0.6 0.9 2.2 3.7 6.3 42.0Kolubara 36 37.6 15.3 4.7 16.6 0.5 3.7 78.4 1.2 0.3 1.1 1.1 1.3 5.0 0.3 0.3 1.2 1.3 3.1 14.5 48.5Kostolac 1 16.9 18.5 4.6 14.3 1.8 4.6 60.7 2.5 0.9 0.4 2.9 4.1 10.8 1.9 0.3 0.4 0.4 3.7 6.7 21.8 45.6Kostolac 2 18.5 20.1 5.6 14.6 2.0 3.0 63.8 2.9 0.5 0.5 4.0 3.0 10.9 0.2 1.4 1.8 3.4 21.9 41.3Kostolac 4 26.2 19.8 3.3 10.6 1.9 3.8 65.6 2.4 0.5 3.3 1.6 7.8 1.9 2.2 1.1 2.4 7.6 19.0 40.8Kostolac 5 29.7 17.9 4.0 9.2 1.2 9.0 71.0 1.0 0.3 0.5 5.6 2.8 10.2 0.2 0.7 0.3 1.4 3.0 5.6 13.2 42.0Kostolac 7 15.6 38.3 5.1 12.6 0.6 4.5 76.7 0.8 0.3 1.1 1.7 0.8 4.7 0.3 0.3 1.4 1.7 3.7 14.9 47.0Kostolac 9 18.3 33.3 3.7 10.4 1.1 6.5 73.3 1.8 0.2 0.7 1.2 2.3 6.2 2.5 0.7 0.2 1.8 3.5 8.7 11.8 44.1Kostolac 11 17.1 23.6 2.9 17.4 3.7 6.1 70.8 1.7 0.7 4.6 2.0 9.0 0.5 0.2 1.7 3.4 5.8 14.4 44.8Kostolac 13 24.9 20.2 2.0 13.4 0.6 7.6 68.7 2.1 0.2 0.3 1.2 1.8 5.6 0.3 0.6 0.2 1.4 1.9 4.4 21.3 41.7Kostolac 15 21.9 19.2 3.2 18.9 1.2 4.7 69.1 2.3 0.2 1.0 4.0 1.2 8.7 0.2 0.7 1.8 3.8 6.5 15.7 43.4Kostolac 16 21.5 24.5 3.1 18.2 1.7 3.4 72.4 1.5 0.2 0.5 1.0 1.2 4.4 1.1 1.2 2.7 5.0 18.2 43.6Kostolac 17 17.0 27.3 4.2 19.4 0.8 5.3 74.0 0.8 0.5 1.1 1.6 1.6 5.6 0.5 1.1 0.3 1.6 2.6 6.1 14.3 44.6Kostolac 22 21.5 24.4 3.1 18.2 1.7 3.4 72.3 1.5 0.2 0.5 1.0 1.2 4.4 1.2 1.2 2.7 5.1 18.2 43.1Kostolac 23 17.0 27.3 4.2 19.4 0.8 5.3 74.0 0.8 0.5 1.1 1.6 1.6 5.6 0.5 1.1 0.3 1.6 2.6 6.1 14.3 42.7

Correlation coefficientKolubara −0.83 0.21 0.58 0.56 0.25 −0.36 −0.65 0.20 −0.59 0.00 −0.22 −0.09 −0.04 −0.15 −0.11 0.48 −0.24 0.24 0.02 0.76Kostolac −0.70 0.62 0.27 0.19 −0.02 −0.07 0.35 −0.38 0.15 0.43 −0.23 −0.07 −0.20 −0.02 −0.49 0.39 −0.14 0.24 −0.01 −0.31

Te — textinite; Ul — ulminite; At — attrinite; De — densinite; Ge — gelinite; Ch — corpohuminite; HUM — total huminite; Sp — sporinite; Cu — cutinite; Re — resinite; Su — suberinite;Ld— liptodetrinite; LIP— total liptinite; Fu— fusinite; SF— semifusinite;Ma—macrinite; Fn— funginite; Id— inertodetrinite; INER— total inertinite;MM— totalmineral correlations, r N |0.55| for Kolubara samples and r N |0.51| for Kostolac samples,significant at the 0.05 level (Pearson test) are given in bold.

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Table 3Group organic geochemical parameters and their correlation with HGI and coal lithotypes.

Sample Corgdb

(wt.%)aCorgdmmfb

(wt.%)bSdb

(wt.%)cExtract yield(mg/g Corg)

Saturated HCd

(wt.%)Aromatic HC(wt.%)

NSOe

(wt.%)Asphaltenes(wt.%)

Kolubara 3 33.83 38.88 0.64 58.5 2.9 1.2 45 51Kolubara 7 29.01 41.81 0.53 37.3 3.9 1.8 60 34Kolubara 8 34.86 39.93 0.74 54.4 2.9 1.1 43 53Kolubara 9 36.48 44.38 0.80 87.2 2.1 0.8 57 40Kolubara 10 37.23 45.13 0.59 79.2 1.8 0.7 60 37Kolubara 13 38.13 43.43 1.03 88.9 1.9 0.8 56 41Kolubara 17 34.73 39.15 0.52 64.7 2.8 1.0 53 43Kolubara 23 45.00 49.56 0.83 84.4 2.2 0.8 52 45Kolubara 29 44.35 49.61 0.99 97.0 1.9 0.8 58 39Kolubara 34 40.15 42.85 0.59 71.4 3.2 1.0 50 45Kolubara 36 34.08 39.86 0.80 124.7 1.7 0.6 75 23Kostolac 1 37.12 47.47 1.66 31.1 7.9 2.2 51 39Kostolac 2 39.85 51.02 1.51 33.3 5.7 3.0 54 37Kostolac 4 38.66 47.73 1.50 35.5 5.7 1.9 54 39Kostolac 5 33.16 38.20 2.56 36.9 7.7 2.3 56 34Kostolac 7 30.69 36.06 2.12 43.2 5.8 2.1 62 30Kostolac 9 34.56 39.18 2.39 35.2 5.7 2.3 57 36Kostolac 11 35.46 41.43 2.25 30.4 6.8 1.9 57 34Kostolac 13 36.68 46.61 1.83 40.5 4.8 1.6 58 35Kostolac 15 35.75 42.41 1.96 48.7 5.8 1.5 55 38Kostolac 16 35.28 43.13 2.05 37.4 5.7 1.5 65 28Kostolac 17 34.29 40.01 1.91 33.1 5.5 1.9 60 32Kostolac 22 38.37 46.91 1.76 40.6 6.0 1.7 60 32Kostolac 23 37.73 44.03 1.91 42.9 5.3 1.8 56 36

Correlation coefficients (r) for samples from the Kolubarar with HGI −0.90 −0.64 −0.58 −0.39 0.50 0.60 0.30 −0.38r with matrix coal −0.49 −0.53 −0.33 −0.02 0.06 0.02 0.15 −0.17r with yellow type of xylite-rich coal 0.72 0.63 0.39 0.33 −0.39 −0.44 −0.06 0.12r with brown type of xylite-rich coal 0.25 0.31 0.23 −0.30 0.22 0.24 −0.37 0.35r with black type of xylite-rich coal 0.53 0.29 0.30 0.47 −0.58 −0.62 0.00 0.07r with (brown + black) type of xylite-rich coal 0.29 0.34 0.26 −0.27 0.17 0.19 −0.37 0.36r with total xylite-rich coal 0.67 0.63 0.41 0.12 −0.21 −0.23 −0.22 0.26r with mineral-rich coal −0.66 −0.37 −0.36 −0.42 0.55 0.74 0.17 −0.24

Correlation coefficients (r) for samples from the Kostolacr with HGI −0.65 −0.58 0.32 −0.05 0.27 −0.08 0.29 −0.38r with matrix coal 0.26 0.33 −0.43 −0.40 −0.09 0.28 −0.18 0.23r with yellow type of xylite-rich coal 0.37 0.30 −0.15 0.21 −0.56 −0.07 0.10 0.00r with brown type of xylite-rich coal −0.13 −0.17 0.36 0.32 0.16 −0.17 0.25 −0.37r with black type of xylite-rich coal −0.42 −0.62 0.53 0.08 0.09 −0.21 0.02 0.04r with (brown + black) type of xylite-rich coal −0.31 −0.44 0.56 0.31 0.18 −0.25 0.23 −0.31r with total xylite-rich coal −0.08 −0.23 0.42 0.39 −0.14 −0.26 0.26 −0.28r with mineral-rich coal −0.64 −0.65 0.41 0.56 −0.09 −0.30 0.44 −0.42

a Corgdb — organic carbon content, dry basis.

b Corgdmmfb — organic carbon content, mineral-matter-free and dry basis.

c Sdb — total sulfur content, dry basis.d HC— hydrocarbons.e NSO— polar fraction, which contains nitrogen, sulfur, and oxygen compounds; correlations, r N |0.55| for Kolubara samples and r N |0.51| for Kostolac samples, significant at the 0.05

level (Pearson test) are given in bold.

351D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

4. Results

4.1. Lithotype composition

Matrix coal (Fig. 3a) is the prevailing lithotype in all the studiedsamples, except one from the Kolubara basin (Kolubara 23; Table 1),for which the xylite-rich coal is the dominant lithotype. Brown xylitetype (Fig. 3d) is the dominant xylite-rich type in almost all samples.Only in the Kolubara 23 sample dark yellow (Fig. 3c) type prevails.The coal from Kostolac has a higher content of black type (Fig. 3e)than the coal from Kolubara (Table 1).

4.2. Maceral composition

Huminite is the prevailing maceral group in all studied samples(57.2–85.7 vol.%; Fig. 4; Table 2). The most abundant macerals aretextinite (13.3–53.6 vol.%), ulminite (3.8–38.3 vol.%) and densinite(9.2–25.5 vol.%) with a variable amount of attrinite (0.9–10.1 vol.%)and corpohuminite (1.6–9.0 vol.%) (Fig. 4; Table 2). Content of gelinite

is low (b5 vol.%). Telohuminite is strongly impregnated by resinous-like substances, while detrohuminite occurs as groundmass surround-ing liptinite or inertinite macerals. In some cases, detrohuminite isinterbeddedwith clayminerals. Gelinite appears inmany cases interbed-dedwith ulminite or densinite, sometimes as thick layers. Corpohuminiteis disseminated throughout textinite and ulminite, sometimes indensinite, mainly as phlobaphinite of globular or tabular shape.

Liptinite ranges from 3.2 to 10.9 vol.% with suberinite (0.8–5.6 vol.%),sporinite (0.5–3.2 vol.%) and liptodetrinite (0.5–4.1 vol.%) being themostabundant macerals. Cutinite and resinite are present in variableamounts (Table 2). Suberinite usually appears as cell wall tissue associ-ated with phlobaphinite. Liptodetrinite is usually associated withdetrohuminite. Resinite mostly occurs as cell filling or isolatedsmall globular bodies and associated with telohuminite as singlebodies, commonly as impregnation in telohuminite and less withdetrohuminite.

The percentages of inertinite, mainly inertodetrinite, funginite,fusinite, semifusinite and macrinite range from 3.0 to 8.7 vol.% in allsamples (Table 2). Funginite is especially abundant in sample Kolubara

Fig. 5.TIC (total ion chromatogram) of saturated fraction typical for investigated samples from theKolubara (a) andKostolac (b). Peak assignments: n-Alkanes are labeled according to theircarbon number; Pr — pristane; Ph — phytane; S1 — nordrimane; S2 — C14 bicyclic sesquiterpene; S3 — elemane; S4 — C15 bicyclic sesquiterpene; S5 — muurolane; S6 — eudesmane;S7 — cedrane; S8 — dihydrovalencene; S9 — cuparane; S10 — longifolane; S11 — cadinane; S12 — dihydroselinene; D1 — 4β(H)-19-norisopimarane; D2 — isopimaradiene; D3 —

isopimaradiene, isomer; D4 — 4α(H)-18-norisopimarane; D5 — norabietane; D6 — beyerane; D7 — norpimarane; D8 — dihydrorimuene; D9 — isophyllocladene; D10 — isopimarane;D11 — fichtelite; D12 — pimarane; D13 — 16β(H)-phyllocladane; D14 — abietane; D15 — 16α(H)-phyllocladane; D16 — 16α(H)-kaurane; T1 — des-A-olean-13(18)-ene; T2 — des-A-olean-12-ene; T3 — des-A-olean-18-ene; T4 — des-A-urs-13(18)-ene; T5 — des-A-urs-12-ene; T6 — des-A-lupane; ββ, βα and αβ designate configurations at C17 and C21 in hopanes,(R) designates configuration at C22 in hopanes; Std — standard.

352 D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

23 (2.7 vol.%). Inertodetrinite is disseminated throughout the coalsamples. Funginite, including single and multi-celled fungal sporesand sclerotia, occurs as single bodies or as colonies. The pores areusually filled with mineral matter, rarely with resinite. Fusinite mostlyoccurs in thick bands. The pores are usually empty but sometimesthey are filled with mineral matter.

The content of mineral matter varies between 6.3 and 30.6 vol.%(Table 2).

4.3. Organic geochemical properties

4.3.1. Bulk organic geochemical parametersOrganic carbon contents (dry basis and dry, mineral-matter-free

basis) in lignites from the Kolubara and Kostolac vary in the range of29.01–45.00% and 36.06–51.02%, respectively (Table 3). Total sulfurcontent is low in samples from the Kolubara (0.52–1.03%), while coalfrom the Kostolac has a higher sulfur content (1.50–2.56%; Table 3).

Fig. 6. TIC (total ion chromatogram) of aromatic fraction typical for investigated samples from the Kolubara (a) and Kostolac (b). Peak assignments: 1 — dihydro-ar-curcumene;2 — gamma-cadinene; 3 — cadina-4,9-diene; 4 — cuparene; 5 — calamenene; 6 — α-calacorene; 7 — cadina-1(10),6,8-triene; 8 — 5,6,7,8-tetrahydrocadalene; 9 — cadalene;10 — isocadalene; 11 — 19-norabieta-8,11,13-triene; 12 — 19-norabieta-6,8,11,13-tetraene; 13 — 16,17-bisnordehydroabietane; 14 — hibaene; 15 — norabietatetraene; 16 — 16,17-bisnorsimonellite; 17 — 18-norabieta-8,11,13-triene; 18 — dehydroabietane; 19 — 1,2,3,4-tetrahydroretene; 20 — 2-methyl, 1-(4′-methylpentyl), 6-i-propyl-naphthalene; 21 —

simonellite; 22 — totarane; 23 — sempervirane; 24 — retene; 25 — 4-methyl-4,5-dihydropyrene; 26 — 3-oxosimonellite; 27 — 2-methylretene; 28 — pentamethyldecahydrochrysene;29 — 3,4,7,12a-tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysene, isomer; 30 — 3,4,7,12a-tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysene; 31 — 3,3,7,12a-tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysene; 32— triaromatic des-A-lupane; 33— 3,3,7-trimethyl-1,2,3,4-tetrahydrochrysene; 34— perylene; 35— 24,25-dinoroleana-1,3,5(10),12-tetraene;36 — D-ring monoaromatic hopane; 37 — 24,25-dinorlupa-1,3,5(10)-triene; 38 — norlanosta(eupha)hexaene; 39 — 24,25-dinorursa-1,3,5(10),12-tetraene; 40 — tetramethyl-1,2,3,4,4a,5,6,14b-octahydropicene; 41 — 4-methyl, 24-ethyl, 19-norcholesta-1,3,5(10)-triene; 42 — 4-methyl, 24-ethyl, 19-norcholesta-1,3,5(10)-triene, isomer; 43 — 7-methyl,3′-ethyl, 1,2-cyclopentanochrysene; 44— 1,2,9-trimethyl-1,2,3,4-tetrahydropicene; 45 — 2,2,9-trimethyl-1,2,3,4-tetrahydropicene; Std — standard.

353D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

The extract yield of the soluble organicmatter (bitumen) is high andranges from 30.4 to 124.7 mg/g Corg (Table 3), attributed to the highproportion of biogenic and diagenetic compounds. A higher content ofbitumen is observed in the Kolubara samples.

In the studied samples, the contents of saturated hydrocarbons (1.7–7.9 wt.%) and aromatics (0.6–3.0 wt.%) are low, while the contentsof asphaltenes (23–53 wt.%) and NSO compounds (43–75 wt.%)(Table 3) are high, as expected for immature terrestrial organicmaterial.

Lignite from the Kostolac has a higher content of hydrocarbons than theKolubara one.

4.3.2. Molecular composition of the organic matter

4.3.2.1. n-Alkanes and isoprenoids. n-Alkanes are relatively abundant insaturated fraction of investigated samples (Fig. 5; Table 4) and are iden-tified in the range from C13 to C33. The Kolubara lignite has a higher

Fig. 7.GC–MSmass chromatograms of hopanoids,m/z 191 typical for investigated samples from theKolubara (a) and Kostolac (b). (S) designates configuration at C22 in hopanes; for otherpeak assignments, see Fig. 5 legend.

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content of n-alkanes, than those from the Kostolac (Table 4). Then-alkane patterns of all samples are dominated by a long-chain homo-logues (C27–C31) with a maximum at n-C29 (Fig. 5; Table 5) and amarked odd-over-even predominance, indicating a significant contribu-tion of epicuticular waxes. Mid-chain n-alkanes (n-C21–C25) originatingfrom vascular plants, microalgae, cyanobacteria, sphagnum and aquaticmacrophytes (Ficken et al., 2000; Matsumoto et al., 1990; Nott et al.,2000) are present in a notably lower amount in comparison to long-chain homologues (Fig. 5; proxy ratio, Paq b Pwax; Σn-C21 − n-C25 /Σn-C13 − n-C33 b Σn-C26 − n-C33 / Σn-C13 − n-C33; Table 5). Thepredominance of odd-over-even carbon-numbered n-alkanes in themid-range n-alkanes (Fig. 5) suggests a microbial origin, consistentwith the presence of hopanoid biomarkers (Table 4).

The values of the CPI (Carbon Preference Index; Bray and Evans,1961) range between 2.09 and 3.37 (Table 5) and are in accordancewith the low rank of the lignite and the predominant terrestrial originof the organic matter (Bechtel et al., 2002, 2007; Zdravkov et al., 2011).

Taking into account the very low abundance of the acyclicisoprenoids pristane and phytane in the samples (Fig. 5), which can re-sult in considerable errors in peak integration, the pristane/phytane ra-tios must be interpreted with care. According to Didyk et al. (1978),pristane/phytane ratios below 1 indicate anaerobic conditions during

early diagenesis, and values between 1 and 3 were interpreted asreflecting dysaerobic environments. For this sample set, the influenceof maturity on the pristane/phytane ratio can be ruled out. Therefore,a slightly predominant role of pristane over phytane (Table 5) in almoststudied samples can be an indication of the presence of free oxygen,though in limited quantity.

4.3.2.2. Steroids and hopanoids. The analysis of the aliphatic fractionreveals very low contents of steroids (Table 4). All studied sampleshave low steroid/hopanoid ratio (Table 6) particularly coal from theKolubara.

Hopanoids are more abundant in the Kolubara than in the Kostolacsamples (Table 4). The hopane composition in all samples is character-ized by the presence of 17α(H)21β(H), 17β(H)21α(H) and 17β(H)21β(H) compounds with 27–31 carbon atoms, with an exception ofC28 homologues. All samples contain unsaturated C27-, C28- and C30

hop-13(18)-ene, as well as C27-, C29- and C30 hop-17(21)-ene (Fig. 7).C3117α(H)21β(H)22(R)-hopane is the most abundant hopanoid in thesaturated fraction of the all studied samples, with exception of Kostolac7 and Kostolac 9 where the C30 hop-17(21)-ene predominates hopanedistribution (Table 6).

Table 4Contents of major biomarkers in saturated fraction and aromatic fraction (μg/g Corg) and their correlation with HGI and coal lithotypes.

Sample Saturated fraction Aromatic fraction

n-Alkanes Sesquiterpenoids Diterpenoids Non-hopanoidtriterpenoids

Steroids Hopanoids Sesquiterpenoids Diterpenoids Non-hopanoidtriterpenoids

Steroids Hopanoids

Kolubara 3 66.22 14.14 479.35 3.01 1.69 60.96 7.44 69.52 28.20 3.95 8.58Kolubara 7 40.70 12.13 452.23 3.39 1.43 59.51 9.19 66.05 33.92 4.79 6.74Kolubara 8 61.34 24.18 666.87 2.08 1.77 55.34 4.97 39.22 28.01 3.25 7.19Kolubara 9 66.62 27.01 755.08 1.29 3.14 57.51 6.15 39.06 34.00 3.42 9.54Kolubara 10 49.80 22.53 570.63 1.39 2.36 45.03 4.50 34.87 27.95 2.99 7.48Kolubara 13 59.34 25.10 749.81 2.07 0.94 50.35 7.63 52.15 37.05 3.28 7.54Kolubara 17 63.29 27.38 699.56 2.49 3.10 51.88 4.89 37.73 41.48 3.59 9.16Kolubara 23 49.62 34.63 868.93 2.28 2.26 37.81 5.20 36.39 35.69 3.25 9.46Kolubara 29 60.97 30.80 837.32 3.56 2.17 52.46 5.27 30.43 48.45 3.43 9.03Kolubara 34 60.28 34.12 1321.68 3.12 1.24 73.64 5.18 34.55 29.74 2.41 5.97Kolubara 36 82.64 32.46 1170.81 3.32 1.45 61.74 4.58 39.73 35.42 2.60 9.22Kostolac 1 25.21 25.83 770.09 6.37 0.55 9.34 4.72 102.93 19.51 1.72 11.75Kostolac 2 23.92 30.43 482.46 7.29 0.68 10.78 7.45 111.48 22.83 2.33 17.42Kostolac 4 17.98 22.37 508.23 6.19 0.86 10.13 6.33 107.65 20.22 2.37 15.35Kostolac 5 19.59 38.51 880.57 5.33 0.59 8.02 6.23 157.84 18.85 1.49 9.66Kostolac 7 43.74 39.47 727.87 16.06 1.84 11.98 6.76 130.37 30.88 3.24 21.06Kostolac 9 27.08 30.76 493.26 9.98 0.60 9.31 5.49 126.56 20.64 2.15 13.86Kostolac 11 20.64 25.83 517.65 4.56 0.50 7.86 4.06 95.44 15.54 1.47 10.64Kostolac 13 17.33 23.71 536.17 5.14 0.99 8.90 4.27 102.12 21.04 1.96 13.57Kostolac 15 15.64 21.69 573.73 3.84 0.76 6.38 6.89 130.62 18.23 1.64 12.99Kostolac 16 19.32 31.30 625.15 5.82 1.13 7.12 5.64 116.10 15.16 1.34 11.30Kostolac 17 19.69 23.82 436.91 8.12 0.46 8.31 6.42 102.98 22.51 2.53 15.95Kostolac 22 14.45 22.05 488.63 4.48 0.32 5.73 9.11 127.76 19.00 2.08 14.14Kostolac 23 46.38 43.82 1640.89 18.71 2.01 21.24 8.47 141.68 18.08 1.62 13.01

Correlation coefficients (r) for samples from the Kolubarar with HGI −0.09 −0.66 −0.30 0.23 −0.30 0.46 0.40 0.52 −0.39 0.38 −0.48r with matrix coal 0.35 −0.27 0.05 0.02 −0.18 0.63 −0.12 0.04 −0.44 −0.23 −0.43r with yellow type of xylite-rich coal −0.13 0.62 0.21 −0.18 0.27 −0.72 −0.40 −0.47 0.33 −0.24 0.50r with brown type of xylite-rich coal −0.43 0.02 −0.16 −0.15 0.14 −0.24 0.44 0.10 0.40 0.37 0.03r with black type of xylite-rich coal 0.43 0.41 0.16 −0.01 0.47 −0.17 −0.65 −0.56 0.49 −0.36 0.50r with (brown + black) type of xylite-rich coal −0.41 0.05 −0.15 −0.15 0.18 −0.25 0.40 0.05 0.45 0.35 0.06r with total xylite-rich coal −0.30 0.48 0.08 −0.21 0.28 −0.65 −0.10 −0.32 0.46 −0.01 0.40r with mineral-rich coal −0.24 −0.74 −0.43 0.50 −0.39 0.25 0.73 0.85 −0.23 0.69 −0.19

Correlation coefficients (r) for samples from the Kostolacr with HGI 0.40 0.15 −0.01 0.31 0.15 −0.05 −0.21 −0.11 0.35 0.27 0.22r with matrix coal 0.01 −0.13 −0.27 −0.02 0.08 0.08 −0.25 −0.48 0.19 0.34 0.34r with yellow type of xylite-rich coal −0.12 0.05 0.15 −0.02 0.11 0.19 0.18 0.05 −0.30 −0.31 −0.13r with brown type of xylite-rich coal −0.02 0.17 0.22 −0.04 −0.09 −0.12 0.32 0.43 −0.16 −0.30 −0.28r with black type of xylite-rich coal 0.00 −0.08 −0.07 0.08 −0.19 −0.12 0.01 0.24 0.00 0.12 −0.08r with (brown + black) type of xylite-rich coal −0.01 0.11 0.16 0.00 −0.17 −0.16 0.28 0.49 −0.14 −0.21 −0.28r with total xylite-rich coal −0.08 0.13 0.22 −0.01 −0.10 −0.04 0.36 0.47 −0.29 −0.36 −0.33r with mineral-rich coal 0.18 0.25 0.13 0.23 0.26 −0.07 0.25 0.48 0.20 0.09 0.14

Correlations, r N |0.55| for Kolubara samples and r N |0.51| for Kostolac samples, significant at the 0.05 level (Pearson test) are given in bold.

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Table 5Parameters calculated from distribution and abundance of n-alkanes and isoprenoids and their correlation with HGI and coal lithotypes.

Sample Paqa Pwaxb n-C23 /

(n-C27 + n-C31)Σn-C13 − n-C20 /Σn-C13 − n-C33

Σn-C21 − n-C25 /Σn-C13 − n-C33

Σn-C26 − n-C33 /Σn-C13 − n-C33

CPIc Pr/Phd

Kolubara 3 0.33 0.74 0.21 0.15 0.24 0.61 2.79 0.57Kolubara 7 0.30 0.77 0.17 0.15 0.21 0.63 2.36 1.11Kolubara 8 0.42 0.67 0.32 0.12 0.31 0.57 2.64 1.47Kolubara 9 0.36 0.71 0.27 0.11 0.27 0.62 2.72 1.09Kolubara 10 0.39 0.69 0.28 0.11 0.29 0.60 2.09 1.07Kolubara 13 0.40 0.68 0.31 0.08 0.30 0.61 3.11 1.18Kolubara 17 0.42 0.67 0.30 0.12 0.30 0.58 2.80 0.98Kolubara 23 0.39 0.69 0.28 0.12 0.28 0.59 2.40 0.77Kolubara 29 0.43 0.66 0.33 0.07 0.33 0.59 2.32 1.04Kolubara 34 0.35 0.72 0.25 0.09 0.27 0.64 2.43 0.74Kolubara 36 0.38 0.69 0.27 0.10 0.28 0.62 2.52 1.04Kostolac 1 0.40 0.69 0.28 0.30 0.22 0.48 2.79 0.23Kostolac 2 0.40 0.69 0.27 0.24 0.24 0.52 2.65 1.09Kostolac 4 0.40 0.69 0.25 0.23 0.25 0.52 2.94 1.15Kostolac 5 0.40 0.70 0.28 0.22 0.24 0.54 3.32 1.22Kostolac 7 0.41 0.68 0.26 0.36 0.21 0.44 3.33 1.43Kostolac 9 0.40 0.69 0.26 0.19 0.25 0.56 3.32 0.87Kostolac 11 0.40 0.69 0.28 0.35 0.21 0.44 3.06 1.20Kostolac 13 0.38 0.71 0.26 0.26 0.22 0.52 3.22 1.00Kostolac 15 0.39 0.71 0.24 0.30 0.21 0.49 2.97 1.44Kostolac 16 0.37 0.71 0.25 0.29 0.21 0.50 3.37 1.30Kostolac 17 0.42 0.68 0.27 0.26 0.24 0.50 3.28 1.07Kostolac 22 0.40 0.69 0.26 0.32 0.22 0.46 2.79 0.19Kostolac 23 0.49 0.62 0.37 0.21 0.30 0.49 2.65 0.92

Correlation coefficients (r) for samples from the Kolubarar with HGI −0.56 0.58 −0.62 0.49 −0.59 0.39 −0.43 0.27r with matrix coal −0.17 0.15 −0.13 −0.03 −0.06 0.29 0.03 0.21r with yellow type of xylite-rich coal 0.48 −0.48 0.45 −0.19 0.40 −0.47 0.30 −0.13r with brown type of xylite-rich coal 0.04 0.01 0.09 −0.09 0.01 −0.05 −0.04 0.00r with black type of xylite-rich coal 0.66 −0.67 0.64 −0.52 0.72 −0.44 0.21 −0.13r with (brown + black) type of xylite-rich coal 0.09 −0.05 0.14 −0.13 0.07 −0.08 −0.02 −0.01r with total xylite-rich coal 0.40 −0.38 0.40 −0.21 0.33 −0.39 0.21 −0.10r with mineral-rich coal −0.78 0.81 −0.86 0.70 −0.84 0.41 −0.71 −0.20

Correlation coefficients (r) for samples from the Kostolacr with HGI 0.04 −0.09 −0.04 0.63 −0.40 −0.60 −0.03 −0.05r with matrix coal −0.18 0.08 −0.22 0.00 −0.01 0.02 0.06 0.23r with yellow type of xylite-rich coal 0.11 −0.05 0.20 −0.50 0.34 0.47 −0.14 0.25r with brown type of xylite-rich coal 0.11 −0.06 0.23 0.25 −0.14 −0.27 −0.24 −0.34r with black type of xylite-rich coal 0.18 −0.12 −0.05 −0.07 0.12 0.02 0.43 −0.01r with (brown + black) type of xylite-rich coal 0.18 −0.10 0.18 0.18 −0.06 −0.23 −0.01 −0.30r with total xylite-rich coal 0.22 −0.12 0.27 −0.11 0.13 0.05 −0.08 −0.13r with mineral-rich coal 0.06 0.02 −0.11 0.15 −0.15 −0.09 −0.33 0.43

a Paq = (n-C23 + n-C25) / (n-C23 + n-C25 + n-C29 + n-C31) (Ficken et al., 2000).b Pwax = (n-C27 + n-C29 + n-C31) / (n-C23 + n-C25 + n-C27 + n-C29 + n-C31) (Zheng et al., 2007).c CPI — Carbon Preference Index determined for full distribution of n-alkanes C15–C33, CPI = 1/2 [Σodd(n-C15 − n-C33) / Σeven(n-C14 − n-C32) + Σodd(n-C15 − n-C33) / Σeven(n-

C16 − n-C34)] (Bray and Evans, 1961).d Pr/Ph— pristane/phytane; correlations, r N |0.55| for Kolubara samples and r N |0.51| for Kostolac samples, significant at the 0.05 level (Pearson test) are given in bold.

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Prominent C31αβ(R) hopane is often reported in low rankcoals (Stefanova et al., 2005a; Vu et al., 2009). Higher proportion ofC31αβ(R) hopane in the sum of total hopanoids in the Kolubarathan in the Kostolac samples (Fig. 7; Table 6) suggests thatpeatification of Kolubara lignite took place in more oxic and acidicenvironments. This result is in accordance with the low sulfurcontent (Table 3).

The contents of hopanes are considered to reflect the level of thedegradation of organic matter by aerobic bacteria, whereas for theC30hop-17(21)-ene a microbial origin from anaerobic (iron-reducing)bacteria is assumed (Wolff et al., 1992). The C30 hop-17(21)-ene/C30

hopane ratio considerably higher than 1, followed by predominanceof C31αβ(R)-hopane in almost all samples (Table 6) may implypeatification firstly in a suboxic environment, and after some time amicrobial degradation of organic matter by anaerobic (iron-reducing)bacteria. Higher values of C30 hop-17(21)-ene/C30 hopane in coal fromthe Kostolac indicates that Kostolac lignite was formed under morereducing conditions in comparison to the Kolubara lignite.

The ratio of 17β(H)21β(H) to (17β(H)21β(H) + 17α(H)21β(H))C30-hopanes is within the limits established for lignite (0.5–0.7;

Mackenzie et al., 1981; Table 6), consistent with low thermal maturityof the OM.

4.3.2.3. Sesquiterpenoids, diterpenoids and triterpenoids with non-hopanoid skeleton. In all studied samples, sesquiterpenoids occur atrelatively low quantities (Figs. 5, 6). These biomarkers are generallymore abundant in saturated than in aromatic fraction (Table 4), indicat-ing relatively low degree of aromatization. Saturated fractions containnordrimane, elemane, muurolane, eudesmane, cedrane, cuparane,dihydrovalencene, longifolane, dihydroselinene and cadinane (Fig. 5),whereas sesquiterpenoid type constituents of aromatic fractions aredihydro-ar-curcumene, gamma-cadinene, calamenene, cuparene,cadina-1(10),6,8-triene, tetrahydrocadalene, cadalene and isocadalene(Fig. 6). Cadalene predominates among the aromatic sesquiterpenoidsin all samples. The presence of cuparene in all samples (Fig. 6) clearly in-dicates contribution of conifers family Cupressaceae, and family generaCupressus, Thuja and Juniperus as precursors to OM(Haberer et al., 2006;Otto and Wilde, 2001).

Diterpenoids are main constituents of both, saturated and aro-matic fractions in all samples, with exception of the Kolubara 17

Table 6Parameters calculated from distributions and abundances of hopanoid and steroid biomarkers and their correlation with HGI and coal lithotypes.

Sample Hopanoid maximum(m/z 191)

% C31αβa C30ββ/C30

(ββ + αβ)bC30Hop-17(21)-ene/C3017α(H)21β(H)-hopane

Steroids/hopanoidsc

Kolubara 3 C31 αβ(R) hopane 71.98 0.63 10.90 0.03Kolubara 7 C31 αβ(R) hopane 72.99 0.66 11.48 0.02Kolubara 8 C31 αβ(R) hopane 69.39 0.65 11.17 0.03Kolubara 9 C31 αβ(R) hopane 68.90 0.68 10.62 0.05Kolubara 10 C31 αβ(R) hopane 69.35 0.67 11.74 0.05Kolubara 13 C31 αβ(R) hopane 68.41 0.74 16.64 0.02Kolubara 17 C31 αβ(R) hopane 68.86 0.74 13.45 0.06Kolubara 23 C31 αβ(R) hopane 65.63 0.64 13.52 0.06Kolubara 29 C31 αβ(R) hopane 68.41 0.75 14.10 0.04Kolubara 34 C31 αβ(R) hopane 73.03 0.69 11.70 0.02Kolubara 36 C31 αβ(R) hopane 69.15 0.78 17.18 0.02Kostolac 1 C31 αβ(R) hopane 36.89 0.64 16.59 0.06Kostolac 2 C31 αβ(R) hopane 33.53 0.57 14.72 0.06Kostolac 4 C31 αβ(R) hopane 34.84 0.68 23.21 0.09Kostolac 5 C31 αβ(R) hopane 26.95 0.57 21.51 0.07Kostolac 7 C30Hop-17(21)-ene 19.94 0.60 18.66 0.15Kostolac 9 C30Hop-17(21)-ene 18.34 0.73 27.56 0.06Kostolac 11 C31 αβ(R) hopane 35.09 0.67 22.93 0.06Kostolac 13 C31 αβ(R) hopane 39.29 0.65 24.30 0.11Kostolac 15 C31 αβ(R) hopane 33.44 0.72 24.22 0.12Kostolac 16 C31 αβ(R) hopane 30.65 0.65 26.29 0.16Kostolac 17 C31 αβ(R) hopane 36.84 0.71 19.73 0.05Kostolac 22 C31 αβ(R) hopane 39.58 0.83 54.55 0.06Kostolac 23 C31 αβ(R) hopane 42.46 0.73 26.26 0.09

Correlation coefficients (r) for samples from the Kolubarar with HGI 0.71 −0.06 −0.18 −0.45r with matrix coal 0.62 0.10 −0.22 −0.43r with yellow type of xylite-rich coal −0.84 −0.01 0.29 0.56r with brown type of xylite-rich coal −0.19 −0.04 −0.03 0.18r with black type of xylite-rich coal −0.38 0.41 0.13 0.41r with (brown + black) type of xylite-rich coal −0.23 −0.01 −0.02 0.22r with total xylite-rich coal −0.73 −0.02 0.21 0.52r with mineral-rich coal 0.56 −0.30 −0.13 −0.42

Correlation coefficients (r) for samples from the Kostolacr with HGI −0.37 0.03 −0.12 0.18r with matrix coal −0.10 −0.36 −0.36 0.06r with yellow type of xylite-rich coal 0.35 −0.01 −0.08 0.11r with brown type of xylite-rich coal 0.17 0.28 0.55 −0.09r with black type of xylite-rich coal −0.34 0.52 0.30 −0.27r with (brown + black) type of xylite-rich coal −0.01 0.48 0.61 −0.20r with total xylite-rich coal 0.18 0.44 0.51 −0.12r with mineral-rich coal −0.35 0.06 −0.07 0.49

a % C31αβ = C3117α(H)21β(H)22(R)-hopane/Σhopanoids, calculated frommass chromatogram, m/z 191.b C30ββ/C30(ββ + αβ) = C3017β(H)21β(H)-hopane/(C3017β(H)21β(H)-hopane + C3017α(H)21β(H)-hopane), calculated from mass chromatogram,m/z 191 (Mackenzie et al.,

1981).c Steroids/hopanoids = Σsteroids/Σhopanoids, calculated from the TIC of saturated fraction; correlations, r N |0.55| for Kolubara samples and r N |0.51| for Kostolac samples, significant

at the 0.05 level (Pearson test) are given in bold.

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and Kolubara 29, where non-hopanoid triterpenoids predominate inaromatic fraction (Figs. 5, 6; Table 4). Moreover, total diterpenoids(sum of diterpenoids in saturated and aromatic fractions) representthe most abundant hydrocarbons in bitumen, indicating a significantcontribution of gymnosperms to the precursor OM (Table 7). As inthe case of sesquiterpenoids, diterpenoids are more abundant insaturated than in aromatic fraction, suggesting relatively low degreeof aromatization (Table 4). In all samples distributions of individualditerpenoids in saturated fractions are similar (Fig. 5). The 16α(H)-phyllocladane and pimarane are dominant by far. Other diterpenoidtype constituents of saturated fraction are isopimaradienes,norisopimarane, norpimarane, norabietane, beyerane, dihydrorimuene,isopimarane, isophyllocladene, abietane, 16β(H)-phyllocladane and16α(H)-kaurane (Fig. 5). A high amount of 16α(H)-phyllocladane indi-cates that coal forming plants belonged to one or several of the coniferfamilies Taxodiaceae, Podocarpaceae, Cupressaceae, Araucariaceae,Sciadopityaceae and Phyllocladaceae, while the high abundance ofpimarane suggests Pinaceae, Taxodiaceae and/or Cupressaceae (Ottoand Wilde, 2001; Otto et al., 1997; Stefanova et al., 2002; Stefanovaet al., 2005b).

Distributions of individual diterpenoids in aromatic fractions ofall samples are similar (Fig. 6). The aromatic diterpenoids consistof norabieta-6,8,11,13-tetraenes, norabieta-8,11,13-trienes, 16,17-bisnordehydroabietane, 16,17-bisnorsimonellite, dehydroabietane,simonellite, retene, sempervirane, totarane, hibaene and 2-methylretene. Simonellite and dehydroabietane are predominantcompounds in all samples from the Kolubara, while simonellite is adominant aromatic diterpenoid in all samples from the Kostolac(Fig. 6). The presence of totarane and hibaene in the aromatic frac-tion of all samples (Fig. 6) clearly indicates the contribution ofCupressaceae, Taxodiaceae, Podocarpaceae and/or Araucariaceae toprecursor biomass (Otto and Wilde, 2001). This is consistent withthe observation derived from the analysis of saturated biomarkers.

The non-hopanoid triterpenoids are present in saturated fraction ofall samples in relatively low amounts (Table 4). On the other hand,these compounds predominate in the aromatic fraction of two samplesfrom the Kolubara (17 and 29) and after diterpenoids represent themost abundant aromatic biomarkers in the remaining samples(Table 4). This result shows that angiosperms also contributed to theorganic matter. Considerably higher abundance of aromatized in

Table 7Total content of major biomarkers in saturated and aromatic fractions (μg/gCorg), parameters calculated from distribution and abundance of diterpenoids and non-hopanoid triterpenoids,Conifer Wood Degradation Index and their correlation with HGI and coal lithotypes.

Sample Sesquiterpenoids Diterpenoids Non-hopanoidtriterpenoids

Steroids Hopanoids Di/(Di + Tri)sata

Di/(Di + Tri)aromb

Di/(Di + Tri)sat + arom

CWDIc

Kolubara 3 21.58 548.87 31.21 10.28 69.55 0.994 0.71 0.946 0.014Kolubara 7 21.32 518.28 37.30 8.17 66.25 0.993 0.66 0.933 0.009Kolubara 8 29.15 706.09 30.09 8.96 62.53 0.997 0.58 0.959 0.012Kolubara 9 33.17 794.14 35.29 12.69 67.05 0.998 0.53 0.957 0.016Kolubara 10 27.04 605.50 29.34 9.84 52.52 0.998 0.56 0.954 0.014Kolubara 13 32.73 801.96 39.12 8.48 57.89 0.997 0.58 0.953 0.010Kolubara 17 32.27 737.29 43.96 12.26 61.04 0.996 0.48 0.944 0.018Kolubara 23 39.83 905.32 37.98 11.72 47.27 0.997 0.50 0.960 0.019Kolubara 29 36.07 867.75 52.02 11.20 61.49 0.996 0.39 0.943 0.018Kolubara 34 39.30 1356.23 32.86 7.21 79.61 0.998 0.54 0.976 0.011Kolubara 36 37.05 1210.53 38.74 10.67 70.96 0.997 0.53 0.969 0.021Kostolac 1 30.55 873.03 25.88 12.30 21.10 0.992 0.84 0.971 0.003Kostolac 2 37.88 593.94 30.12 18.10 28.20 0.985 0.83 0.952 0.003Kostolac 4 28.70 615.88 26.41 16.22 25.48 0.988 0.84 0.959 0.004Kostolac 5 44.74 1038.41 24.18 10.25 17.68 0.994 0.89 0.977 0.002Kostolac 7 46.23 858.24 46.94 22.91 33.04 0.978 0.81 0.948 0.006Kostolac 9 36.25 619.82 30.62 14.47 23.18 0.980 0.86 0.953 0.004Kostolac 11 29.89 613.09 20.10 11.14 18.50 0.991 0.86 0.968 0.003Kostolac 13 27.98 638.29 26.19 14.57 22.48 0.990 0.83 0.961 0.002Kostolac 15 28.58 704.35 22.07 13.75 19.37 0.993 0.88 0.970 0.002Kostolac 16 36.95 741.25 20.98 12.43 18.42 0.991 0.88 0.972 0.002Kostolac 17 30.24 539.89 30.63 16.40 24.25 0.982 0.82 0.946 0.003Kostolac 22 31.16 616.39 23.49 14.46 19.86 0.991 0.87 0.963 0.002Kostolac 23 52.29 1782.57 36.78 15.02 34.25 0.989 0.89 0.980 0.003

Correlation coefficients (r) for samples from the Kolubarar with HGI −0.67 −0.28 −0.34 −0.45 0.42 −0.53 0.57 −0.27 −0.42r with matrix coal −0.35 0.06 −0.42 −0.37 0.61 −0.01 0.26 0.20 −0.22r with yellow type of xylite-rich coal 0.62 0.20 0.29 0.46 −0.68 0.41 −0.51 0.19 0.50r with brown type of xylite − rich coal 0.13 −0.16 0.37 0.07 −0.25 −0.02 −0.13 −0.33 −0.32r with black type of xylite-rich coal 0.33 0.13 0.47 0.54 −0.11 0.32 −0.67 0.03 0.63r with (brown + black) type of xylite − rich coal 0.15 −0.15 0.41 0.12 −0.26 0.00 −0.18 −0.33 −0.27r with total xylite-rich coal 0.53 0.07 0.41 0.40 −0.63 0.30 −0.46 −0.02 0.24r with mineral − rich coal −0.68 −0.40 −0.16 −0.29 0.24 −0.87 0.71 −0.49 −0.30

Correlation coefficients (r) for samples from the Kostolacr with HGI 0.10 −0.02 0.38 0.23 0.09 −0.38 −0.29 −0.20 0.48r with matrix coal −0.17 −0.29 0.09 0.25 0.33 −0.23 −0.44 −0.32 0.40r with yellow type of xylite-rich coal 0.08 0.14 −0.17 −0.11 0.06 0.13 0.24 0.11 −0.53r with brown type of xylite-rich coal 0.22 0.24 −0.11 −0.28 −0.24 0.32 0.40 0.34 −0.43r with black type of xylite − rich coal −0.07 −0.06 0.05 −0.10 −0.13 −0.23 0.16 −0.12 0.37r with (brown + black) type of xylite-rich coal 0.16 0.18 −0.07 −0.29 −0.27 0.17 0.42 0.24 −0.20r with total xylite-rich coal 0.19 0.24 −0.16 −0.32 −0.21 0.22 0.51 0.28 −0.47r with mineral − rich coal 0.28 0.15 0.25 0.17 0.03 −0.18 0.16 −0.01 0.11

a Di/(Di + Tri) sat = Σditerpenoids / (Σditerpenoids + Σtriterpenoids), calculated from the TIC of saturated fraction, Σditerpenoids = (16α(H)-phyllocladane + pimarane +isopimaradienes + 4β(H)-19-norisopimarane +4α(H)-18-norisopimarane + norpimarane + norabietane + beyerane + dihydrorimuene + isopimarane + abietane +isophyllocladene + 16β(H)-phyllocladane + 16α(H)-kaurane), Σtriterpenoids = (des-A-olean-13(18)-ene + des-A-olean-12-ene + des-A-urs-13(18)-ene + des-A-olean-18-ene + des-A-urs-12-ene + des-A-lupane).

b Di/(Di + Tri) arom = Σaromatic diterpenoids / (Σaromatic diterpenoids + Σaromatic triterpenoids), calculated from the TIC of aromatic fraction (Haberer et al., 2006; Nakamuraet al., 2010), Σaromatic diterpenoids = (19-norabieta-6,8,11,13-tetraene + 18-norabieta-6,8,11,13-tetraene + 19-norabieta-8,11,13-triene + 18-norabieta-8,11,13-triene + 16,17-bisnordehydroabietane + 16,17-bisnorsimonellite + dehydroabietane + simonellite + retene + 1,2,3,4-tetrahydroretene + sempervirane + totarane + hibaene + 2-methylretene), Σaromatic triterpenoids = (24,25-dinoroleana-1,3,5(10),12-tetraene + 24,25-dinorursa-1,3,5(10),12-tetraene + 24,25-dinorlupa-1,3,5(10)-triene +3,4,7,12a-tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysene + 3,3,7,12a-tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysene + 3,4,7-trimethyl-1,2,3,4-tetrahydrochrysene + 3,3,7-trimethyl-1,2,3,4-tetrahydrochrysene + 1,2,4a,9-tetramethyl-1,2,3,4,4a,5,6,14b-octahydropicene + 2,2,4a,9-tetramethyl-1,2,3,4,4a,5,6,14b-octahydropicene + 1,2,9-trimethyl-1,2,3,4-tetrahydropicene + 2,2,9-trimethyl-1,2,3,4-tetrahydropicene + triaromatic des-A-lupane + pentamethyldecahydrochrysene).

c CWDI— ConiferWood Degradation Index; CWDI = perylene / (perylene + cadalene + retene + simonellite + dehydroabietane) (Marynowski et al., 2013); correlations, r N |0.55| forKolubara samples and r N |0.51| for Kostolac samples, significant at the 0.05 level (Pearson test) are given in bold.

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comparison to non-aromatized angiosperm triterpenoids (Table 4) in-dicates intense aromatization of triterpenoids during diagenesis. Thesame observation was also reported by Kalkreuth et al. (1998) andNakamura et al. (2010), who showed that aliphatic angiosperm-derived triterpenoids are more easily altered to aromatic derivativesthan gymnosperm-derived diterpenoids, resulting in the selective lossof analogous aliphatic compounds.

The non-hopanoid triterpenoids in saturated fraction of all samplesare present in low amounts (Table 4) and consist of des-A-olean-13(18)-ene, des-A-olean-12-ene, des-A-olean-18-ene, des-A-urs-13(18)-ene, des-A-urs-12-ene and des-A-lupane (Fig. 5). Degradationof triterpenoids' A-ring implies intense microbial activity.

Distributions of individual non-hopanoid terpenoids in aromaticfractions of all samples are similar (Fig. 6). The following aromatictetra- and pentacyclic triterpenoids occur in the aromatic hydrocarbonfractions: ring-A-monoaromatic triterpenoids (24,25-dinoroleana-1,3,5(10),12-tetraene, 24,25-dinorursa-1,3,5(10),12-tetraene,24,25-dinorlupa-1,3,5(10)-triene), tetramethyloctahydrochrysenes,trimethyltetrahydrochrysenes, tetramethyloctahydropicenes,trimethyltetrahydropicenes, triaromatic des-A-lupane andpentamethyldecahydrochrysene. 24,25-Dinoroleana-1,3,5(10),12-tetraene and 24,25-dinorlupa-1,3,5(10)-triene are predominantcompounds in all samples (Fig. 6). Pentacyclic triterpenoids are no-tably more abundant than tetracyclic chrysene derivatives in all

Fig. 8. Correlation diagrams of Hardgrove Grindability Index (HGI) vs. yellow xylite-rich coal (a), total xylite-rich coal (b), matrix coal (c) and organic carbon content (d) for the Kolubaraand Kostolac samples.

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samples. This result shows that the main pathway of aromatizationwas progressive aromatization (Stout, 1992). The most prominentpeaks are ring-A-monoaromatic triterpenoids (Fig. 6), whichconfirms its relatively high stability observed also in earlier investi-gations (Stefanova et al., 2005a; Stout, 1992).

Due to the enhanced aromatization of angiosperm derivedtriterpenoids, the ratio of diterpenoids to sum of di- and terpenoid in

Fig. 9. Correlation diagrams of Hardgrove Grindability Index (HGI) vs. textinite content(vol.%) for the Kolubara and Kostolac samples.

saturated fraction, Di/(Di + Tri)sat shows extremely high and uniformvalues (above 0.97; Table 7) in all studied samples, indicating that coni-fers nearly exclusively contributed to coal formation. Therefore, in orderto estimate the contribution of gymnosperm and angiosperm vegetationthe ratio of diterpenoids and angiosperm-derived triterpenoids aromaticbiomarkers (Di/(Di + Tri)arom) (Haberer et al., 2006; Nakamura et al.,2010) and the ratio of total diterpenoids and total non-hopanoidtriterpenoids (Di/(Di + Tri)sat + arom) (Bechtel et al., 2002, 2003;Table 7) were used. The lower values of both ratios, particularly thosebased on aromatic counterparts in coal from the Kolubara indicate in-creasing proportion of angiosperms within the peat-forming vegetation.This result suggests that peatification of the Kolubara lignite in dryerand more oxic environment is consistent with low sulfur content(Table 3) and prominent C31 17α(H)21β(H)22(R)-hopane (Table 6).

4.3.2.4. Perylene. Perylene is a common aromatic biomarker in lowmatu-rity organic matter (OM) of terrestrial origin (e.g. Aizenshtat, 1973;Golovko et al., 1999; Jiang et al., 2000). It is identified in low amountsin all studied samples (Fig. 6). The probably biological precursors ofthe perylene are fungi (Grice et al., 2009; Louda and Baker, 1984;Suzuki et al., 2010). Recent investigations showed that terrestrial OMcould be degraded after deposition by fungi under aquatic conditions(Fan et al., 2011; Itoh et al., 2012). Also, white rot fungi and brown rotfungi develop a unique wood-degrading system that can split bothcellulose and lignin into metabolisable fragments (Enoki et al., 1997).

Conifer Wood Degradation Index (CWDI; Marynowski et al., 2013)expressed as ratio of contents of perylene and sum of perylene,

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cadalene, retene, simonellite and dehydroabietane (Table 7) wasproposed as a parameter for the degree of fossil wood degradation bywood-degrading fungi during decay, transport and early diagenesis.Higher values of CWDI in the samples from the Kolubara than in coalfrom the Kostolac (Table 7) imply more intense degradation of coniferby wood-degrading fungi, which most probably resulted from moreoxic environment. This result is consistent with low sulfur content(Table 3), higher contributionof angiosperms (Tables 4, 7) andprominentC31 17α(H)21β(H)22(R)-hopane (Table 6) in the Kolubara samples.

5. Discussion

5.1. Lithotype composition and Hardgrove Grindability Index

Hardgrove Grindability Index (HGI) has uniform values in thesamples from the Kostolac. The samples from the Kolubara also showrelatively uniform values of HGI, with an exception of two samplesKolubara 23 and Kolubara 29 which are characterized by HGI b35.8,and sample Kolubara 7, reaching HGI 56.9 (Table 1).

A significant negative linear correlation between dark yellow type ofxylite-rich coal and Hardgrove Grindability Index (r = −0.77) forKolubara coals is observed (Table 1).Moderate negative linear correlationbetween dark yellow type and HGI (r =−0.57) is observed in coal fromtheKostolac (Table 1). All xylite types, aswell as total xylite-rich coal fromthe Kolubara have a negative impact on the grindability properties (r =−0.79), while only yellow types (pale and dark yellow type of xylite-rich coal) from the Kostolac have a negative influence on HGI (r =−0.57) (Table 1; Fig. 8a, b). Therefore it can be concluded that the yellowtypes of xylite-rich coal have the worst impact on grindability properties,particularly in the Kolubara basin.

Matrix coal from theKolubara demonstrated a positive effect onHGI,whereas in the case of the Kostolac coal a slight negative influence isobserved (Table 1; Fig. 8c). The variation of the matrix coal impact ongrindability is related to its great heterogeneity as recently reportedby Oikonomopoulos et al. (2013).

Dopplerite coal, black very bright non-stratified lithotype, mademostly from gelinite and char coal seem to not have a significant impacton HGI (Table 1).

Mineral-rich coal, especially from the Kolubara has a positive impacton HGI (r = 0.69) (Table 1).

Mineral matter, determined over the stereo microscope, as allmentioned lithotypes and xylite types, particularly from the Kolubarahas a positive impact on HGI (r = 0.72) (Table 1). HGI is controlled byamount and type of mineral matter. The mineral matter from theKolubara mainly consists of quartz, while clays and feldspar are lessabundant. The mineral matter from the Kostolac mainly consists ofbentonitic clays (Simić et al., 1997) and quartz. Therefore, our resultsare in concordance with the results of previous investigations (Howeret al., 1987; Ural and Akyildiz, 2004) which also showed that quartzhas a positive impact on grindability.

5.2. Maceral composition and Hardgrove Grindability Index

Significant negative linear correlations for Kolubara (r=−0.83) andfor Kostolac (r = −0.70) between textinite content and HGI (Table 2;Fig. 9) are observed, suggesting a negative impact on HGI. It is consistentwith a high content of xylite-rich yellow type (Table 1). Contents of othermacerals do not show clear and coherent influence on grindabilityproperties of the Kolubara and Kostolac samples (Table 2).

5.3. Organic geochemical properties and Hardgrove Grindability Index

5.3.1. Bulk organic geochemical parameters and Hardgrove GrindabilityIndex

Negative linear correlations for the Kolubara (r = −0.90 and−0.64) and for the Kostolac (r = −0.65 and −0.58) between organic

carbon contents (Corg, dry basis and dry, mineral-matter-free basis, re-spectively) and HGI (Table 3; Fig. 8d) suggest a negative impact of Corgon HGI. The correlation analysis showed a positive correlation betweenCorg and yellow type, as well as total xylite-rich coal in the Kolubarasamples, whereas a slight positive correlation between Corg and yellowxylite type is observed in the Kostolac samples (Table 3). Since it wasshowed that the yellow type and total xylite-rich coal in the Kolubarasamples, as well as the yellow type in the Kostolac samples hindergrindability properties (Table 1), the negative impact of mentionedlithotypes can be attributed to an elevated content of organic carboncontent.

Slight negative correlations between contents of soluble organicmatter (bitumen) and HGI (r = −0.39) are observed for the Kolubarasamples (Table 3). This result is expected, due to unfavorable mechani-cal properties of bitumen. Considering slight positive correlations ofbitumen content with yellow and black types of xylite-rich coal in theKolubara samples (Table 3), it can be assumed that a negative impactof the xylite-rich coal on HGI in part can be related to the high contentof soluble organic matter.

The contents of saturated and aromatic hydrocarbons showed posi-tive impact on HGI in the Kolubara samples, whereas in the case of theKostolac lignite there is no correlation. The content of asphaltenesshowed a slight negative influence on HGI in both basins (Table 3).However, no statistically significant relationship between asphaltenecontent and different coal lithotype is observed (Table 3).

Matrix coal does not show any significant correlation with bulk geo-chemical parameters in both basins, whereas, as expected, a negativecorrelation between Corg and mineral-rich coal is observed (Table 3).

5.3.2. Molecular composition of the organic matter and HardgroveGrindability Index

5.3.2.1. n-Alkanes and HGI. The content of total n-alkanes does not influ-ence grindability properties of both the Kolubara and Kostolac lignites(Table 4). The Kolubara samples showed a negative correlation betweenHGI and parameters related to abundance of mid-chain n-alkanes (Paq,n-C23 / (n-C27+ n-C31),Σn-C21− n-C25 /Σn-C13− n-C33), and a positivecorrelation HGI vs. Pwax (Table 5). This result is followed by a slight pos-itive correlation between the yellow type, black type and total xylite-rich coal from the Kolubara and Paq, n-C23/(n-C27 + n-C31), as well asΣn-C21 − n-C25/Σn-C13 − n-C33 (Table 5). On the other hand, xylite-rich coal in the Kolubara samples demonstrated a negative correlationwith Pwax (Table 5). Therefore, it can be assumed that the negative im-pact of the Kolubara xylite-rich coal on HGI in part can be related to themid-chain n-alkanes. In samples from the Kostolac short-chain n-alkanes (parameter Σn-C13 − n-C20/Σn-C13 − n-C33) have positiveimpact on HGI, whereas yellow type of xylite-rich coal which showedthe most negative impact on HGI demonstrated a negative correla-tion with content of short-chain n-alkanes (Table 5). Matrix coalfrom both basins does not show any correlation with distributionof n-alkanes. Mineral-rich coal from the Kolubara positively corre-lated with Pwax, amount of short- and long-chain n-alkanes, where-as a negative correlation with content of mid-chain n-alkanes andCPI is observed (Table 5).

5.3.2.2. Hopanoids, steroids and HGI. The contents of hopanoids in satu-rated and aromatic fractions, as well as the content of total hopanoids(in saturated and aromatic fractions) do not influence grindability prop-erties of Kostolac lignite (Tables 4, 7). In the samples from the Kolubaraa slight positive correlation between HGI and contents of hopanoids insaturated fractions as well as total hopanoids is observed, whereas thecontent of aromatic hopanoids showed a slight negative impact onHGI (Tables 4, 7). The aromatic hopanoids have a slight positive correla-tion with yellow and black types of xylite-rich coal, as well as totalxylite-rich coal in the Kolubara (Table 4), indicating that a negative im-pact of xylite-rich coal on HGI in the Kolubara samples can be partly

361D. Životić et al. / International Journal of Coal Geology 131 (2014) 344–362

related to aromatization of hopanoids. On the other hand, Kolubaraxylite-rich coal showed a negative correlation with the contents ofhopanoids in saturated fractions and total hopanoids, which demon-strated a positive impact on HGI (Tables 4, 7). Matrix coal from theKolubara positively correlated with the contents of hopanoids insaturated fractions and total hopanoids, whereas a slight negative corre-lation with the content of aromatic hopanoids is observed (Tables 4, 7),implying that a positive impact of matrix coal on HGI in the Kolubarasamples (Table 1) in part can be related to the content of non-aromatic hopanoids.

The content of steroids in saturated and aromatic fractions aswell asthe content of total steroids do not show influence onHGI in both basins(Tables 4, 7).

5.3.2.3. Sesquiterpenoids, diterpenoids, triterpenoids with non-hopanoidskeleton and HGI. Significant statistical correlations between HGI andthe contents of total sesquiterpenoids, diterpenoids, and triterpenoidswith non-hopanoid skeleton (in saturated and aromatic fractions) aswell as the contents of these biomarkers in saturated and aromatic frac-tions do not exist for lignites from the Kostolac (Tables 4, 7). On theother hand, the content of total sesquiterpenoids and the content ofsesquiterpenoids in saturated fraction demonstrated a negative correla-tion with HGI in the Kolubara samples (Tables 4, 7). A positive correla-tion of total sesquiterpenoids and the content of sesquiterpenoids insaturated fraction with yellow type of xylite-rich coal, as well as withtotal xylite-rich coal in the Kolubara samples (Tables 4, 7) indicatethat a negative impact of yellow type and total xylite-rich coal on HGIcan be attributed to the content of sesquiterpenoids. Matrix coalshowed a slight negative correlation with total sesquiterpenoids,whereas mineral coal demonstrated a significant negative correlationwith total sesquiterpenoids in the Kolubara samples (Table 7).

The correlation analysis of the Kolubara samples indicated a positiveimpact of the content of aromatic diterpenoids on HGI, and a slightnegative impact of the content of aromatic non-hopanoid triterpenoidson grindability properties, which resulted in a positive correlation ofDi/(Di + Tri)arom ratio and HGI (Tables 4, 7). This result is followedby a negative correlation of the content of aromatic diterpenoids, aswell as Di/(Di + Tri)arom ratio with the yellow type, black type andtotal xylite-rich coal and a slight positive correlation between totalxylite-rich coal and content of aromatic non-hopanoid triterpenoids(Tables 4 and 7), indicating that the negative impact of xylite lithotypeon HGI in the Kolubara samples resulted from elevated contents of aro-matic non-hopanoid triterpenoids. Matrix coal from the Kolubarashowed a slight negative correlation with the content of aromaticnon-hopanoid triterpenoids, whereas a significant positive correlationbetween mineral-rich coal and content of aromatic diterpenoids, aswell as Di/(Di + Tri)arom ratio is observed (Tables 4 and 7).

CWDI showed a slight negative correlation with HGI in the Kolubarasamples, whereas for Kostolac lignite a slight positive correlation is observed(Table 7). A positive correlation between CWDI and xylite-rich coal (yellowand black type) indicates that the negative impact of xylite lithotype fromthe Kolubara on HGI in part can be related to more intense degradation ofwood tissues. Kostolac lignite showed an opposite trend, i.e. negative correla-tion is observed between xylite-rich coal and CWDI, whereas matrix coaldemonstrated a slight positive correlation with CWDI (Table 7).

6. Conclusions

The impact of different coal lithotypes (matrix coal, yellow-, brown-and black types of xylite-rich coal) on grindability properties has beenstudied. Coal lithotypes were isolated from lignite from two mainUpperMiocene lignite basins in Serbia (Kolubara andKostolac). The yel-low type of xylite-rich coal demonstrated the most negative impact onHardgrove Grindability Index (HGI). All xylite types, as well as totalxylite-rich coal from the Kolubara basin have a negative impact on thegrindability properties, while only the yellow xylite types from the

Kostolac showed a negative impact on HGI. Matrix coal does not showa consistent effect on HGI.

Textinite content hinders HGI in both basins, whereas contents ofother macerals do not show influence on grindability properties.

The negative impact of the yellow type and sum of total xylite-richcoal on the grindability properties in part can be attributed to highamounts of total organic carbon and soluble organic matter. Matrixcoal does not show any significant correlation with bulk geochemicalparameters in both basins.

Peat-forming vegetationof all samples frombothbasins dominated bydecay of resistant gymnosperm (coniferous) plants. Coal forming plantsbelonged to one or several of the gymnosperm families Taxodiaceae,Podocarpaceae, Cupressaceae, Araucariaceae, Phyllocladaceae andPinaceae. All samples from the Kolubara are characterized by a highercontribution of angiosperm vegetation than coal from the Kostolac.Peatification of the Kolubara coal was performed in a more oxic environ-ment undermore intensemicrobial activity. Molecular organic geochem-ical data indicated that the negative impact of all xylite types from theKolubara on HGI in part can be related to higher proportion of angio-sperms and peatification in more oxic environments which generallycontributed to more intense degradation of wood tissues. Moreover, thecorrelation analysis of HGI, xylite-rich coal and biomarker parameterssuggested that elevated abundance of mid-chain n-alkanes andsesquiterpenoids, as well as aromatization of non-hopanoid triterpenoidsand hopanoids also caused lowering of grindability properties. The posi-tive impact of matrix coal on HGI in the Kolubara samples in part can beattributed to the elevated amount of non-aromatic hopanoids and lowcontents of aromatic non-hopanoid triterpenoids and sesquiterpenoids.

Variable correlations of biomarker compositions with HGI and coallithotypes in the Kolubara and Kostolac samples indicate that mechani-cal properties of coal considerably depend on its chemical composition.

Acknowledgments

This work was financed by the Ministry of Education and Science ofthe Republic of Serbia (Project No. 176006 and III 42010) and theEarth-Science Studies in Central and South-Eastern Europe (CEEPUS),which are gratefully acknowledged. We are also grateful to the anony-mous reviewers.

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