Micro-Raman Spectroscopy of Selected Macerals of the Huminite Group: An Example from the Szczerców Lignite Deposit (Central Poland)

Lignite (ulminite reflectance Rr = 0.27%) from the Szczerców deposit (Central Poland) is dominated by huminite group macerals, containing a high proportion of attrinite and densinite. Densinite and ulminite are more abundant in small aromatic units than attrinite, which may result from their stronger gelification. The differences in Raman spectral characteristics between attrinite and ulminite are more pronounced than between attrinite and densinite. Fusinite, in comparison with the huminite group macerals, is composed of larger, more varied aromatic systems. The D4 (1190–1200 cm−1) and D5 bands (1280–1290 cm−1), most likely, correspond to different chemical structures, and their origin should be further investigated.


Introduction
Lignite is an important energy carrier in many countries. Its chemical and technological features, which depend to a large extent on the petrographic composition, have been intensively studied for many years [1][2][3][4]. However, the microstructure of lignite macerals is still poorly known. So far, Raman spectroscopy studies aimed at understanding brown coal's microstructure have been conducted almost exclusively on bulk samples [5][6][7][8][9][10][11].
The purpose of this work was to reveal, for the first time, and compare the microstructural features of attrinite, densinite, ulminite, and, additionally, fusinite, based on the example of the Szczerców deposit. It is one of two deposits currently exploited by the Polska Grupa Energetczna (PGE) open-pit mine Bełchatów-the largest lignite mine in Europe.

Geological Setting
The Szczerców lignite deposit is located in the Szczecin-Łódź-Miechów synclinorium, which is located in the western part of the Kleszczów Graben [12] (Figure 1). The Szczerców deposit is approximately 7 km long and 1.5 to 3 km wide [13]. From the east, it is adjacent to the Bełchatów lignite deposit. They are separated by the Dębina salt dome [13]. The Szczerców lignite overlies Upper Jurassic, Upper Cretaceous, and, locally, Paleogene sediments. The coal-bearing sediment is divided into [13]: a sub-coal complex, a coal complex, a clay and coal complex, and a clay-sand complex (Figure 1). The thickness of the coal complex reaches up to 100 m in the central part of the deposit; the average thickness is approximately 30-40 m [14,15].
The main seam consists of detritic and xylodetritic coal. The average content of xylites is approximately 20%. Occasionally, paratonstein (clay kaolinite) interbeddings are observed [14]. The lower part of the seam is of Lower Miocene age. Stratigraphically, it is part of the lowest part of theŚcinawa Formation [16]. The hanging wall part of the seam is of Middle Miocene age [17]. Quaternary deposits are glacial, interglacial, and sandy sediments with lenses of gravels and glacial clays [13,18].  [13] and Kasiński et al. [19]; modified.

Methods
The sample for microscopic and Raman spectroscopic investigation was collected as a representative channel sample from the seam in the western part of the Szczerców deposit. Two polished sections-one for each type of examination-were prepared according to the ISO 7404-2:2009 standard [20]. Petrographic analysis was conducted under incident white light and blue light excitation (i.e., fluorescence mode) using a Zeiss Axioplan polarizing microscope.
The random reflectance of ulminite B and other macerals was determined using monochromatic light with a wavelength of 546 nm and immersion oil (n = 1.518), and a Zeiss MPM-400 reflectometer according to the ISO 7404-5:2009 standard [21]. The petrographic composition of coal was determined according to ICCP guidelines [22][23][24]. The analysis was performed in accordance with the ISO 7404-3:2009 standard [25].
In addition, proximate and ultimate analyses were performed in an accredited laboratory in accordance with applicable standards for total moisture [26], ash content [27], volatile matter content [28], sulfur content [29], gross calorific value [30], and carbon and hydrogen content [31].
Raman spectroscopy of attrinite, densinite, ulminite, and fusinite macerals was performed in 27 to 30 randomly chosen points for each maceral. It should be noted that Raman spectra measurements were made on pure macerals. The selected measurement points are shown in the photographs in Appendix A ( Figures A1-A4). The spectra were recorded with a Thermo Scientific DXR Raman (Faculty of Geology, Geophysics and Environment Protection, AGH University of Science and Technology, Kraków, Poland) microscope with a 900 grooves/mm grating and a Charge Coupled Device (CCD) detector. The Olympus 10× (NA 0.25) objectives (spot sizes 2.1 µm and 1.1 µm, respectively) were used. Excitation was activated with a 532 nm diode laser with a maximum power of 10 mW. Measurements were performed in a spectral range of 400-3500 cm −1 , at a spectral resolution of 1 cm −1 , and an area of 1 µm 2 . The laser power was set at 1-2 mW. The spectrometer was calibrated using a polystyrene standard. The accumulated measurement time was 30 s for each spectrum. Peak-fitting was conducted in the range of 1000-1800 cm −1 by GRAMS32, based on the earlier Raman studies of low-rank coal (brown coals) [6][7][8][9][10][11]). The second derivative of the spectra was also considered to find the initial positions of the Raman bands. Lorentzian curves were applied. The goodness of fit was checked by the χ2 test. The I D1 /I G ratio was determined from the D1 and G band intensities. Furthermore, the A D3 + A D6 /A ALL , A D4 /A ALL , and A D5 /A ALL ratios were calculated from the band areas, where "A ALL " denotes the sum of all band areas in a given spectrum. Raman band separation (RBS)-i.e., the distance between the G and D1 peak-was also determined.
The nomenclature of the bands was taken from Henry et al. [32]. The additional band D7 (~1095 cm −1 ), which was earlier found by Li et al. [6] and labeled as S R , was introduced.
The results of attrinite examinations were compared with those of densinite, ulminite, and fusinite using Student's t-test with a significance level of α = 0.05. This was preceded by the Shapiro-Wilk test to check the normality of distribution, and the Fisher-Snedecor test to assess the equality of variances.

Petrographic Composition
Based on the macroscopic analysis (Table 1), the tested coal is xylodetritic coal with a xylite content of 22%. The coal from the Szczerców deposit is dominated by macerals of the huminite group (80.7% vol.). The huminite macerals are dominated by attrinite (29.9% vol.) and densinite (23.9% vol.). The sum of these macerals indicates the dominant share of the detritic component in coal. Petrographic components of woody remnants, namely textinite (11.7% vol.) and ulminite (12.1% vol.), were observed. The huminite components of the tested coal also include gelinite and corpohuminite (1.5 and 1.6% vol., respectively) ( Table 1).
The mineral matter content of the examined coal is 10.5% vol. (dry basis. The most commonly observed minerals are clay and quartz, which are dispersed among attrinite and densinite. The content of these minerals is 9.2% by volume. Pyrite, which accounts for 0.8% by volume, is less abundant, and is most commonly found in the form of framboidal pyrite. The presence of carbonates, which are associated with lake chalk, is typical for coal from the Szczerców deposit [14]. The mineral matter is dispersed in the macerals and locally impregnates intracellular spaces. Dispersed clay minerals are most commonly observed in attrinite.

Random Reflectance and Proximate and Ultimate Parameters of Lignite from the Szczerców Deposit
The random reflectance was measured on five macerals according to the ISO 7404-5 standard [21]. The measurements on huminite macerals were made on textinite, ulminite, attrinite, and densinite. In the case of macerals of the inertinite group, the random reflectance was measured on fusinite ( Table 2). The obtained results correspond well with the results obtained by Sýkorová et al. (2005). The huminite macerals textinite (0.2%) and densinite (0.31%) have the lowest and highest random reflectance, respectively. Ulminite B's random reflectance is 0.27%.  [34], the investigated lignite was low-rank coal (lignite C)-as the average reflectance of ulminite B is less than 0.40% (R r = 0.27%), the total moisture of the coal is >35% (51.3%), and the ash-free moisture is less than 75% (43.8%). The results obtained are consistent with other chemical measures of the rank of coal (C daf = 67.39%) and the gross calorific value (as received) (GCV maf = 13.9 KJ/kg) (Table 3), which classifies the examined coal as ortholignite.

General Characteristics of the Raman Spectra
Representative Raman spectra of attrinite, densinite, ulminite, and fusinite from the studied coal are given in Figure 2. Eight bands of absorption are usually identified (Figure 3). The G band (~1580 cm −1 ) is typically assigned to a graphitic lattice mode E 2g [35,36].   However, in the case of lignite, it is mainly related to aromatic ring breathing (Li et al., 2006). The D2 band (~1615 cm −1 ) makes a shoulder on the G band, being also attributed to the graphitic E2g mode [37,38]. It always occurs when the D1 band is present [39]. The D3 band (~1540 cm −1 ), also referred to as GR (Li et al., 2006), originates from out-of-plane vibration due to structural defects and heteroatoms [40,41]. It is assigned to aromatic systems (composed of three to five rings) and amorphous carbon [6,37,42,43].
The D6 band (~1465 cm −1 ), also denoted as VL [6], is related to methyl or methylene, small aromatic systems, and amorphous carbon [6]. The D1 band (~1285 cm −1 ) is assigned to in-plane defects or occurrence of heteroatoms and is associated with the breathing mode (A1g) of sp 2 atoms in aromatic rings [35,38,41]. The origin of the D5 band (~1285 cm −1 ) or SL band [6] is not fully understood. It is believed to represent aryl-alkyl ethers [6]or CH in aliphatic chains [44]. The D4 band (~1195 cm −1 ) (S band-in Li et al., 2006) is attributed mainly to the occurrence of sp 3 -rich or sp 3 -sp 2 carbon structures, alkyl-aryl groups, aliphatic (or aromatic) ethers, as well as CH in aliphatic chains or on aromatic rings [6,[43][44][45]. Frequently, especially in bituminous coals and cokes, only one band in the 1200-1240 cm −1 region, named D4, is indicated. [43,[46][47][48][49]. A very weak D7 band at ~1095 cm −1 , marked SR by Li et al. [6], is found in most of the spectra. It is assigned to C-H on aromatic rings [6]. Additionally, a very weak band at ~1700 cm −1 , related to the carbonyl group However, in the case of lignite, it is mainly related to aromatic ring breathing (Li et al., 2006). The D2 band (~1615 cm −1 ) makes a shoulder on the G band, being also attributed to the graphitic E 2g mode [37,38]. It always occurs when the D1 band is present [39]. The D3 band (~1540 cm −1 ), also referred to as G R (Li et al., 2006), originates from out-of-plane vibration due to structural defects and heteroatoms [40,41]. It is assigned to aromatic systems (composed of three to five rings) and amorphous carbon [6,37,42,43].
The D6 band (~1465 cm −1 ), also denoted as V L [6], is related to methyl or methylene, small aromatic systems, and amorphous carbon [6]. The D1 band (~1285 cm −1 ) is assigned to in-plane defects or occurrence of heteroatoms and is associated with the breathing mode (A 1g ) of sp 2 atoms in aromatic rings [35,38,41]. The origin of the D5 band (~1285 cm −1 ) or S L band [6] is not fully understood. It is believed to represent aryl-alkyl ethers [6] or CH in aliphatic chains [44]. The D4 band (~1195 cm −1 ) (S band-in Li et al., 2006) is attributed mainly to the occurrence of sp 3 -rich or sp 3 -sp 2 carbon structures, alkyl-aryl groups, aliphatic (or aromatic) ethers, as well as CH in aliphatic chains or on aromatic rings [6,[43][44][45]. Frequently, especially in bituminous coals and cokes, only one band in the 1200-1240 cm −1 region, named D4, is indicated [43,[46][47][48][49]. A very weak D7 band at 1095 cm −1 , marked S R by Li et al. [6], is found in most of the spectra. It is assigned to C-H on aromatic rings [6]. Additionally, a very weak band at~1700 cm −1 , related to the carbonyl group (C=O) vibration, was also observed in some of the spectra (not used in the curve-fitting procedure).
In the case of densinite, the G band is found at~1580 cm −1 , and the D1 band at 1374 cm −1 . The FWHM of these bands is~78 cm −1 and~140 cm −1 , respectively ( Table 4). The RBS reaches a value of~206 cm −1 ( Table 5).
The values of other spectral parameters are summarized in Tables 4 and 5.
The sum of the A D4 /A ALL + A D5 /A ALL ratio calculated herein is similar to the value of the A D4 /A ALL ratio (derived from a typical five-band curve fitting method) obtained for semifusinite, but lower than that of fusinite in bituminous coals [47,50]. Furthermore, the sum of the A D3 /A ALL + A D6 /A ALL ratios determined in this study corresponds well to the value of the A D3 /A ALL ratio (one band was indicated in the valley between the D1 and G bands) in the spectra of semifusinite from bituminous coals [47,50]. Explanation: n-number of measurements; the standard deviation of the measurements is given in italics. Explanation: n-number of measurements; the standard deviation of the measurements is given in italics.

Comparison of the Microstructural and Chemical Properties of the Analyzed Macerals
Comparisons were performed for the following pairs of macerals: attrinite-densinite, attrinite-ulminite, and attrinite-fusinite. To find out if the differences in spectral parameters between the macerals were statistically valid, the Shapiro-Wilk, Fisher-Snedecor, and, finally, Student's t-test were performed.
The Shapiro-Wilk test shows that all data sets of individual Raman-derived spectral parameters obtained from the curve fitting procedure have normal or close to normal distribution. The Fisher-Snedecor test demonstrates that variances are equal in all analyzed pairs of spectral parameters.
Considering this, Student's t-test was conducted for all parameters. Each time that the difference in spectral parameters is mentioned in the text, it is statistically significant, as inferred from the t-test (p value < 0.05).
Attrinite and densinite show the highest similarity of microstructural properties among all of the analyzed macerals, which is in line with the results of the FTIR investigations revealing that the chemical composition of detrohuminite subgroup macerals is largely independent of its degree of gelification [51]. However, there are some statistically significant (p < 0.05) differences between their spectral parameters. This applies to the position of the D1 band, which appears at lower wavenumbers in the densinite spectra, whereas the D4 band falls at higher wavenumbers (Figure 4a,b). The densinite spectra also reveal higher D6 and D4 band FWHMs and higher A D3+D6 /A ALL and A D4 /A ALL ratios (Figure 4c-f). This reflects higher amounts of small aromatic units and alkyl-aryl and CH groups in the chemical structure of densinite compared with attrinite [6,[43][44][45], probably resulting from stronger gelification of densinite. An increase in aromaticity of the huminite group macerals with increasing gelification was earlier reported, based on the FTIR examination [5,[52][53][54]. This is also in line with the difference in R r value between densinite (0.31%) and attrinite (0.26%). Similarly, the difference in D1 band position observed herein corresponds with a shift of the D1 band to lower wavelengths with increasing reflectance, as reported by Kelemen and Fang [55] and Guedes et al. [8]. The RBS value, which correlates well with the R r value [32,[55][56][57][58][59], is higher for densinite, which also corresponds with higher reflectance values of this maceral (Tables 2 and 5). Where detrohuminite's microstructure is concerned, it should be remembered that the primary plant material and the initial peat-forming conditions have a great impact on the features of macerals belonging to this subgroup.
Statistical analysis shows that the microstructural features of attrinite and ulminite differ significantly. The position of some of the Raman bands (D2, G, and D4) in ulminite spectra is moved to higher wavenumbers, whereas the D6, D1, and D7 bands fall at lower ones (Figure 5a,b; Table 4). Ulminite exhibits a higher FWHM of the D3, D6, D1, and D4 bands and higher A D3+D6 /A ALL and A D4 /A ALL ratios in comparison with attrinite (Figure 5c-e; Tables 4 and 5). On the other hand, the A D5 /A ALL ratio is lower in ulminite than in attrinite (Figure 5f). Taking the origin of the D3, D4, and D6 bands [6,[43][44][45], the differences in the values of the spectral ratios imply that the chemical structure of ulminite is more abundant in small aromatic systems composed of three to five rings, C aromatic -C alkyl systems, and CH groups. This may be due to gelification of ulminite, as aromaticity increases with increasing gelification [5,[52][53][54]. Ulminite spectra also exhibit higher Raman band separation (RBS) values ( Table 5) than those of attrinite. The differences observed may also result from different origins of starting materials; attrinite may come from the soft, cellulose-rich tissues of plants, and ulminite may come from lignin-rich xylem. Lignin remnants may affect the aromaticity of lignite at the early stage of coalification [60]. Statistical analysis shows that the microstructural features of attrinite and ulminite differ significantly. The position of some of the Raman bands (D2, G, and D4) in ulminite spectra is moved to higher wavenumbers, whereas the D6, D1, and D7 bands fall at lower ones (Figure 5a,b; Table 4). Ulminite exhibits a higher FWHM of the D3, D6, D1, and D4 bands and higher AD3 + D6/AALL and AD4/AALL ratios in comparison with attrinite (Figure 5c-   As should be expected, there are major differences in microstructure between attrinite and fusinite. Most of the Raman bands occur at lower wavenumbers when fusinite is considered, but the position of the G band moves to higher wavenumbers (Figure 6a,b; Table 4). Fusinite spectra also have lower FWHM values of the D2 and G bands and a As should be expected, there are major differences in microstructure between attrinite and fusinite. Most of the Raman bands occur at lower wavenumbers when fusinite is considered, but the position of the G band moves to higher wavenumbers (Figure 6a,b; Table 4). Fusinite spectra also have lower FWHM values of the D2 and G bands and a higher FWHM of the D3, D1, D5, and D7 bands in comparison with attrinite (Figure 6c,d; Table 4). However, the D3, D6, and D5 bands reveal low height; thereby, the A D3+D6 /A ALL and A D5 /A ALL ratios are lower for fusinite (Figure 6e,f; Table 5). The RBS value determined for fusinite is also higher ( Table 5). The lower G band FWHM indicates better organization of the fusinite microstructure in relation to the attrinite microstructure [8,32,39,47,48,55,61,62]. The higher D1 band FWHM in the fusinite spectra reveals that aromatic clusters in fusinite are more varied in terms of their order and dimensions [63]. The higher RBS value determined for fusinite corresponds with the higher mean random reflectance (R r ) of this maceral (Tables 2 and 5) [32,[55][56][57][58][59]. The lower A D3 /A ALL , A D6 /A ALL and A D5 /A ALL ratio values determined for fusinite (when compared with attrinite) are a consequence of the lower content of smaller aromatic systems-aliphatics as well as mixed structures and alkyl-aryl groups [6,37,[42][43][44]. Instead, larger aromatic ring systems dominate the chemical structure of fusinite.  Ferralis et al. [44] found a linear relationship between the (I D4 +I D5 )/(I G +I D2 ) Raman band area ratio (Symbols taken from Ferralis et al. [44]) and the elemental H:C ratio. Considering the above, a similar calculation of the (I D4 + I D5 )/(I G + I D2 ) ratio was performed here, which shows much lower values obtained for fusinite (0.26) than for attrinite (0.37), densinite (0.46), or ulminite (0.50). This reflects the much lower hydrogen content in the chemical structure of fusinite in comparison with other examined macerals. However, no positive correlation between the A D4 /A ALL and A D5 /A ALL ratios is observed-which should be expected if the D4 and D5 bands are related to the same chemical structures in coal. This may suggest that the origin of the D4 and D5 bands is different.

Conclusions
Lignite (ulminite reflectance R r o = 0.27%) from the Szczerców deposit (Central Poland) was subjected to micro-Raman spectroscopy examination. The study revealed differences in the microstructure of the huminite group macerals (attrinite, densinite, and ulminite). Densinite and ulminite are more abundant in small aromatic units than attrinite, which may result from their gelification or the kind of starting material. The differences in the Raman spectral characteristics between attrinite and ulminite are more pronounced than between attrinite and densinite. As expected, fusinite, in comparison with the huminite group macerals, is composed of larger, more varied aromatic systems. Similarities to semifusinite's microstructure in bituminous coals are observed. The D4 band (1190-1200 cm −1 ) and D5 band (1280-1290 cm −1 ) most likely correspond to different chemical structures, and their origin should be further investigated.
On the basis of petrographic analysis, the directions of use and the best ways of exploitation can also be determined. The presence of fibrous xylites is particularly unfavorable during mining and grinding. Therefore, in practice, the best solution is to determine the xylite content as early as possible, even at the stage of deposit exploration. The petrographic composition of coal has a direct impact on its suitability for the gasification process, where the degree of coal gelification and the concentration of carbonate and quartz minerals are of particular importance.