Variations in chemistry of macerals as refl ected by micro-scale analysis of a Spanish coal

An Oligocene lignite (Ebro Basin, Spain) and its density fractions were analyzed petrographically and with microscale techniques (electron microprobe and micro-FTIR) to gain insight into differences between individual macerals of low rank high-sulfur coal. The density of the alginite-dominated fraction is below 1.26g/cm3, and that of the huminite-dominated fraction is above 1.38g/cm3. Densities within 1.26-1.38g/cm3 represent mixtures of liptinite and huminite macerals. With regard to elemental composition, alginite has the highest carbon content (75.6% on average) and the lowest oxygen content (6.1% on average). Corpohuminite is characterized by the lowest carbon content (62.3% on average) and the highest oxygen content (21.5% on average). Nitrogen contents for corpohuminite and ulminite (~1%) are similar, but are signifi cantly lower in alginite (0.2% on average). Sulfur content is highest in alginite (13.4% on average), followed by corpohuminite (9.8%) and ulminite (7.7%). Functional group analysis documents large differences between macerals of the huminite and liptinite group, but also indicates differences between individual macerals within both the huminite and liptinite group. These differences are most notable in aromaticity, degree of aromatic ring condensations, and hydrocarbon potential. Lignite. Macerals. Elemental composition. Functional groups.


INTRODUCTION
The late Oligocene Mequinenza lignite is found in the eastern Ebro Basin of northeast Spain (Fig. 1).Detailed geologic descriptions of the coal and its setting are found in Cabrera and Saez (1987), Querol et al. (1996), Arenas et al. (1999), and Cabrera et al. (2002).Briefl y, the Ebro Basin is a foreland basin of the Pyrenees that between early Oligocene and the late Neogene was fi lled with nonmarine sediments.Sedimentary environments included large alluvial fans, fl uvial systems along tectonically active basin margins, and shallow lacustrine systems that include both evaporitic sediments and lignite beds.The lignite deposits have, on as-received-basis, high ash (30-35% on average) and very high sulfur (3-8% on average) contents, and the heating value ranges from 14.6 to 23.0kJ kg -1 (Cabrera and Saez, 1987).Petrographically, these coals are rich in huminite (and corpohuminite, in particular) and have low liptinite and inertinite content (White et al., 1994;Querol et al., 1996).They were deposited in a lacustrine environment ranging in salinity from fresh to saline, as indicated by biomarker studies (Cabrera et al., 2002).
The present study provides details of the chemistry of individual macerals in the lignite.Because of the very high sulfur content (Chou, 2012), this coal provides a rare opportunity to discuss the effect of the abundance of sulfur on maceral chemistry.Both bulk lignite and Density-Gradient-Centrifugation (DGC) separated fractions were analyzed using standard petrographic and in-situ microscale (electron microprobe and micro-FTIR) techniques.

Density-gradient centrifugation
A sample of the lignite from the eastern Ebro Basin in Spain (obtained from Colin Snape, University of Nottingham; at University of Strathclyde at time of sample procurement) was crushed to <100 mesh, demineralized by HF, and separated by density-gradient centrifugation.This technique is based on procedures developed by Dyrkacz et al. (1981Dyrkacz et al. ( , 1991) ) and modifi ed by Robl et al. (1987).A Beckman model J2-21 centrifuge with a 1900-ml Titanium JCF-Z zonal core rotor, located at the University of Kentucky Center for Applied Energy Research, was used for the DGC separation.For each run, an ultrasonically dispersed mixture containing of about 8g of the coal, 200ml of water, and 2.0+/-0.05g of Brij-35 surfactant was pumped to the outer wall of the centrifuge rotor.After centrifugation at up to 25,000G's on the outer wall, Fluurinert (3M Corp.), a high-density, water-immiscible fl uorocarbon was used to force the density gradient from the rotor and through a density meter.The fractions were fi ltered under vacuum through a 0.45-μm fi lter, rinsed, dried overnight at 60 o C, and weighed.Multiple runs were performed to obtain suffi cient amounts of material for further analysis.

Petrology
Fractions of the density-gradient-centrifugation separates were mixed with epoxy in 2.54cm molds and prepared to a fi nal 0.05μm polish for examination on a Leitz Orthoplan microscope using a 50x, oil-immersion, refl ected-light objective and 10x eyepieces.Vitrinite refl ectance was measured using a Zeiss Photoscope microscope.

Micro-FTIR spectroscopy
Micro-FTIR measurements were performed on polished blocks using a Nicolet 6700 spectro-photometer connected to a Nicolet Continuum microscope operated in refl ectance mode.The microscope was linked to a liquid-nitrogencooled mercury-cadmium-telluride (MCT) detector (Nicolet Instrumentations Inc., Madison, WI, USA) and a computer-controlled mapping stage.Micro-FTIR spectra were obtained at a resolution of 4cm -1 , using a gold plate as background.The 15x infrared objective and the measuring aperture of 100 x 100μm were used in most cases, except for sporinite and cutinite, the small size of which required the use of a 32x objective and apertures suitable for these macerals.The OMNIC program was used for spectral de-convolution, curve-fi tting, and determination of peak integration areas.All micro-FTIR spectra were subjected to Kramers-Kroning transformation, and the peak  assignments of spectra were based on previously published work on coals (Painter et al., 1985;Wang and Griffi ths, 1985;Mastalerz and Bustin, 1993).
For semi-quantitative FTIR analysis, many indices are used to evaluate the chemical characteristics of coal macerals (Kister et al., 1990;Mastalerz and Bustin, 1996).The ratios selected for this study are listed in Table 1.Curve-fi tting of the ~1560-1800cm -1 region peak provided the integration areas of oxygenated and aromatic carbon groups.Self-deconvolution of the 2800-3000cm -1 region was performed to calculate CH2/CH3 ratios.

Electron Microprobe Analysis
The same polished blocks as used for optical and FTIR analyses were sputter-coated with carbon for use in the microprobe.The samples were analyzed for the abundances of carbon, oxygen, nitrogen, sulfur, iron, calcium, and chlorine using a Cameca SX-100 electron microprobe equipped with four wavelength-dispersive spectrometers.The microprobe provides compositional data in weight percent for each element, based on the total number of elements analyzed.A PC2 crystal was used to measure carbon and oxygen; a PET crystal for sulfur, calcium, and silicon; a PC0 for nitrogen, and an LiF crystal for Fe.Ten-second beam count times were used for C, O, and S, and 20 seconds for N. Detailed procedures for light element determination are found in Bustin et al. (1993) and Mastalerz and Gurba (2000).Beam conditions included a beam size of 20μm, a current of 20nA, and an accelerating voltage of 10kV.Depending on the maceral availability and its identifi cation under the probe optics, the number of analyses for individual macerals per sample varied from 5 to 25.
In density fractions, density progresses from alginitedominated light fractions (density <1.26g/cm 3 , particularly the <1.18g/cm 3 fraction, Table 2) (Fig. 2A, B), through textinite-ulminite/corpohuminite/liptinite/alginite mixes in the intermediate fractions (density 1.26-1.38g/cm 3), and textinite-ulminite-corpohuminite-rich fractions (1.38 -1.43g/cm 3 ), to almost pure (>93%) textinite-ulminite heavy fractions of 1.43-~1.59g/cm 3(huminite-rich fractions shown on Fig. 2C-H).In comparison, for Miocene lignite from Indonesia, Stankiewicz et al. (1996) obtained densities of 1.43g/cm 3 and above for almost pure huminite, whereas the fraction of 1.10g/cm 3 was liptinite-rich.In that study, density fractions of 1.26 and 1.35g/cm 3 were mixed fractions rich in both huminite and liptinite.This comparison suggests that for coal of low rank, densities of 1.38g/cm 3 and above are characteristic of huminite.Discrepancies between densities of the liptinite-dominated fraction between the Indonesian coals (<1.10g/cm 3 ) and the Spanish coals studied here (<1.26g/cm 3 ) may result from compositional differences in liptinite macerals; in the Indonesian coal, liptinite did not include alginite, which is a prominent component of the Spanish coals studied (Table 2).Alginite is not a common component of coal and, therefore, its density in coal has not been studied to the same extent as in shales, where several studies indicated densities below 1.15g/cm 3 (Mastalerz et al., 2012, and references therein).Hower et al. (1994) described similar fractionation of a high-volatile A bituminous coal from Kentucky, with liptinite being the most abundant in the <1.17g/cm 3 density fraction (total vitrinite exceeding 98% from the 1.286-1.301g/cm 3fractions) and fusinite + semifusinite >98% in the >1.388g/cm 3 fractions.For high volatile bituminous Eocene coal from Indonesia, Stankiewicz et al. (1996) 1 0 . 1 3 4 4 / 1 0 5 . 0 0 0 0 0 2 0 5 4 Micro-analysis of coal  almost pure liptinite (resinite) fractions, and found almost pure vitrinite in the density fractions of 1.25g/cm 3 and above.This comparison, as well as other similar studies on coal (e.g.Rimmer et al., 2006) suggest that densities of maceral groups can shift slightly in response to rank, specifi cs of macerals composition within maceral groups, and possibly other factors such as, for example, the type of original vegetation.

Distribution of functional groups
Several macerals of the huminite group were analyzed using micro-FTIR and they include ulminite, corpohuminite, levigelinite, and porigelinite (Fig. 3).Micro-FTIR spectra of each maceral were obtained both on the bulk lignite and density fractions and several spectra were used to generate the average spectrum that was used for interpretation.Qualitatively, spectra of huminite macerals look similar (Fig. 3), being characterized by prominent bands in the aliphatic stretching region (2800-3000cm -1 ), aromatic carbon region (peak at 1620cm -1 ), and aliphatic bending region (1350-1460cm -1 ).Bands of smaller absorbance occur in the oxygenated group region (peak at 1705cm -1 ) and aromatic out-of-plane region (700-900cm -1 ).
In the liptinite group, sporinite, cutinite, and alginite were analyzed using micro-FTIR (Fig. 4).FTIR spectra of sporinite and cutinite were collected on the bulk lignite, whereas the lightest density fraction (1.00-1.18g/cm 3 ) was used to collect alginite spectra because alginite could not be identifi ed with the dry infrared objective in the bulk coal.This coal has also areas where ulminite was fi nely interfi ngered with cutinite; these areas were also analyzed (marked as cutinite-ulminite in Figures 5-7).Inertinite macerals, although present in the coals studied in minor quantities (Figure 2D), either were not identifi ed with certainty using the dry infrared objective or were too small to measure and, therefore, are not discussed in this paper.Spectra of liptinites (Fig. 4) are very different from those of huminite because they have: i) much more defi ned and prominent aliphatic stretching bands (2800-3000cm -1 ); ii) much lower absorbance of aromatic carbon (peak at 1620cm -1 ); iii) generally well-defi ned bands of carboxyl/carbonyl at ~1710cm -1 ; and iv) non-detectable aromatic bands in the aromatic out-of-plane region (700-900cm -1 ).These are expected differences and result from the aliphatic character of liptinite group macerals in contrast to more aromatic huminite (e.g.Mastalerz and Bustin, 1993).While notable differences in spectra exist between macerals of the huminite and liptinite groups, the differences among the macerals within the liptinite group alone are less pronounced upon qualitative inspection of the spectra (Fig. 4).One exception is noted for the cutinite spectrum, which has a prominent aromatic carbon band at ~1610cm -1 ; this band, however, is from the more aromatic ulminite associated with cutinite in this coal.The spectra of alginite show prominent aliphatic stretching bands, their narrow peaks resulting from an overwhelming contribution of CH2 and a very minor CH3 contribution in this region, which, in turn, suggests long and straight aliphatic chains (Lin and Ritz, 1993).
The latter peaks may also suggest cyclic aliphatics, confi rmed by the absence of a peak at 1375cm -1 attributed to bending by terminal methyl groups.

Micro-analysis of coal
Figures 5-7.Aromaticity (AR1, Table 1) of the huminite group macerals is higher than that of liptinite (Fig. 5A).Within the vitrinite group, ulminite and corpohuminite have the lowest and highest aromaticity, respectively.Within the liptinite group, sporinite has a higher aromaticity than cutinite and alginite.Increase in aromaticity is accompanied by an increase in maceral refl ectance, with correlation coeffi cient R 2 of 0.50.The degree of aromatic ring condensation (DAC, Table 1) is highest in porigelinite and lowest in sporinite (Fig. 5B), and no correlation existed with maceral refl ectance.Hydrocarbon potential (FA, Table 1) is much higher in the liptinite than in the huminite group macerals (Fig. 6A), expectedly revealing a strong negative correlation with maceral refl ectance (R 2 = 0.82).Within the huminite group this ratio is highest in porigelinite and decreases in this order: porigelinite>ulminite> levigelinite> corpohuminite.Aliphatic chains (ACL, Table 1) are the longest in alginite (Fig. 6B), and there was no correlation with maceral refl ectance.Within the huminite group, levigelinite has the longest and least-branched chains and porigelinite the shortest chains.Finally, ratios that utilize oxygenated groups (Fig. 7A, B) show that liptinite group macerals, as compared to vitrinite group macerals, have lower relative contributions of oxygenated groups relative to the aliphatic C-H groups (OX1, Fig. 7A) but higher contribution of oxygenated groups relative to aromatic carbon (OX2, Fig. 7B).The juxtaposition of ulminite material to cutinite results in a rapid decrease in these ratios, although the decrease is likely not characteristic of cutinite itself (Fig. 7A, B).Among huminite group macerals, porigelinite has the highest contribution of oxygenated groups and corpohuminite the lowest.These two ratios that utilize oxygenated groups have strong relationships with maceral refl ectance; a positive correlation with R2 of 0.68 exists with OX1, whereas a negative correlation (R2 of 0.96) exists with OX2.
While the relationships between maceral refl ectance and some functional group ratios (for example aromaticity, AR1, or hydrocarbon potential, FA) are expected based on different maceral chemistries, an interesting observation   for the coal studied is its high contribution of aliphatic groups (both in aliphatic stretching and bending regions) in vitrinite macerals (Fig. 3) at this low rank.This supports the suggestion that high-S coals follow a different evolution path during diagenesis than low sulfur coals, possibly because of replacement of the oxygen by sulfur in the molecular structure and the consequent slower elimination of aliphatic chains during maturation (Li et al., 2010).

Elemental composition
This study also provides information on the elemental chemistry of individual macerals in the low-rank coal.While knowledge of the chemistries of individual macerals in bituminous coals has advanced over last decades, attributable to both maceral separation (Dyrkacz et al., 1984(Dyrkacz et al., , 1991;;Stankiewicz et al., 1996) and in-situ analysis using the electron microprobe (Mastalerz and Bustin, 1993;Gurba and Ward, 2000;Ward et al., 2005), these techniques have not yet been applied to the same degree in lower-rank coals (Drobniak and Mastalerz, 2006;Li et al., 2007Li et al., , 2010)).
The elemental compositions of corpohuminite, ulminite, and alginite macerals for the bulk samples and density fractions in the coal studied are listed in Table 4.Other macerals were diffi cult to identify under the electron microprobe and, therefore, were not analyzed.Out of the three macerals analyzed, alginite has the highest carbon content (75.6% on average, Table 5) and the lowest oxygen content (6.1% on average).Corpohuminite is characterized by the smallest carbon content (62.3% on average) and the largest oxygen content (21.5% on average).Corpohuminite and ulminite have similar nitrogen contents (~1%), but are signifi cantly lower in alginite (0.2% on average).These are high-sulfur coals and sulfur content is greatest in alginite (13.4% on average), followed by corpohuminite (9.8%) and ulminite (7.7%).In comparison, analysis of ulminite in Polish Miocene lignitic wood (Drobniak and Mastalerz, 2006) results in very similar carbon contents (~64-69%) to that of the ulminite studied (~70%, Table 4).However, the ulminite studied has signifi cantly lower oxygen, higher nitrogen, and signifi cantly higher sulfur contents that the Miocene wood.Those elemental differences are consistent with the very different depositional environments of these two   deposits: freshwater for the Polish deposit (Wagner, 1996) and fresh to saline water for the Spanish deposit (Cabrera et al., 2002).Li et al. (2010), based on microprobe and FTIR studies, demonstrated that macerals in high-S coals from Australia had different functional group characteristics than low-S coals, specifi cally showing increased aliphatic and reduced aromatic functionalities.
With the exception of N content, which is low in the alginite-rich light fractions, there is no regular shift in the proportions of C, O, N, and S in density fractions.The differences in elemental composition among individual macerals result in different elemental ratios.Specifi cally, the O/C and N/C ratios are highest in corpohuminite, followed by ulminite and alginite, whereas the S/C ratio is highest in alginite, followed by corpohuminite and ulminite.
These elemental data document the large differences between alginite and huminite macerals (corpohuminite and ulminite).Lower O/C and N/C and much higher S/C ratios in alginite may result from differences in original biomass as well as different ability to incorporate sulfur during diagenesis.The alginite S/C ratio of 0.17 to 0.18 (Table 5) significantly exceeds even that of alginite from very high-sulfur Type II-S kerogen.For example, S/C of 0.036 and 0.028 was documented for alginite from the Duwi Formation (Campanian/Maastrichtian, Egypt) and the Monterey Formation (Miocene, California), respectively (Stankiewicz et al., 1996).This very high C/S ratio suggests that, in addition to its original high-sulfur content, the alginite of the coal studied might have experienced the incorporation of significant amounts of sulfur during early diagenesis (Stankiewicz et al., 1997).

CONCLUSIONS
i) For the Oligocene Mequinenza (Spain) lignite studied, the density of liptinite dominated by alginite is below 1.26g/cm 3 (particularly the <1.18g/cm 3 ), the density of liptinite dominated by a mixture of alginite and other liptinite has a range of 1.26-1.32g/cm 3, the density of the huminite/liptinite mix is 1.32-1.43g/cm 3, and a fraction with more than 93% huminite (textinite/ulminite) has density of 1.43-~1.50g/cm 3.Comparing these density ranges to coals from other locations demonstrates that densities of maceral groups can shift in response to rank, composition of individual macerals within maceral groups and, likely, other local factors.
ii) In-situ elemental analysis of macerals in this coal indicates that alginite has the largest carbon content (75.6% on average) and smallest oxygen content (6.1% on average).
Corpohuminite is characterized by the smallest carbon content (62.3% on average) and the largest oxygen content (21.5% on average).Nitrogen content is similar for corpohuminite and ulminite (~1%), but is signifi cantly lower in alginite (0.2% on average).These are high-organic sulfur coals and sulfur content is high in all the macerals analyzed.Among macerals, sulfur content is greatest in alginite (13.4% on average), followed by corpohuminite (9.8%) and ulminite (7.7%).We suggest that, in addition to the inherent high sulfur content, the alginite of the coal studied experienced the incorporation of additional sulfur during early diagenesis.
iii) Functional group analysis documents expected differences between macerals of huminite and liptinite groups, but also indicates differences between individual macerals within both the huminite and liptinite groups.These differences are best refl ected in aromaticity and hydrocarbon potential, and can be correlated to the refl ectance of macerals.Abundance of aliphatic functional groups in vitrinite macerals may relate to the abundance of sulfur in the molecular structure.

ACKNOWLEDGMENTS
We wish to thank Colin Snape, University of Nottingham, for supplying the coal sample.Comments of two reviewers are greatly appreciated.

Spain
Location of the Ebro Basin in Spain.Star in the eastern part of the Ebro Basin indicates the location of the lignite studied.Alluvial systems, H: Huesca, GM: Guadalope-Matarranya, LM: Los Monegros, and Mo: Montsant.Modified after Cabrera et al. (2002).

FIGURE 2
FIGURE 2 spectra of huminite macerals.Each spectrum is an average spectrum based on six to ten individual spectra.
FIGURE 3 FIGURE 4 and B) degree of aromatic ring condensation of the macerals studied.

FIGURE 6
FIGURE 6 Oxidation indices of the macerals studied.

FIGURE 7
FIGURE 7 obtained densities <1.11% for Petrographic composition (in volume %) of the bulk coal and its density fractions.Other liptinite category includes sporinite and cutinite

TABLE 3
Microprobe elemental analyses (in weight %) of selected macerals in the bulk coal and density fractions.Each value is an average of 5 to 25 analyses and values in brackets are standard deviations Summary of microprobe analysis in bulk lignite and density fractions (in weight %) and selected elemental ratios