Micro-Raman spectroscopy of microscopically distinguishable

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Micro-Raman spectroscopy of microscopically distinguishable components of naturally graphitized coals from central Hunan Province, China Kuo Li, Susan M. Rimmer, Qinfu Liu, and Yinmin Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04065 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Micro-Raman spectroscopy of microscopically distinguishable components of naturally graphitized coals from central Hunan Province, China Kuo Li1,2*, Susan M. Rimmer2, Qinfu Liu1, Yinmin Zhang3 1College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing, 100083, China 2Department of Geology, Southern Illinois University Carbondale, Carbondale IL 62901, USA 3Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, 010051, China

Abstract Micro-Raman spectroscopic analysis of microscopically distinguishable components in a series of high-rank coals (anthracite to graphitized coal) adjacent to a granitic pluton was used to assess the structural evolution of coal during natural graphitization. Microscopically identifiable components were differentiated into six groups: vitrinite; inertinite; microcrystalline graphite with a fine, granular texture and a low reflectance; pyrolytic carbon with layering normal to particle edges; needle graphite and flake graphite, both of which are similar to commercial synthetic graphite. Approaching the intrusion, Raman spectra exhibit a distinctly different evolution for vitrinite and microcrystalline graphite: the D1 band of the first-order Raman spectrum becomes narrower and more intense for vitrinite, whereas the D1 band intensity decreases for the granular, microcrystalline graphite. A plot of full width at half maximum for the D1 band versus R1 (intensity ratio of the D1 to the G band) indicates that structural evolution of vitrinite occurs during carbonization, whereas that of the microcrystalline graphite components occurs during graphitization. The increase in the intensity of the 2D1 band and the appearance of the 2450 cm-1 band in the second-order Raman spectrum for the microcrystalline graphite components also suggest that they have reached the graphitization stage. Structural heterogeneity in the metamorphosed coals initially decreases with increased coal rank, but then increases when fine granular particles (microcrystalline graphite) are seen in the highly graphitized coals. The structural heterogeneity of the most graphitized coals increases due to the formation of new components (pyrolytic carbon, needle and flake graphite). Insights on the structural features and evolution of natural graphitized coals at a maceral scale presented here may be important in 1

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future applications including the production of synthetic graphite from coal and coaly microcrystalline graphite. Key words

Microcrystalline graphite, micro-Raman spectroscopy

graphitized

coal,

igneous

intrusion,

graphitization,

* Corresponding Author: email: [email protected] Introduction Coal macromolecular structure can be affected significantly when igneous rocks intrude coal-bearing strata. The organic molecules in coal are subject to pyrolysis and condensation, and can transform into triperiodic graphite under high temperatures and pressures.1,2 According to Bonijoly et al.,3 natural geothermal gradients cannot supply sufficient activation energy to result in graphitization of carbonaceous materials, but shear stress will promote the reorientation and alignment of the polyaromatic hydrocarbon sheets,4 eventually producing microcrystalline graphite. Microcrystalline graphite that occurs in coal-bearing strata (also called coal graphite5) shows a high graphitization degree, and consists of graphite microcrystals with different orientations.6 The formation of natural coal graphite is mainly the result of igneous intrusion.5,7 During the process of graphite formation from coal, the physical structure of the coal becomes well-ordered and its chemical composition approaches pure carbon.8,9 Thus, a series of samples from anthracite through meta-anthracite, and semi-graphite to graphite provides an ideal rank range to study the structural transformations that occur during natural graphitization of coal. Li et al.7 reported that new components form and some inertinite is preserved in highly graphitized coals. In the most graphitized coals, microcrystalline graphite is the predominant component. Standard methods for measuring reflectance on microcrystalline graphite result in reflectance values that are lower than reported values for graphite,10–12 although X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and C content indicate these samples are graphite.7,13 Approaching the intrusion, C content increases, whereas H, N, and O contents generally decrease. Hydrogen appears to be more readily removed than oxygen from 2


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the coal macromolecule during natural graphitization.7 The geochemical evolution of graphitized coal may differ from that of other intruded coals (for example, where the intrusive body is relatively small) or from that of coals matured by normal burial.7 Average crystallite parameters derived from XRD do not change significantly between anthracite and meta-anthracite ranks, although the meta-anthracite samples have significantly lower H/C atomic ratios. However, the crystallite parameters (La and Lc) for higher rank (graphitized) coals increase progressively with the degree of metamorphism.7 Previously, we reported on the bulk coal characteristics of a series of coals metamorphosed by a large granitic intrusion.7 What remains unclear is how the structures of specific macerals change during natural graphitization. In this study, micro-Raman spectroscopy was used to analyze individual organic components. The objectives of this project were to: (1) study the structural heterogeneity in a series of metamorphosed coals from Hunan Province, China, by comparing the structural features of individual components; and (2) investigate coal structural evolution at the maceral level over the course of metamorphism. 1.1 Attribution of Raman bands in carbonaceous materials (CMs) As a non-destructive, time-saving method that requires only minimal sample preparation, Raman spectroscopy has become an important tool in the characterization of carbonaceous materials (CMs).1,8,9,14–19 Raman spectroscopy provides crystalline lattice information for materials relying on the inelastic collision of photons. The scattered photons have a different frequency and energy than incident photons,18 and this technique is highly sensitive to subtle changes in lattice vibrations and properties of the crystal.21,22 Wopenka and Pasteris9 reported that Raman spectroscopy appears to be more sensitive than XRD in determining the maturity of CMs. Micro-Raman spectroscopy allows in situ analysis of specific components, as opposed to powder XRD analysis that only obtains average structural parameters for the bulk coal. Fundamental studies on band attribution have been published by many authors.14,16,17,23–25 In the region of the first-order Raman spectrum (800-2000 cm-1), only a graphite band (G band) at around 1580 cm-1 is observed when Raman measurements are collected on the basal planes of 3


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well-crystallized graphite.17 The G band is attributed to the in-plane vibration of carbon atoms in graphene sheets, which is a doubly degenerate phonon mode of E2g with D46h symmetry.26 The other pronounced band in the first-order Raman for most CMs is the disorder band (D1 band) at around 1350 cm-1; the specific D1 band position depends on the laser wavelength of the Raman equipment. This band is seen in Raman spectra measured on the edge planes of well-crystallized graphite,17 microcrystalline graphite,25 semi-graphite,22,27 and amorphous CMs, such as low-rank coal, char, carbon black, etc.16,23,28–30 The attribution of the D1 band remains controversial,31,32 but the common explanation is the symmetry in the sp2 graphene plane is broken by an edge and/or defects resulting in the D1 band.25,26,33,34 A detailed structural interpretation is possible using spectral deconvolution. In the first-order Raman region, two additional sub-bands can be identified in the broad defect band by curve-fitting:35 the D3 band at ~ 1500 cm-1 originates due to carbon with interstitial defects with sp3 hybridized carbon links outside or inside aromatic ring planes forming amorphous carbon phases,36–38 and the D4 band at around 1200 cm-1 is related to aliphatic components bound to the aromatic rings in the coal macromolecule,39,40 but there is not universal agreement on the attribution of these defect sub-bands.31 In addition, a D2 band attributed to the disorder inside the graphite planes always shows up as a shoulder peak on the G band at 1620 cm-1 in highly graphitized CMs.20,41 Bands in the second-order region (2200-3400 cm-1) are due to the overtone and combination of the first-order lattice vibration modes.42 The band at 2700 cm-1 is interpreted to be an overtone of the D1 band (2D1), the band at 2950 cm-1 results from the combination of the G and D1 bands (G+D1), and the 3240 cm-1 band is attributed to the overtone of the D2 band (2D2).42 A band at around 2450 cm-1 is seen in the Raman spectra of graphitic CMs, but its attribution is still disputed.43,44 1.2 Raman parameters for CMs Raman spectral profiles change with increased structural order in CMs; thus, Raman spectroscopy has been used in the qualitative and quantitative interpretation of the structural organization of CMs.45 Parameters derived from first-order Raman spectra, including the position, 4


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FWHM, and ratio (peak amplitude or area) of the G and D1 bands, can provide structural evolution information on CMs.20,27,42,46,47 Several studies have used Raman parameters in semi-quantitative studies of coal structural evolution with rank.9,15,20,48,49 Tuinstra and Koenig first described the inversely proportional relationship between the band intensity ratio (R1) and crystal size of polycrystalline graphite.26 A decrease in the D1 band intensity with a gradual increase in crystalline size (La) has also been shown in laboratory studies of synthetic graphite.32 However, previous studies reported the formula proposed by Tuinstra and Koenig is only valid when La > 2 nm, and the opposite trend is observed when La < 2 nm.23,50 Cuesta et al.21 emphasized that the Tuinstra and Koenig formula is only an approximation for determining La values of CMs, and the accuracy in highly disordered materials and graphite is very low. The Raman parameter R1 is not a suitable indicator of the level of organization for amorphous carbon.51 In the Raman spectra of CMs from sub-bituminous to semi-graphite rank, the D1 band becomes narrower, whereas the G band position decreases from 1610 cm-1 to 1582 cm-1.35,46 The marked decrease in the position and width of the G band has been associated with increased structural organization in high rank coals15 and it has been used to distinguish high rank coals from graphitic materials. The broadening of the G and D1 bands in lower rank coals was observed by Zerda et al.,52 but no clear relationship has been found between coal rank and the positions of these bands. Previously, it has been reported that the band separation between the G and D bands increases with increased reflectance of both vitrinite and solid bitumen and, therefore, this can be used as a robust maturity indicator for organic matter.2,53–55 It is clear that changes in Raman parameters are due to structural changes in CMs. Deldicque et al.31 documented that the structural changes in CMs during graphitization are completely different to those that occur during carbonization. Carbonization of coal corresponds to the release of heteroatoms and the formation of nanometer-sized polyaromatic carbon structures yet still the coal structure remains highly disordered; during graphitization, long-range orientation of condensed aromatic layers occurs and structural defects are eliminated gradually. These two structural evolution processes in carbon materials can be distinguished from each other based on 5


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changes in Raman parameters.56 Sampling and methodology 2.1. Samples and preparation Ten Carboniferous coal samples were collected from six coal mines located at the Hanpo'ao mining area, Xinhua County in Hunan Province, China (Fig. 1, Table 1). Details on the geological settings, sampling methods, and pellet preparation have been described previously.7 Briefly, these samples had been altered by the Tianlongshan granitic intrusion (Fig. 1) (~202-208 Ma) during the Indosinian period. Samples were collected at varying distances (~0.3-21 km) from the intrusion (body size ~130 km2) and range in rank from anthracite to graphite;7 distances and reflectance values are summarized in Table 1. The most metamorphosed samples were collected from mines close to the intrusion, an area where the near-vertical coal beds had been highly deformed; further away from the intrusion, the coal beds are near horizontal (Fig. 1). A microcrystalline graphite sample from Lutang in Hunan Province, China was used as a comparison. 2.2. Petrographic analysis Reflectance measurements (maximum, minimum, and random) were collected using a Leica DM2500P microscope linked with a J&M MSP200 reflectance measurement system. At least 100 measurements per sample were collected, using a 546 nm light source and a 50× oil immersion objective. Random reflectance measurements were collected under non-polarized light; minimum and maximum reflectances were taken under polarized light, rotating the stage through 360°. For the SHL and Lutang coal graphite, reflectance readings were collected on the microcrystalline graphite. To assess petrographic composition, 1000 points were counted per sample (using 2 pellets per sample). Photomicrographs were taken with a Leica DFC 7000T camera mounted on a Zeiss Universal reflected-light microscope. The reader is referred to Li et al.7 for additional information on petrographic methods. 2.3. Raman spectroscopy Raman spectroscopic analysis was conducted using a Renishaw InVia Raman spectrometer fitted with a Leica microscope and a 50× objective lens (N.A. 0.50); the diameter of the laser (532 6


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nm excitation wavelength of a semiconductor laser) beam spot on the sample was ~2 μm, and Raman scattering was dispersed by a 2400 lines/mm grating (resolution up to ~1 cm-1) and detected by a CCD camera. The Raman system was calibrated using a Si wafer (520.4 cm-1 line). Spectra were recorded over the range 500–3500 cm-1, which covers the first- and second-order Raman bands. Three to eight Raman spectra were collected per petrographic component in each sample. Macerals were distinguished based mainly on their brightness and morphology under the Raman microscope. Fusinite was slightly brighter than the vitrinite and the cell lumens in pyrofusinite aided identification; macrinite was identified by its higher reflectance, its slightly higher relief, and its morphology. Raman spectra were collected on polished pellets (see Li et al. 7 for polishing details). A crystalline graphite sample from Sri Lanka was also polished and analyzed to determine whether polishing had any effect on the Raman spectra. No defect band was detected in the Raman spectra of the polished Sri Lanka graphite (shown in supplemental Fig. S) suggesting no adverse effects from the polishing. To avoid possible photodegradation of the samples, laser power was limited to < 0.2 mW at the sample measuring point; in addition, consecutive spectra of the same point were compared to assess whether the sample had been altered by the laser. It was found that the baselines of the Raman spectra for the graphitized coals and microcrystalline graphite were very low, and that the consecutive Raman spectra collected from the same spot were visually similar. The peak-fitting process can influence the results derived from Raman spectrum, thus, care must be taken during this process.57 All spectra were peak-fitted utilizing the Peak Fitting Module of Origin Lab® software. During peak-fitting, no smoothing was needed because of the high signal-to-noise ratio in the Raman spectra of graphitized coals, and a third-order polynomial was used for baseline correction. A second derivative function was used to find the position of each band, with some bands being added manually based on previous work,42 such as the D3 band at ~ 1500 cm-1, and the D2 band at ~ 1620 cm-1. All bands were assigned a Lorentzian shape except for the D3 band that has a Gaussian shape according to the literature;42,45,49 band positions and widths were allowed to vary during iterations. The coefficient of determination (R2) was > 0.99 7


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for each curve-fitting run. Raman parameters R1, R2, and R3 were calculated as follows: R1 = ID1/IG, where ID1 and IG are the band intensities of D1 and G band, respectively; R2 = AD1/AD+AG, where AD1 is the integrated intensity (area) of the D1 band, and AD and AG are the D and G bands area, respectively; R3 = A2D1/A2450 +A2D1 + AD1+G +A2D2, where A2D1 is the area of the 2D1 band, and A2450 +A2D1 + AD1+G +A2D2 is the area of all the second-order Raman bands. 2.4. Micro-FTIR analysis Micro-FTIR spectral data were collected with a Nicolet iS50 FT-IR spectrometer equipped with a mercury cadmium telluride detector; the system included a Nicolet Continuum FT-IR microscope that was used to focus on vitrinite in the samples using a 15× objective (N.A. 0.58). Five to ten spectra were collected per petrographic component in each sample in reflectance mode, 1200 scans per collection at a 4 cm-1 resolution was used, following the methods of Presswood et al.58 Maceral identification was based on morphology and relative reflectance. As water in the air can affect data collected in reflectance mode, data collection was limited to times of low humidity (~10%). Background spectra were collected in air on a polished gold slide every hour. The Kramers-Kronig transformation was used to correct the band shifts and the influence of transreflectance in reflectance mode. FTIR data were used only for qualitatively analysis because of the weak band intensities. Results 3.1 Petrographic analysis Based on reflectance, morphological features, and interference colors, organic components in the metamorphosed coals were divided into six groups: vitrinite, inertinite, microcrystalline graphite (MG), pyrolytic carbon (PC), needle graphite (NG), and flake graphite (FG).7 Vitrinite and inertinite have a homogenous texture (Fig. 2a, b, c, d, and e), whereas microcrystalline graphite

6,7

has a fine granular texture (Fig. 2f, g, h, and j). The newly formed pyrolytic carbon

lines fractures and pores, with inner layering normal to the rim of the particles or fractures (Fig. 2h), and shows similar morphologic characteristics to pyrolytic carbon reported previously.59,60 The needle graphite that formed in these graphitized coals has a texture (Fig. 2f and i) that is 8


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similar to that of synthetic graphite with a needle-like structure.61 Part of the newly formed graphite shows broad layers (Fig. 2j), and has a texture similar to flake graphite.17 The petrographic characteristics of these samples change with decreasing distance to the Tianlongshan intrusion. In samples collected further away from the intrusion, macerals such as collotelinite, collodetrinite, pyrofusinite, degradofusinite, macrinite, and inertodetrinite were identified based on their reflectance and morphology (Fig. 2a and b). As distance to the intrusion decreases, it becomes increasingly difficult to distinguish vitrinite and inertinite from one another; however, the use of an antiflex objectice (under cressed polarizers) produces interference colors that aid in the identification of the macerals (Fig. 2d and e). In the highly altered coals, there is an increase in microcrystalline graphite (Table 1) that is optically similar to the microcrystalline graphite from Lutang, China (Fig. 2g), which is also formed from coal5 and is currently mined commercially. Microcrystalline graphite becomes the dominant component in SHL-3 and SHL-5, samples that were collected closest to the intrusion (>80%, Table 1). In these most graphitized coals, newly formed components PC, NG, and FG are present in small amounts, < 10% (Table. 1). Reflectance values of vitrinite show a general increase approaching the granitic intrusion, with Rmax values of 4-5% in JZS-3 and LX-3 (the lowest rank samples) increasing to over 8 or 9% closer to the intrusion (e.g., CM-5, SL-5, BC-3) (Table 1). Maximum reflectance values are then significantly lower in the most graphitized samples (SHL) closest to the pluton (4.5-5%) as measured on the microcrystalline graphite (the predominant component). The commercially mined microcrystalline graphite from Lutang has a maximum reflectance of only 3.95% (Table 1). 3.2 Micro-Raman spectroscopy analysis 3.2.1 Vitrinite and inertinite Several representative Raman spectra measured on homogenous vitrinite (such as that shown in Fig. 2a) in the metamorphosed coals are shown in Fig. 3. Raman parameters including band position and FWHM and band ratio are shown in Table 2, and more detailed Raman parameters 9


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are summarized in the supplemental materials (Table S). For samples JZS-3 and LX-3 collected farthest away from the intrusion, Raman spectra of vitrinite show a broad D band at ~ 1330 cm-1 and a sharp G band at ~ 1600 cm-1 (Fig. 3a and Table 2). The broad D band is composite and can be resolved into D3 and D4 bands at 1500 cm-1 and 1200 cm-1 by curve fitting (Fig. 3a). Approaching the granitic intrusion, the D1 band becomes significantly narrower and more intense (Fig. 3d). A weak D4 band is visible as a slight shoulder on the D1 band; the D3 band is no longer present in the Raman spectra of samples closer to the intrusion (CM-3, CM-5, SL-3, SL-5, BC-3, and BC-5). A well-defined D2 band shows up clearly in the samples collected from the SL and BC mines (Fig. 3c and d). In the second-order Raman spectra, three bands at ~ 2670 cm-1 (2D1), 2930 cm-1 (D1+G) and 3200 cm-1 (2D2) are seen; these become better defined approaching the granitic intrusion (Fig. 3), but the intensity of the 2D2 band is fairly weak in the SL and BC mine samples (Fig. 3c and d). From sample JZS-3 (furthest away from the intrusion) to samples BC-3 and BC-5 (close to the intrusion), the D1 band position increases from 1333 cm-1 to 1348 cm-1 and the D1 band FWHM decreases from 116 cm-1 to 47 cm-1 (Fig. 4a and b). The position of the G band does not show very much variation in this series, but the G band FWHM for samples JZS-3 and LX-3 is less than that in samples closer to the intrusion (Table 2, Fig. 4d and e). The Raman parameter R1 for vitrinite increases significantly from anthracite (0.59) to higher rank coals (3.09) (Table 2, Fig. 4c). The Raman spectral profiles for inertinite (fusinite and macrinite) show changes with rank much like that for vitrinite (Fig. 5). A broad D band width as well as a strong G band intensity are seen in samples farthest away from the intrusion (JZS-3 and LX-3), whereas a sharp D band and a relatively weak G band show up in the inertinite from sample CM-5 (closer to the intrusion). However, the Raman spectral profiles of inertinite and vitrinite in the same sample are similar (Fig. 3a and b, Fig. 5), and the Raman parameters obtained from their Raman spectra are comparable (Table 2 and S). In the much higher rank coals, Raman spectra were not collected on inertinite because it was difficult to identify under the Raman microscope. 10


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3.2.2 Microcrystalline graphite and other newly formed components Figures 6a, 6b, and 6c show representative Raman spectra of the particles with a fine granular texture (i.e., the MG particles shown in Fig. 2f) in the graphitized coals. Fig 6d shows the Raman spectrum for microcrystalline graphite from Lutang, China.5 In the first-order Raman, all graphite particles exhibit a narrow and strong G band, and the D1 band intensity decreases approaching the intrusion (Fig. 6a, b and c, Tables 3 and S). Both the D3 and D4 bands that are observed in vitrinite (Fig. 3) are negligible in the spectra of microcrystalline graphite (Fig. 6). The D2 band becomes less intense approaching the intrusion. The positions of the D1 and the G bands at around 1348 cm-1 and 1581 cm-1, respectively, do not change noticeably in the microcrystalline graphite (Fig. 4a and d). The FWHM of the D1 and G bands for microcrystalline graphite are lower than those of vitrinite (Fig. 4b and e), and the G band FWHM decreases in samples closer to the granitic intrusion (Fig. 4e). In the second-order Raman spectra of the microcrystalline graphite, the 2D1 band (~2700 cm-1) is more intense than that of the vitrinite (Fig. 3 and 6, Table S). R3, the area ratio of the 2D1 band over that for all the second-order bands, increases approaching the intrusion (Table 3, Fig. 4f). In addition, a minor band at ~2450 cm-1 appears (Fig. 6, Table S). Representative Raman spectral profiles for NG and PC from sample SHL-3 are shown in Fig. 7. The structural characteristics of PC and NG are similar to those of MG: a narrow and strong G band with a D2 band that occurs as a shoulder on the first-order Raman, and four second-order bands (Figs. 6 and 7). However, Raman parameters (R1 and R2) are different in these components as shown in Table 3. 3.3 Micro-FTIR analysis The FTIR spectral bands for vitrinite in this study are fairly weak (Fig. 8); thus, FTIR data were used only to qualitatively assess coal structure. For the lower rank coals (JZS-3 and LX-3), the aromatic hydrocarbon stretching band (3100-3000 cm-1) and the aliphatic hydrocarbon stretching band (3000-2800 cm-1) are seen, whereas all of the 3100-2800 cm-1 region bands are lost in the much higher rank coals (Fig. 8). The aromatic out-of-plane deformation bands 11


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(900-700 cm-1) are present as reported previously for intruded coals,58 but the bands in this area become broader for vitrinite in samples with a higher reflectance. This suggests that micro-FTIR may not provide clear structural information for coals with Rr > 5.9% (Fig. 8). Discussion 4.1 Compositional heterogeneity of metamorphosed coals In lower rank coals (lower than anthracite), micro-FTIR data show that functional groups in different macerals (vitrinite, liptinite, and inertinite) vary in both type and amount.62–64 The structural differences between vitrinite and inertinite also are demonstrated by their micro-Raman spectra.48 With an increase in rank, the differences between macerals diminish: in high rank coals, liptinite cannot be distinguished easily from vitrinite and the reflectance of vitrinite merges with that of inertinite.58,62,65,66 In the present study, for coal samples collected further away from the intrusion (JZS-3 and LX-3), despite optical differentiation of collotelinite, pyrofusinite, and degradofusinite based on reflectance and/or morphology (Fig. 2a and c), the Raman spectra and Raman parameters for these components are similar. This suggests that although vitrinite and inertinite preserve some of their original morphological characteristics, other physical and chemical properties of these macerals merge at anthracite stage. Microcrystalline graphite co-exists with vitrinite in the SL and BC mine samples. The Raman spectra of microcrystalline graphite are different to those of vitrinite (Fig. 3 and 6). For example, in sample SL-3, vitrinite shows a high intensity D1 band in the first-order Raman and a low intensity 2D1 band in the second-order, whereas the Raman spectra of microcrystalline graphite exhibit relatively strong G and 2D1 bands (Fig. 3c and 6a). Structural differences are suggested by Raman parameters, including the G band position, D1 band FWHM, and the R1 and R3 ratios (Table 2 and 3). The presence of both microcrystalline graphite and homogenous vitrinite in the same sample attest to the compositional heterogeneity of graphitized coals. In samples collected from sites relatively close to the intrusion (BC and SHL mines), pyrolytic carbon (PC) and needle graphite (NG) are common. Raman parameters (R1 and R2) are different in these components (Table 3). Previous studies have shown that R1 and R2 values 12


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decrease with an increase in La,26,67 suggesting PC has a relatively larger crystalline size than NG (at a nanometer scale). The differences in crystallite sizes for the PC and NG add to the structural heterogeneity in the most graphitized coals. 4.2 Structural evolution of metamorphosed coals Approaching the granitic intrusion, vitrinite reflectance values indicate an increase in coal rank between the JZS and BC mines (Table 1). However, the intensity ratio R1 for vitrinite and that of the microcrystalline graphite show opposite trends (Fig. 4c). The increase in intensity of the D1 band of vitrinite with rank is similar to that seen in carbonized chars as reported by Deldicque et al.31 who showed an increase in D band intensity with increased treatment temperature. Similar phenomena for kerogen and carbonized char were also reported previously.28,68 This suggests that structural evolution of vitrinite in these metamorphosed coals occurred during carbonization.31,56 A plot of the D1 band FWHM versus R1 was used to distinguish between the carbonization stage and the graphitization stage: the D1 band FWHM decreases with an increase in R1 during carbonization (Fig. 9). During graphitization, the D1 band intensity shows the opposite trend due to crystal growth caused by coalescence of neighboring crystallites.31,69 Therefore, continued structural evolution of the microcrystalline graphite occurs in the graphitization stage. In addition, four bands appear in the second-order Raman spectrum of the microcrystalline material; these are comparable to those of microcrystalline graphite collected from Lutang, carbon black,70 and graphitic carbon materials.17 The increase in the 2D1 band intensity from vitrinite to microcrystalline graphite also suggests the latter has reached the graphitization stage.71 Anthracites are composed of nanometric micropores and bi-periodic, turbostratic ordering layers that consist primarily of distorted polyaromatic rings.3,72 The increase in D1 band intensity in vitrinite between samples JZS-3 and BC-3 implies the boundary density between polyaromatic units increases during carbonization,31,73 which is probably due to more sp2 hybridized carbon forming as volatile matter is released, but the polyaromatic rings retain a turbostratic texture during this process. In samples that experienced relatively low levels of metamorphism (JZS-3 13


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and LX-3), the D3 and D4 bands (~ 1500 cm-1 and 1120 cm-1) are observed (Fig. 3a). The D3 band was used as an indicator of the amount of amorphous carbon in carbon blacks and active sites in coals in previous studies.44,70,74 The D3 band is only seen in the Raman spectra of vitrinite in less metamorphosed coals (JZS-3 and LX-3) and disappears in those of higher rank coals (Fig. 3). This suggests that interstitial defects connected by sp3 hybridized carbon decrease during carbonization, and almost disappear in coals with Rr > 5.9%. The decrease in D4 band intensity demonstrates the aliphatic portion of the coal structure is lost during carbonization; this is consistent with the micro-FTIR results that show the aliphatic functional groups are only detected in the least metamorphosed coals (JZS-3 and LX-3) (Fig. 8). The aromatic CH absorption band suggests aromatic ring condensation. The decrease in intensity of the D3 and D4 bands also corresponds to the elemental change reported by Li et al.7 on the same sample set: a significant decrease in H and N content from the CM mine samples, and increase in C content in samples closer to the intrusion. For the fine granular particles that have reached the graphitization stage, the disappearance of the first-order D3 and D4 bands means that aliphatic chains and amorphous carbon are negligible in microcrystalline graphite. The degree of structural order in graphitized CMs is characterized by the Raman parameter R1 reported in previous studies.26,67 Additionally, the effective crystallite size La and the D1 band intensity have been shown to be inversely proportional.75 The decrease in the R1 ratio of the fine granular particles suggests that the structural order of microcrystalline graphite increases between samples from the SL and SHL mines. This result is consistent with the average crystallite parameters (La and Lc) that are derived from XRD data (XRD data from Li et al.7) (Fig. 10). However, it should be noted that the XRD crystallite parameters were obtained on bulk coal, whereas the Raman parameter R1 was collected on microcrystalline graphite. Based on the Raman spectral profile and parameters, the structure of coals collected from the SHL mine is close to that of the well-known commercial microcrystalline graphite from Lutang. Conclusions 14


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Coal samples collected from Xinhua County, central Hunan Province, China have undergone significant metamorphism due to emplacement of the Tianlongshan granitic intrusion. Macerals such as collotelinite, collodetrinite, fusinite, and macrinite are observed in the lower rank coals further away from the intrusion, and new components like PC, NG, and FG are relatively common in the graphitized coals close to the intrusion. Coals from the SHL mine (collected closest to the intrusion) have similar petrographic and structural characteristics to those of the well-known microcrystalline graphite from Lutang, China. Although vitrinite and inertinite macerals still preserve some of their original morphological features, Raman data show that differences in maceral structure in the same sample diminish with increased rank at the anthracite stage. The presence of microcrystalline graphite and vitrinite in the same samples, components that have distinctly different Raman spectra, confirms the compositional and structural heterogeneity of highly graphitized coals. The components PC, NG, and FG add to the structural heterogeneity in the most graphitized coals. Based on optical features and Raman spectral characteristics, the structural evolution of metamorphosed coal ranging between anthracite and coal graphite can be divided into two phases: carbonization for the vitrinite and graphitization for the microcrystalline graphite. Although the amorphous carbon phase and the aliphatic components decrease with increased coal rank, structural disorder in the vitrinite increases approaching the intrusion during carbonization. The increase in the 2D1 band intensity and the appearance of an additional band at 2450 cm-1 in the second-order Raman of the microcrystalline component, may suggest that these components have reached the graphitization stage. The decrease in both the R1 ratio and the D2 band intensity in spectra of the microcrystalline graphite suggests that crystallite sizes increase with increased proximity to the intrusion. Supporting Information Raman spectrum of polished Sri Lanka graphite (Figure S), detailed Raman parameters (Table S). Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of 15


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China (41672150, 51034006). The Spackman Award from The Society for Organic Petrology and the Antoinette Lierman Medlin Scholarship from the Geological Society of America, Energy Division (both to KL) are also acknowledged. The authors thank Dr. Alian Wang of Washington University in St. Louis, MO, for access to her Laser Raman Imaging Laboratory; Jian Chen also of Washington University in St. Louis for help with micro-Raman measurements; Justin Filiberto at SIUC for use of his micro-FTIR; and Dandan Hou at The Pennsylvania State University for help with Raman spectral peak-fitting. The authors also thank John Crelling at SIUC, for enlightening discussions on graphite. References

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List of Figure Captions. Fig.1. Geological sketch map of the study region showing sample locations (modified after Li et al. 7). a) Study area in China; b) and c) location of mines sampled for this study; inserts d) and e) schematic representation of coal beds far away from (horizontal) and close to (near vertical) the intrusion, respectively. Fig. 2. Representative photomicrographs of coal samples. All images taken under white light, 40× antiflex objective, oil immersion; for d, e, g, h, j and l, crossed nichols were used. Scale bar length is 20m. a) Collodetrinite (Cd), pyrofusinite (Pf) and degradofusinite (Df), JZS-3, Rr = 4.36%; b) pyrofusinite (Pf), JZS-3, Rr = 4.36%; (Pf); c) collodetrinite (Cd), macrinite (Ma), and fusinite (Fs), SL-5, Rr = 8.23%; d) degradofusinite (Df), CM-3, Rr = 5.91%; e) collodetrinite (Cd) and macrinite (Ma), SL-5, Rr = 8.23%; f) microcrystalline graphite (MG) and needle-like graphite (NG), SHL-3, Rr = 4.58%; g) microcrystalline graphite (MG), Lutang, Rr = 3.63%; h) microcrystalline graphite (MG) and pyrolytic carbon (PC), SHL-3, Rr = 5.01%; i) needle-like graphite (NG), SHL-3, Rr = 4.58%; j) flake graphite (FG) and microcrystalline graphite (MG), SHL-5, Rr = 5.01%. Fig. 3. Representative Raman spectra of vitrinite in metamorphosed coals. a) JZS-3, Rr = 4.36%; b) CM-5, Rr = 7.67%; c) SL-3, Rr = 6.09%; d) BC-3, Rr = 6.62%. Fig. 4. Raman parameters versus distance from the intrusion (plotted on a log scale); error bar shows one standard deviation. Fig.5. Representative Raman spectra of inertinite in metamorphosed coals. Pyrofusinite (Pf) from JZS-3; degradofusinite (Df) from LX-3; macrinite (Ma) from CM-5. Fig. 6. Representative Raman spectra of graphite with different degrees of graphitization. a) SL-3, Rr = 6.09%; b) BC-3, Rr = 6.62%; c) SHL-5, Rr = 5.01%; d) Lutang, Rr = 3.63%. Fig. 7. Representative Raman spectra of newly formed components. Needle-like graphite (NG) and pyrolytic carbon (PC) from SHL-3. Fig.8. Micro-FTIR spectra for vitrinite of three representative samples. Spectra are baseline corrected and normalized to a maximum absorbance intensity of 1.00. JZS-3, Rr = 4.36%; LX-3, Rr = 4.7%; BC-5, Rr = 7.48%. Fig. 9. Plot of D1 band FWHM versus R1, that can be used to discriminate between carbonization and graphitization paths 56. Fig. 10. Crystallite parameters versus R1 of microcrystalline graphite. Crystallite parameters La and Lc derived from XRD spectra according to Scherrer formula: La = 1.84λ/ Ba cos(φa); Lc = 0.89λ/ Bc 22


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cos(φc), where Ba and Bc are the full width at half maximum (FWHM) of the (100) and (002) peaks, and φa and φc are the corresponding scattering angles (Li et al. 7). Table 1 Reflectance values and maceral composition of a series of naturally graphitized coals and commercial microcrystalline graphite (Lutang). Dist, distance from intrusion (km). Rr, mean random reflectance; Rmax, mean maximum reflectance (%); Rmin, mean minimum reflectance (%); MG, microcrystalline graphite; PC, pyrolytic carbon; NG, needle graphite; FG, flake graphite. Includes data from Li et al. 7. Samples

JZS-3

LX-3

CM-3

CM-5

SL-3

SL-5

BC-3

BC-5

SHL-3

SHL-5

Lutang

Dist (km)

21.0

6.5

2.0

2.0

1.1

1.1

0.6

0.6

0.3

0.3

–

Rr

4.36

4.70

5.91

7.67

6.09

8.23

6.62

7.48

4.58

5.01

3.63

s.d.

0.51

0.43

1.14

1.81

1.32

1.84

1.94

2.10

1.70

1.02

0.97

Rmax

5.06

5.26

7.80

9.49

8.14

9.96

8.85

9.11

4.95

5.54

3.95

s.d.

0.31

0.36

0.79

0.65

1.03

1.21

1.00

1.32

1.03

1.40

1.27

Rmin

3.05

3.17

2.40

3.08

2.27

4.19

2.22

2.98

1.98

2.75

1.84

s.d.

0.68

0.66

1.16

1.30

1.30

1.74

1.15

1.84

1.21

1.01

1.02

Vitrinite

54.5

53.5

51.7

48.2

37.7

45.1

17.5

17.0

0

0.9

–

Inertinite

45.2

46.2

48.2

51.1

39.6

50.4

27.3

30.8

0.4

4.3

–

MG

0

0

0

0

15.3

0.2

46.6

41.6

86

82.6

–

PC

0.3

0.3

0.1

0

1.3

1.1

3.8

6.4

9.6

4.8

–

NG

0

0

0.2

0.7

6.0

2.8

4.4

3.9

3.7

6.8

–

FG

0

0

0

0

0.1

0.4

0.4

0.3

0.3

0.6

–

Table 2 Micro-Raman spectroscopy parameters for vitrinite and inertinite in metamorphosed coals. Unit of band position and FWHM is cm-1. R1 = ID1/IG; R2 = AD1/AD+AG; R3 = A2D1/A2450 +A2D1 + AD1+G +A2D2; Pyrofusinite (Pf) is from sample JZS-3; degradofusinite (Df) is from sample LX-3; macrinite (Ma) is from sample CM-5. Each parameter is followed by its standard deviation (s.d.), number of measurements for each petrographic component is in the parentheses following sample ID. Samples

D1 band

G

　　 Position

　 FWHM

2D1 band

　 Position

FWHM

　

D1+G band

　 Position

FWHM

2D2

　

　 Position

Ratio

　

Position

FWHM

FWHM

JZS-3 (7)

1333

116

1598

39

2631

293

2906

238

3192

76

s.d.

1.66

8.76

1.01

0.15

0.53

0.26

0.74

2.98

0.35

LX-3 (4)

1334

111

1600

38

2632

275

2906

250

s.d.

0.97

10.59

2.61

4.87

2.98

4.58

5.19

CM-3 (5)

1347

58

1598

51

2679

147

s.d.

0.44

5.65

0.62

0.28

3.59

CM-5 (5)

1346

63

1598

53

s.d.

0.21

4.64

0.71

0.39

SL-3 (5)

1348

51

1592

s.d.

0.30

4.00

0.04

　 R1

R2

R3

0.62

0.51

0.44

3.05

0.07

0.01

0.02

3196

74

0.59

0.48

0.42

19.27

4.89

9.05

0.10

0.02

0.03

2934

92

3215

85

1.84

0.64

0.55

23.89

0.33

2.32

0.19

0.15

0.06

0.02

0.03

2678

207

2934

123

3199

122

1.76

0.63

0.53

0.01

6.29

0.67

3.56

0.92

9.29

0.04

0.00

0.02

48

2683

124

2938

89

3214

82

2.70

0.67

0.56

0.15

0.21

6.77

0.12

7.82

3.31

21.67

0.00

0.00

0.03

23


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

SL-5 (8)

1348

53

1591

50

2684

142

2937

102

3209

102

2.54

0.67

0.56

s.d.

0.99

6.42

0.13

5.78

2.07

15.84

1.21

6.41

1.98

21.49

0.28

0.02

0.02

BC-3 (3)

1348

47

1589

44

2686

103

2937

91

3216

90

3.09

0.70

0.57

s.d.

0.80

4.80

0.80

0.61

0.12

3.04

0.03

2.36

1.09

7.85

0.09

0.00

0.00

BC-5 (4)

1348

50

1591

48

2685

126

2937

100

3211

89

2.63

0.67

0.56

s.d.

0.72

3.29

0.57

2.28

1.99

5.42

1.18

5.90

1.56

4.42

0.10

0.01

0.02

Pf (4)

1332

115

1598

42

2632

285

2908

239

3192

66

0.62

0.52

0.44

s.d.

0.85

7.62

0.89

2.49

1.23

9.25

3.69

11.32

0.86

2.84

0.02

0.02

0.00

Df (3)

1334

119

1602

38

2644

327

2918

252

3197

74

0.62

0.52

0.47

s.d.

0.79

8.05

1.03

0.77

1.89

0.84

2.46

5.53

3.78

6.17

0.08

0.02

0.03

Ma (3)

1348

65

1600

53

2683

207

2927

114

3200

88

1.90

0.65

0.51

s.d.

0.32

4.32

2.35

1.04

2.39

5.39

14.30

8.26

2.05

3.89

0.10

0.05

0.08

Table 3 Micro-Raman spectroscopy parameters for the graphites and newly formed components. Unit of band position and FWHM is cm-1. R1 = ID1/IG; R2 = AD1/AD+AG; R3 = A2D1/A2450 +A2D1 + AD1+G +A2D2; newly formed components pyrolytic carbon (PC) and needle-like graphite (NG) are from sample SHL-3. Each parameter is followed by its standard deviation (s.d.), number of measurements for each petrographic component is in the parentheses following sample ID.

Samples

D1 band Position

G

　

FWHM

　 Position

　 FWHM

2D1 band

　 Position

FWHM

　

D1+G band

　 Position

FWHM

2D2

　

　 Position

Ratio

　

FWHM

　 R1

R2

R3

SL-5 (3)

1347

50

1583

31

2696

63

2936

100

3232

68

1.18

0.60

0.69

s.d.

1.50

7.25

1.00

3.38

0.02

0.54

0.24

4.94

1.40

6.47

0.06

0.00

0.04

SL-3 (4)

1349

39

1583

31

2695

53

2939

82

3233

61

1.41

0.58

0.73

s.d.

0.84

5.89

0.89

0.82

0.16

1.97

1.04

7.00

0.34

8.14

0.20

0.03

0.03

BC-3 (8)

1349

38

1581

24

2698

53

2944

45

3238

50

0.55

0.44

0.84

s.d.

0.50

4.13

0.72

0.79

0.04

1.60

0.98

2.65

0.56

4.80

0.08

0.01

0.03

BC-5 (6)

1347

50

1580

23

2700

59

2940

71

3241

27

0.46

0.45

0.85

s.d.

0.65

4.11

1.05

0.22

0.08

0.78

1.82

10.05

0.67

8.12

0.09

0.00

0.00

SHL-3 (5)

1349

44

1580

22

2699

62

2941

78

3240

38

0.47

0.44

0.83

s.d.

0.53

3.91

0.38

1.32

0.89

3.95

0.20

28.54

2.24

26.09

0.10

0.04

0.08

SHL-5 (4)

1349

39

1579

21

2700

65

2943

77

3242

27

0.43

0.43

0.86

s.d.

0.69

3.28

0.91

0.28

0.07

1.39

2.01

26.22

0.42

3.43

0.00

0.01

0.02

Lutang (6)

1347

46

1579

21

2702

68

2942

66

3241

23

0.39

0.44

0.87

s.d.

0.76

8.60

0.35

2.61

2.28

0.58

2.02

24.09

2.61

14.22

0.04

0.02

0.07

PC (3)

1351

43

1582

23

2705

73

2945

71

3241

33

0.63

0.53

0.82

s.d.

0.67

4.89

0.54

1.33

3.09

2.36

0.60

3.32

4.16

9.45

0.06

0.03

0.01

NG (3)

1351

39

　 0.94

0.61

0.83

s.d.

0.54

2.03

0.03

0.01

0.01

　 1583 0.37

25 0.65

　 2702 0.81

68 4.87

　 2947 1.38

77 10.74

　 3244 1.32

26 10.32

24


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Energy & Fuels

Fig.1


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Page 26 of 35

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Energy & Fuels

Fig.2


(Continued)

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 28 of 35

G

D1

D1

G

D4

D3

D1+G

D1+G D4

2D1

D1

2D2

D1

G D4

2D1

2D2

D2

G

D1+G 2D1

D4 2D2

Fig.3


D1+G

D2 2D1

2D2

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Energy & Fuels

Fig.4


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 30 of 35

G

D

D1+G 2D1

2D2

JZS-3

LX-3

CM-5

Fig.5


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Energy & Fuels

D1 G

G 2D1 2D1

D1

D2

D1+G

D2

D1+G 2D2 2450

2D2

2450

G

G

2D1 D1

D1 D2 2450

D1+G 2D2

Fig.6


2D1 D2 2450

D1+G 2D2

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 32 of 35

D G 2D1

D2

D1+G 2450 D2

Fig.7


2D2

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Energy & Fuels

Fig.8 ACS Paragon Plus Environment

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Fig.9


Page 34 of 35

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Energy & Fuels

Fig.10


Micro-Raman spectroscopy of microscopically distinguishable

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