Geochemical and petrographic alteration of rapidly heated coals from the Herrin (No. 6) Coal Seam, Illinois Basin

International Journal of Coal Geology 165 (2016) 243–256

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Geochemical and petrographic alteration of rapidly heated coals from the Herrin (No. 6) Coal Seam, Illinois Basin Severin M. Presswood, Susan M. Rimmer ⁎, Ken B. Anderson, Justin Filiberto Department of Geology, Southern Illinois University Carbondale, Carbondale, IL, 62901, USA

a r t i c l e

i n f o

Article history: Received 14 July 2016 Received in revised form 21 August 2016 Accepted 22 August 2016 Available online 5 September 2016 Keywords: Intruded coals Natural coke Reflectance micro-FTIR Vitrinite reflectance Volatile matter

a b s t r a c t Coals altered by rapid heating events (i.e., those intruded by dikes and sills) are thought to follow a different geochemical maturation pathway than coals altered through diagenesis. If an igneous intrusion alters the petrographic and geochemical properties of a coal, the effect should also be observable in the coal's molecular structure. In this study, we evaluate whether coals altered by rapid heating follow distinct maturation trends from coals that were altered by slower heating (burial maturation). Petrographic, geochemical, and micro-FTIR analyses were performed on a series of Pennsylvanian Illinois Basin coal samples, collected at various distances from a Permian igneous dike. Standard coal characterization techniques including vitrinite reflectance and proximate and ultimate analyses provide valuable insights on the maturation pathways experienced by rapidly heated coals. These techniques were coupled with reflectance micro-FTIR to provide a better understanding of the molecular changes that occur in the coal structure during relatively short-lived, intensive heating events. With decreasing distance to the intrusion, coals have higher mean random vitrinite reflectance values (Rr) within the dike alteration zone. Coking textures similar to those observed in industrial cokes are observed within 2 m of the intrusion. Geochemical data for HCl-treated coals indicate an overall loss of H, O, and N and an increase in C approaching the dike. Intruded coals have higher volatile matter (VM) yields at high rank than coals of similar rank that result from normal burial maturation. When plotted on a van Krevelen diagram or Seyler chart, intruded coals follow different coalification trends than coals matured through normal burial diagenesis. Reflectance micro-FTIR analysis of collotelinite shows increased aromaticity with rank: both the ratio of the aromatic CH stretching band at ~3100–3000 cm−1 versus the aliphatic CHx stretching bands between 3000 and 2800 cm−1 (AR1), and the ratio of the aromatic out-of-plane deformation bands between ~900–700 cm−1 versus the aliphatic CHx band (AR2) increase with increasing Rr. Within the 3000–2800 cm−1 region, there is an increase in the area under the asymmetric CH3 peak at ~2960 cm−1 relative to the asymmetric CH2 peak at ~2920 cm−1 with increased rank. Within the 900–700 cm−1 region, the overall intensity of the ~750 cm−1 peak (aromatic rings with four adjacent H atoms) relative to the ~870 cm−1 peak (aromatic rings with one isolated H atom) increases up to 2.5% Rr, likely reflecting a lower degree of substitution (DOS) of alkyl groups on aromatic ring sites. The prevalence of the 750 cm−1 peak at high rank may represent a lower degree of condensation of aromatic rings in the structure of intruded coals compared to normally matured coals. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The effects of igneous intrusions on the geochemical, mineralogical, and petrographic composition of bulk coals and the microscopically distinguishable components of coals (macerals) have received considerable attention (Clayton and Bostick, 1985; Dai and Ren, 2007; Finkelman et al., 1998; Golab and Carr, 2003; Murchison, 2004, 2006; Murchison and Raymond, 1989; Raymond and Murchison, 1992; Rimmer et al., 2009, 2015; Stewart et al., 2005; Yoksoulian et al., 2016). Coals that have been heated rapidly by intruding dikes and sills have been shown to follow different coalification trends than those ⁎ Corresponding author. E-mail address: [email protected] (S.M. Rimmer).

http://dx.doi.org/10.1016/j.coal.2016.08.022 0166-5162/© 2016 Elsevier B.V. All rights reserved.

altered under normal diagenetic conditions (Murchison, 2004, 2006; Murchison and Raymond, 1989; Rahman and Rimmer, 2014; Raymond and Murchison, 1992; Rimmer et al., 2009). When data for coals altered by igneous intrusion are plotted on a van Krevelen diagram, they have lower H/C ratios at given O/C values in comparison to coals matured through burial diagenesis (Rimmer et al., 2009; van Krevelen, 1993). Coals altered by igneous intrusions have higher volatile matter (VM) contents when compared to coals altered by diagenesis (Rahman and Rimmer, 2014; Rimmer et al., 2009). Only limited research has focused on the molecular trends coals experience through interaction with magmatic bodies (Amijaya and Littke, 2006; Dun et al., 2013). This study presents reflectance micro-FTIR spectra of vitrinite macerals from coal samples collected along a transect adjacent to an igneous dike.

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1.1. Previous FTIR studies of coal rank Reflectance micro-FTIR can provide information on changes in the relative abundances of some specific molecular substructures, and it has been demonstrated as a viable tool for the characterization of coal macerals (Chen et al., 2012; Mastalerz and Bustin, 1993a, 1993b, 1995, 1996). This approach allows in-situ characterization of individual macerals, and provides greater detail on the heterogeneity of coals (Chen et al., 2012; Mastalerz and Bustin, 1993a, 1993b, 1995, 1996). Sample preparation for reflectance micro-FTIR is identical to that for vitrinite reflectance analysis, allowing for detailed comparisons between petrographic and molecular features (Mastalerz and Bustin, 1995). The reflectance micro-FTIR process is non-destructive, is very reproducible when the same area on a maceral is measured (Mastalerz and Bustin, 1996), and does not require separation techniques such as density-gradient centrifugation (Dyrkacz and Horwitz, 1982) in order to analyze individual macerals (Mastalerz and Bustin, 1995). Much of the fundamental work, including the assignment of specific bands within the spectra (Painter et al., 1981, 1982, 1985; Wang and Griffiths, 1985), and the overall applications of FTIR to the characterization of the complex structure of coal and kerogen (Painter et al., 1982; Solomon et al., 1982) were discussed extensively in the 1980's. Several advances have been made recently in the FTIR characterization of coals (Chen et al., 2012; Dun et al., 2013; Li et al., 2007; Petersen et al., 2007), facilitated by enhanced signal processing techniques and detector capabilities, as well as advanced software capable of rapid spectral deconvolution (Chen et al., 2012; Dun et al., 2013; Li et al., 2007; Petersen et al., 2007). Several studies have used FTIR data to characterize the distributions of chemical functional groups within coals of different ranks (Amijaya and Littke, 2006; Chen et al., 2012; Dun et al., 2013). Previous FTIR studies of coals have found aromaticity, condensation of aromatic rings, and out-of-plane aromatic C\\H responses increase with increasing coal rank (Chen et al., 2012). Aliphatic chain length, as well as the absorbance of aromatic C_C ring stretch, decreases with increasing rank (Amijaya and Littke, 2006). FTIR studies on intruded coals have generally been conducted using KBr-FTIR techniques (Amijaya and Littke, 2006; Dun et al., 2013; Saikia et al., 2007) and are therefore representations of bulk molecular trends. Reflectance micro-FTIR is capable of rapid data collection on individual macerals in-situ (Mastalerz and Bustin, 1993a, 1993b, 1995, 1996), and could be used to complement bulk geochemical data and vitrinite reflectance trends that have been identified in rapidly heated coals. 1.2. Characteristic IR absorption bands in the FTIR spectra of coal Several mid-infrared absorption bands are commonly used for coal studies (Chen et al., 2012; Mastalerz and Bustin, 1995). Typically, these are selected on the basis of being well resolved (i.e., occurring in regions of the spectrum where there are relatively few overlapping absorbances) and useful for investigation of specific structural characteristics of interest (i.e., they correspond to specific, well defined, molecular sub-structures). Among the most useful absorption bands in the microFTIR spectra are: (1) the 3100–3000 cm−1 region, representing aromatic C\\H stretching bands; (2) the 3000–2800 cm−1 region, representing aliphatic C\\H stretching; and (3) the 900–700 cm−1 bandwidth, representing aromatic out-of-plane deformation modes. These absorptions can provide semi-quantitative information regarding the evolution of aromatic and aliphatic structures with increased coal rank (Chen et al., 2012; Li et al., 2007). Due to the inherent bandwidth of most mid IR absorption bands and the structural complexity and heterogeneous nature of coal structure, coals express broad and overlapping peaks in their IR spectra. Most observed absorption bands are actually comprised of several individual bands (Painter et al., 1982). The most prominent issue in studying coals via FTIR techniques involves the separation of these overlapping

absorptions into their individual components using deconvolution functions (Painter et al., 1982). Absorption observed in the 3000–2800 cm−1 range is composed of the symmetric and asymmetric stretching vibrations of aliphatic CH structures. Previous studies (Chen et al., 2012; Ibarra and Miranda, 1995) have attempted to use the ratio of the area under the asymmetric methyl peak (~ 2955 cm−1) to the area under the asymmetric methylene peak (~2920 cm−1) as a semi-quantitative measurement of aliphatic chain length (Ibarra and Miranda, 1995), although this is complicated by differences in the extinction coefficients associated with these discrete absorbances. Deconvolution of absorbances between 900 and 700 cm−1, which largely correspond to outof-plane deformations of aromatic C\\H groups, can be used to measure the degree of substitution of aromatic sites with alkyl groups, through the comparison of the area under the 870 cm−1 peak (typically associated with deformation of isolated aromatic C\\H structures) versus the area under the 750 cm−1 peak (typically associated with deformations of aromatic CH structures with 4 adjacent aromatic protons) (Chen et al., 2012; Iglesias et al., 1995). The absorption from ~1800–1550 cm−1 represents both oxygenated (carbonyl/carboxyl) structures and ring-stretching modes of aromatic carbon groups. The prominent peak centered at ~1600 cm−1 is representative of aromatic ring stretching modes, but several challenges exist within the interpretation of this peak as a quantitative measurement of aromatic groups or oxygenated structures (Li et al., 2007; Painter et al., 1983, 1985). Water bands overlap within this region of the spectra. Water bands can significantly alter the results of deconvolution if precise atmospheric substitution is not achieved, or if the atmospheric conditions in the laboratory change during analysis (Venyaminov and Prendergast, 1997). The number of bands that makes up the 1800–1550 cm−1 region makes deconvolution very difficult (Li et al., 2007; Painter et al., 1983, 1985). The separation of this region differs considerably from author to author as well (Chen et al., 2012; Ibarra and Miranda, 1995). Regardless of the degree of separation of the 1800–1550 cm−1 bandwidth, there is still considerable overlap of the aromatic C_C band by highly conjugated C_O bands (Ibarra and Miranda, 1995; Painter et al., 1983, 1985). Similarly, there is overlap of absorption due to aromatic C_C structures by absorption due to carboxyl groups. This is especially significant in lower rank coals where COO– groups are commonly prevalent (Li et al., 2007). It is therefore difficult to use the aromatic C_C band at ~1600 cm−1 as a quantitative indicator of aromaticity (Li et al., 2007). 2. Methods 2.1. Sampling Thirty-six samples were collected along a transect of the Herrin (No. 6) Coal at various distances from a northwest-trending igneous intrusion from an underground mine in northern Saline County, Illinois (Fig. 1). Grab samples of approximately 200 g each were collected along the transect. NEM-1 was collected as close to the intrusion as possible. Samples NEM-2 through NEM-15 were collected in 3- to 10-cm intervals. 30- to 50-cm intervals were used for collection of NEM-16 through NEM-25. Beyond this range, the distance between sample points increased to 0.2 to 0.3 m to ensure sufficient collection of unaltered background coal. The sampling interval was more closely spaced near the intrusion due to exponential maturation reported by Rimmer et al. (2009) for another intruded coal seam in this region. 2.2. Sample preparation Coals were processed to minus 20-mesh for petrographic and microFTIR analysis, and to minus 60-mesh for geochemical analysis according to ASTM standards (ASTM, 2011a). Ground coals were embedded in epoxy resin and polished to a final 0.06 μm colloidal silica polish by

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245

Fig. 1. Map showing location site of sampling site, the New Era Mine near Galatia, IL.

methods outlined in Pontolillo and Stanton (1994) for both petrographic and micro-FTIR analysis. 2.3. Petrographic and geochemical analysis Point-count data were collected using a Zeiss Universal reflectedlight microscope fitted with a 40/0.85 Pol oil antiflex objective. To distinguish coke textures, crossed nicols were used. Both white- and blue-light illumination was used during point-count analysis, specifically in samples below a vitrinite reflectance of ~1.36% where liptinite fluorescence aided in the identification of macerals. Categories counted included vitrinite, liptinite, and inertinite, and anisotropic and isotropic coke textures. For all samples, 500 counts were collected on each of two pellets. Variations in point-count results between the two pellets did not vary by N2–3%, and the average of the two point counts is reported for each sample. Samples were analyzed using a Leica DM 2500P petrographic microscope with integrated TIDAS CCD UV/NIR hardware and software to determine mean random vitrinite reflectance (Rr). Defect-free and homogenous collotelinite particles (and in altered samples, the coked counterpart) were used for reflectance measurements, with 50 measurements being averaged from each of two pellets per sample. Only one measurement was taken on any given piece of collotelinite. Carbon, H, and N analyses were performed at ALS Environmental in Tucson, AZ, by combustion/thermo-conductivity with IR detection using a LECO TruSpec Macro in accordance with ASTM D5373-08 (ASTM, 2008). Proximate and total sulfur analyses were performed at the Kentucky Geologic Survey in Lexington, KY, using standard ASTM methodologies D7582-10 and D4239-11 (ASTM, 2010, 2011b). The coals in this study had been altered by the intrusion of the igneous dike and therefore likely contain carbonates, which can form due to devolatilization of organic carbon and reactions of hydrothermal fluids

associated with an igneous intrusion (Mastalerz et al., 2009; Schimmelmann et al., 2009; Stewart et al., 2005; Rimmer et al., 2009). A subset of samples was selected for carbonate removal with acid and then was reanalyzed to determine the effect of the intrusion on carbonate content. Approximately 10 g of each sample was treated with 100 ml of 6 N HCl and allowed to react over a 12-h period. The samples were then centrifuged and filtered, and the process was repeated until no effervescence was observed upon addition of HCl. The samples were then washed repeatedly with distilled water to a pH of 5.5. This process was reported to remove all carbonate by Rimmer et al. (2009) and follows the procedure outlined by Yoksoulian (2010). Acid-treated samples were then submitted to the Kentucky Geological Survey for proximate and total sulfur analysis, and ALS Environmental for C, H, and N analysis. 2.4. Reflectance micro-FTIR analysis Reflectance micro-FTIR spectra of pure collotelinite were collected using a Nicolet iS50 FTIR spectrometer coupled with a DTGS detector, and equipped with a Nicolet Continuum FTIR microscope linked to a liquid nitrogen-cooled mercury cadmium telluride detector. A 15× air objective was used for all micro-FTIR spectra collection. The microscope was operated in reflectance mode. For micro-FTIR analysis, 1200 scans at a 4 cm−1 resolution were collected and co-added. Atmospheric background spectra were collected on a polished gold mirror and subtracted from the sample spectra to remove the effects of water and CO2. The program OMNIC was used to control data acquisition, and for deconvolution of spectra, peak area and height determinations, as well as curve-fitting of spectral features. ATLUS software controlled the movement of the motorized stage. The Kramers-Kronig transformation was applied to all reflectance micro-FTIR spectra. This transformation corrects for the influence of transreflectance and shifts spectral bands in reflectance micro-FTIR analysis to positions that are comparable to

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KBr-FTIR analysis (Kramers, 1927; Kronig, 1926). It has been demonstrated that the Kramers-Kronig transformation is a necessary procedure for reflectance-micro FTIR analysis of polished coal pellets, specifically if the spectra are to be compared with KBr spectra (Mastalerz and Bustin, 1996). For deconvolution of individual spectral features within a given region of the spectrum, the second derivative function was used in conjunction with previous work (Ibarra and Miranda, 1995; Painter et al., 1981, 1982). Curve-fitting results can be suspect if care is not taken during the process, and analytical bias can result from improper deconvolution (Painter et al., 1982). To determine the peak center position of individual bands, the second derivative was displayed above the spectra. The minima of the second derivative profile correspond to the position of peaks and shoulders of the absorbance spectra (Maddams, 1980). Once peak positions were established with the second derivative, the best fit was calculated using OMNIC based on a minimization of the difference between the true spectral profile and the derived summary curve of the individual peaks. OMNIC used a combination of Lorentzian and Gaussian band shapes to fit the curves. Consistent deconvolution parameters were used for every coal in the rank suite, and baseline correction was performed prior to deconvolution. Several semi-quantitative FTIR absorption ratios were used to analyze the coals (Table 1). The ratio of the absorption area of the aromatic CH stretch to total aliphatic CHx stretch (including both symmetric and asymmetric stretching) absorption was used as a proxy for aromaticity (AR1) (Chen et al., 2012). Likewise, the ratio of the area of the aromatic out-of-plane deformation modes to total aliphatic CHx stretch was also used as a supporting proxy for aromaticity (AR2) (Chen et al., 2012). The aliphatic CHx stretching modes were deconvoluted, and the ratio of the asymmetric methylene (CH2) stretching (area of peak centered at 2920 cm−1) to asymmetric methyl (CH3) stretching (area of peak centered at 2960 cm−1) was used as an indication of average aliphatic chain length (Chen et al., 2012; Lin and Ritz, 1993). The aromatic outof-plane deformation modes were also deconvoluted, and the ratio of the aromatic rings containing one isolated H atom (area of peak centered at 870 cm− 1) to aromatic rings containing four adjacent H atoms (area of peak centered at 750 cm− 1) was used as a proxy for the degree of condensation of aromatic rings, and degree of substitution of aromatic sites with alkyl groups (DOS) (Chen et al., 2012; Iglesias et al., 1995). 3. Results 3.1. Petrography of intruded coals Mean random vitrinite reflectance of the coal samples ranges from background levels of approximately 0.55% up to 5.00% (Rr, % in oil) at the contact with the igneous intrusion (Table 2, Fig. 2). The alteration zone begins at the point where Rr increases above background levels (NEM-19, Rr = 0.62%); all coals within the alteration zone have higher

Table 2 Petrographic data for intruded coals. D = mid-point of sample from intrusion, m; Rr = mean random vitrinite reflectance (%, in oil); Vit, Lip, Int = vitrinite, liptinite, and inertinite (volume, %), respectively; IC, AC = isotropic coke and anisotropic coke (volume, %), respectively; T = estimated maximum temperature (°C) calculated from Ln(Rr) = 0.0078Tmax − 1.2 (Barker and Pawlewicz, 1994). Sample

D

Rr

Vit

Lip

Int

IC

AC

T

NEM-1 NEM-2 NEM-3 NEM-4 NEM-5 NEM-6 NEM-7 NEM-8 NEM-9 NEM-10 NEM-11 NEM-12 NEM-13 NEM-14 NEM-15 NEM-16 NEM-17 NEM-18 NEM-19 NEM-20 NEM-21 NEM-22 NEM-23 NEM-24 NEM-25 NEM-26 NEM-27 NEM-28 NEM-29 NEM-30 NEM-31 NEM-32 NEM-33 NEM-35 NEM-36 NEM-37

0.04 0.23 0.38 0.58 0.70 0.90 1.17 1.43 1.65 2.02 2.25 2.55 2.78 2.98 3.30 3.62 3.83 4.13 4.43 4.78 5.03 5.43 5.83 6.23 6.68 7.15 7.73 8.38 9.18 10.08 10.93 11.48 13.05 16.18 18.98 21.50

5.00 3.12 3.03 2.90 2.79 2.90 2.55 2.46 2.15 1.83 1.79 1.65 1.63 1.42 1.36 1.21 1.07 1.01 0.62 0.55 0.55 0.55 0.55 0.55 0.56 0.56 0.55 0.56 0.56 0.57 0.55 0.55 0.55 0.57 0.56 0.56

75.2 77.2 78.4 76.4 79.2 78.0 76.4 74.4 72.8 80.2 81.6 82.8 79.2 81.4 85.0 83.4 82.8 82.2 80.6 78.8 76.6 78.4 79.0 74.8 74.0 78.8 81.6 79.2 78.6 83.6 81.2 80.2 76.2 77.0 80.2 81.6

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.4 2.2 2.8 3.2 2.8 2.2 1.8 3.4 3.8 2.2 2.8 2.4 2.6 3.2 3.4 4.0 2.6 2.8 2.8 3.2 2.4

24.8 22.8 21.6 23.6 20.8 22.0 23.6 25.6 27.2 19.8 18.4 17.2 20.8 18.6 15.0 14.2 15.0 15.0 16.2 18.4 21.2 19.8 17.6 21.4 23.8 18.4 16.0 18.2 18.2 13.0 14.8 17.2 21.0 20.2 16.6 16.0

32.4 74.8 76.4 76.4 79.2 78.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

42.8 2.4 2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

360 300 296 290 285 290 274 269 252 232 228 218 216 199 193 178 163 155 93 76 77 77 77 77 80 80 77 80 80 82 77 77 77 82 80 80

vitrinite reflectances than coals outside of the alteration zone, with reflectances increasing towards the contact. Petrographic observations (Fig. 3, Table 2) show the coals are predominantly vitrinite or thermally altered vitrinite, 75–85% by volume. Vitrinite is present primarily as homogenous collotelinite and as matrix collodetrinite, the latter containing fragments of other macerals (Fig. 3a). Devolatilization vacuoles are observed within the coked vitrinite of highly altered coals, (Fig. 3b), and appear to become more

Table 1 Micro-FTIR semi-quantitative ratios (Chen et al., 2012). Semi-quantitative bandwidth ratio

Ratio calculation

Band region (cm−1)

Aromaticity (ARI)

Aromatic CH stretch/aliphatic CHx stretch Aromatic CH out-of-plane deformation/aliphatic CHx stretch Asymmetric methylene/asymmetric methyl Aromatic rings with 1 isolated H atom/aromatic rings with 4 adjacent H atoms

(3100−3000)/(3000–2800)

Aromaticity (AR2)

Aliphatic chain length Degree of condensation of aromatic rings (DOS)

(900–700)/(3000–2800)

2920/2960

870/750 Fig. 2. Mean random vitrinite reflectance (Rr, % in oil) versus distance from dike contact. Within the alteration halo, Rr increases towards the contact with the intrusion.

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prevalent with increased rank inside of the alteration halo. Inertinite is fairly abundant as well, constituting 13–25% by volume of the coal (Table 2). The majority of the inertinite is fusinite, with a smaller quantity of the less reflective semifusinite (Fig. 3c). In high rank samples, differentiation of thermally altered vitrinite and inertinite was based on morphology rather than colour due to the similarity in reflectance (Fig. 3d). Liptinite is generally uncommon, representing 2–4% (by volume) in the unaltered coals. Liptinite occurs mostly as sporinite (Fig. 3e), although cutinite and some resinite are seen as well. The fluorescence of liptinite changes from light yellow in the background samples (Rr = 0.55–0.57%) (Fig. 3e) to dark orange (Fig. 3f) within the outer part of the alteration halo. Liptinite macerals are not visible in samples with reflectances of 1.36% (NEM-15) and above. The overall vitrinite/altered vitrinite and inertinite contents do not change significantly across the rank suite (Table 2). Although there are no distinguishable liptinite macerals in sample NEM-15, some

247

fluorescence is observed within inertinite structures. This could represent hydrocarbons that have been generated and migrated from adjacent shale or from liptinite macerals in less mature areas of coal (e.g., Rimmer et al., 2009). In addition, as rank increases approaching the intrusion, coke textures are observed in the altered vitrinite (Table 2). Isotropic coke is observed in altered vitrinite macerals within ~1 m of the intrusion (Rr = 2.90%). Incipient and fine-grained circular anisotropic coke textures are observed in samples with reflectances N3.03%, and they are more abundant than isotropic coke in samples at the coal/intrusion contact. 3.2. Geochemistry of intruded coals Heating associated with the intrusion affected the chemical composition of the coal out to a distance of 4.5 m from the dike. Volatile matter (VM, on a dry, ash-free basis or daf) in the unaltered coals is ~35–40%,

Fig. 3. Representative photomicrographs of NEM samples. a) Collotelinite (Ct) and collodetrinite (Cd) in unaltered coal, NEM-35, Rr = 0.57%; b) high reflectance vitrinite displaying devolatilization vacuoles in highly altered coal, NEM-2, Rr = 3.12%; c) fusinite (Fs) and semifusinite (Sf), NEM-35; d) fusinite (Fs) and collotelinite (Ct) in altered coal, NEM-9, Rr = 2.15%; e) yellow fluorescing sporinite, NEM-25, Rr = 0.56%; f) dark orange fluorescing liptinite in altered coal, NEM-16, Rr = 1.21%.

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and decreases to b15% at the contact with the intrusion (Table 3). Ash yield (dry basis) increases inside of the dike alteration zone (Fig. 4). Carbonate removal significantly changes the VM trend along the transect. During proximate analysis, carbonates break down to form CO2, which contributes significantly to volatile matter yields (Given and Yarzab, 1978). Proximate analysis after HCl-treatment of the coal samples demonstrates that a significant proportion of the VM within the alteration zone is due to the presence of carbonate minerals. In the untreated samples, there is an initial decrease in VM within the dike alteration zone, followed by an increase and, subsequently, a decrease in VM (Fig. 4). The increase in VM coincides with high ash yield. After HCltreatment, the coal samples show consistently lower VM contents with decreased distance from the intrusion reaching a minimum of 8–9% at the contact (Fig. 4; Table 3). Carbonate removal also significantly

decreases the ash yield of the coals, especially inside the dike alteration zone (Fig. 4). Carbon, H, N, O, and S contents (daf basis) of the coals change significantly approaching the intrusion (Table 3, Fig. 5). Due to the influence of carbonate mineralization on coal composition, which affects carbon and oxygen contents (Rahman and Rimmer, 2014; Rimmer et al., 2009), only the HCl-treated samples will be discussed here. Carbon increases from unaltered levels around ~80% to above 90% at the contact with the intrusion (Fig. 5a). Hydrogen contents decrease from background levels ~5.3% to 2.3% at the contact with the dike (Fig. 5b). Oxygen decreases from background levels of ~ 10% down to 2–3% at the intrusion contact (Fig. 5c). Nitrogen content also decreases approaching the intrusion; however, a much less clear trend is observed when compared to the other elements (Fig. 5d). Nitrogen content across the

Table 3 Geochemical data. D = mid-point of sample from intrusion, m; Mois = moisture (%, as determined); ash yield presented on %, dry basis; VM = volatile matter, %, dry, ash-free (daf) basis; FC = fixed carbon, %, daf basis; ultimate analysis data (C, H, O, N, S) presented on %, daf basis (O calculated by difference); Parr = mineral matter content, %, dry basis. Sample

D

Mois

Ash

VM

FC

C

H

O

N

S

Parr

Untreated coals NEM-1 NEM-2 NEM-3 NEM-4 NEM-5 NEM-6 NEM-7 NEM-8 NEM-9 NEM-10 NEM-11 NEM-12 NEM-13 NEM-14 NEM-15 NEM-16 NEM-17 NEM-18 NEM-19 NEM-20 NEM-21 NEM-22 NEM-23 NEM-24 NEM-25 NEM-26 NEM-27 NEM-28 NEM-29 NEM-30 NEM-31 NEM-32 NEM-33 NEM-35 NEM-36 NEM-37

0.04 0.23 0.38 0.58 0.70 0.90 1.17 1.43 1.65 2.02 2.25 2.55 2.78 2.98 3.30 3.62 3.83 4.13 4.43 4.78 5.03 5.43 5.83 6.23 6.68 7.15 7.73 8.38 9.18 10.08 10.93 11.48 13.05 16.18 18.98 21.50

1.83 1.86 1.46 1.39 1.26 1.32 1.39 1.38 2.09 2.04 2.22 2.40 2.29 2.41 2.30 2.39 2.55 2.51 3.35 3.51 3.93 4.45 4.25 4.57 4.04 4.85 4.94 4.87 4.69 5.20 5.22 4.94 5.30 4.72 5.18 4.85

21.79 19.98 28.45 28.61 30.80 32.19 26.84 24.02 23.40 23.58 23.28 20.64 16.37 16.80 13.11 15.09 10.14 10.51 7.93 14.01 9.60 7.50 19.29 16.53 14.13 9.77 6.65 5.09 8.74 3.56 4.64 5.51 3.35 6.53 4.50 6.61

14.03 16.05 23.31 30.83 36.37 33.32 28.43 24.22 23.42 22.31 21.91 19.13 18.10 21.08 22.50 29.16 26.94 29.95 34.52 38.14 39.49 37.64 36.04 40.40 38.29 38.38 36.88 36.43 37.14 37.09 37.16 37.76 36.82 38.45 37.46 37.95

85.97 83.95 76.69 69.17 63.63 66.68 71.57 75.78 76.58 77.69 78.09 80.87 81.90 78.92 77.50 70.84 73.06 70.05 65.48 61.86 60.51 62.36 63.96 59.60 61.71 61.62 63.12 63.57 62.86 62.91 62.84 62.24 63.18 61.55 62.54 62.05

93.59 91.48 90.23 84.80 79.67 82.27 83.61 87.18 87.04 88.93 88.06 86.72 90.18 88.87 87.52 86.09 84.45 83.46 81.07 79.77 80.01 80.98 82.62 79.80 83.85 81.08 81.17 78.61 83.44 80.77 76.90 76.97 76.55 78.76 76.44 77.39

2.04 2.76 2.63 2.20 2.34 2.31 2.39 2.64 3.30 3.39 3.59 3.70 3.72 4.15 4.30 4.60 4.67 4.75 5.18 5.25 5.21 5.33 5.09 5.47 4.79 5.50 5.41 5.13 5.60 5.33 5.50 5.45 5.41 5.36 5.51 5.44

1.66 1.73 1.80 9.68 15.00 12.01 10.66 7.10 5.93 3.91 4.43 2.53 2.97 1.99 3.56 4.16 6.01 5.83 8.51 9.81 8.75 9.09 4.83 7.56 8.01 8.46 9.05 11.16 6.72 9.19 13.40 12.78 14.34 12.01 14.07 13.22

1.09 1.63 1.63 1.48 1.44 1.46 1.48 1.42 1.77 1.78 1.85 1.75 1.84 1.74 1.70 1.67 1.69 1.67 1.62 1.66 1.66 1.67 1.52 1.66 1.40 1.69 1.66 1.57 1.74 1.69 1.67 1.69 1.66 1.70 1.65 1.71

1.62 2.41 3.72 1.85 1.56 1.95 1.86 1.67 1.97 2.00 2.09 5.30 1.29 3.26 2.92 3.49 3.18 4.29 3.63 3.52 4.37 2.93 5.96 5.52 1.97 3.28 2.72 3.53 2.50 3.03 2.54 3.12 2.04 2.18 2.34 2.25

24.23 22.64 32.19 31.62 33.86 35.49 29.74 26.64 26.10 26.31 26.02 24.61 18.27 19.63 15.56 17.93 12.52 13.46 10.40 16.80 12.54 9.59 23.48 20.39 16.19 12.18 8.58 7.34 10.69 5.45 6.34 7.57 4.70 8.17 6.09 8.29

HCl-treated coals NEM-1 0.04 NEM-2 0.23 NEM-3 0.38 NEM-4 0.58 NEM-5 0.70 NEM-6 0.90 NEM-7 1.17 NEM-8 1.43 NEM-9 1.65 NEM-10 2.02 NEM-11 2.25 NEM-12 2.55 NEM-15 3.30 NEM-18 4.13 NEM-22 5.43 NEM-30 10.08

1.40 1.87 1.58 1.93 1.46 1.47 1.53 1.46 4.04 1.55 1.45 1.49 1.47 1.65 2.02 2.21

16.33 11.82 10.10 8.99 7.50 14.00 8.87 9.19 10.64 14.37 13.15 14.45 7.22 6.22 5.88 3.25

8.33 10.92 9.79 10.76 10.42 10.23 9.93 10.40 12.98 14.14 13.98 15.83 20.85 28.58 38.56 37.60

91.67 89.08 90.21 89.24 89.58 89.77 90.07 89.60 87.02 85.86 86.02 84.17 79.15 71.42 61.44 62.40

92.49 89.26 89.31 89.47 89.81 88.84 90.17 90.38 88.23 87.64 87.51 85.61 84.45 81.55 79.32 79.20

2.29 3.03 2.94 3.04 3.08 3.08 3.01 2.99 3.45 3.59 3.68 3.72 4.25 4.76 5.29 5.33

2.31 3.39 2.28 3.39 3.08 3.66 2.78 3.30 4.16 4.65 4.54 3.82 6.42 7.34 10.53 10.68

1.12 1.64 1.54 1.68 1.70 1.80 1.69 1.42 1.87 1.90 1.96 1.88 1.70 1.73 1.69 1.73

1.79 2.68 3.93 2.42 2.34 2.63 2.35 1.92 2.31 2.22 2.31 4.97 3.19 4.62 3.17 3.06

18.46 14.06 12.85 10.92 9.29 16.36 10.76 10.88 12.62 16.56 15.31 17.94 9.43 9.10 7.99 5.14

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3.3. FTIR analysis of vitrinite in intruded coals

Fig. 4. Volatile matter (%, daf) and ash yield (%, dry) versus distance from dike contact for untreated and HCl-treated samples (plotted on a log scale).

transect is very scattered, and no discernable decreases are observed until the sample directly in contact with the intrusion (NEM-1). Sulfur content shows no correlation with distance from the contact, and very localized increases in S are observed (Fig. 5e).

a)

The aliphatic CHx stretching absorption band at 3000–2800 cm−1 fluctuates slightly up to Rr = 1.21%, beyond which point the aliphatic stretching absorption area decreases with increasing rank (Table 4, Fig. 6). There is also an increase in the intensity of the aromatic CH stretching and aromatic out-of-plane deformation modes at 3100–3000 cm−1 and 900–700 cm− 1, respectively, with increasing rank (Fig. 6). The comparison of the stretching absorption of an individual band across rank is complicated by differences in extinction coefficients. However, comparison of relative band absorptions can provide information on the types of changes that are occurring in the molecular structures that remain as the total hydrogen content within the coals decreases. For the heat-altered samples, there is a nearly linear increase in the ratio of aromatic CH to total aliphatic CHx adsorption (AR1) (R2 = 0.91), and aromatic out-of-plane deformation to aliphatic CHx bandwidth (AR2) (R2 = 0.93) up to ~ 2.5% Rr (Table 4, Fig. 7). Above this maturity, a significant increase in AR1 and AR2 values is observed, possibly representing the semi-anthracite/anthracite coalification jump. Second derivative analysis identified five minima within the 3000–2800 cm−1 absorption region, leading to deconvolution of this region into five individual bands, each of which represents a unique molecular response (Fig. 8). There is an increase in the relative absorption due to asymmetric methyl (CH3) stretching (2960 cm−1) compared to the asymmetric methylene (CH2) stretching absorption (2920 cm−1) with rank (Fig. 9). Although the areas of all five peaks that make up the 3000–2800 cm−1 band region decrease with increased Rr, it appears that CH2 is lost more readily with increased rank (Table 4). The 900–700 cm−1 region can be deconvoluted into three individual bands (Fig. 10). In the unaltered coals, absorption in this region in vitrinite macerals is dominated by the 870 cm−1 and 815 cm−1 peaks. As coals become progressively more mature, the relative abundance of the 870 cm−1 and 815 cm−1 peaks gradually decreases, and absorption becomes dominated by the 750 cm−1 peak. This trend continues until

b)

d)

c)

e)

Fig. 5. a) Carbon, b) hydrogen, c) oxygen, d) nitrogen, and e) sulfur contents (all on a %, daf basis) versus distance from dike contact for HCl-treated samples (plotted on a log scale).

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Table 4 Peak areas for reflectance micro-FTIR spectra. AR1 = area of 3100–3000 cm−1/area of 3000–2800 cm−1. AR2 = area of 900–700 cm−1/area of 3000–2800 cm−1. % 2960 = area of band centered at 2960 cm−1/area of 3000–2800 cm−1. % 2920 = area of band centered at 2920 cm−1/area of 3000–2800 cm−1. CH2/CH3 = area of peak centered at 2920 cm−1 versus area of peak centered at 2960 cm−1. % 870 = area of band centered at 870 cm−1/area of 900–700 cm−1. % 750 = area of band centered at 750 cm−1/area of 900–700 cm−1. 870/750 = area of 870 cm−1 peak versus 750 cm−1 peak. Sample

3100–3000

3000–2800

900–700

AR1

AR2

% 2960

% 2920

CH2/CH3

% 870

% 750

870/750

NEM-1 NEM-2 NEM-3 NEM-4 NEM-5 NEM-6 NEM-7 NEM-8 NEM-9 NEM-10 NEM-11 NEM-12 NEM-13 NEM-14 NEM-15 NEM-16 NEM-17 NEM-18 NEM-19 NEM-20 NEM-21 NEM-22 NEM-23 NEM-24 NEM-25 NEM-26 NEM-27 NEM-28 NEM-29 NEM-30 NEM-31 NEM-32 NEM-33 NEM-35 NEM-36 NEM-37

4.20 3.78 4.08 3.80 4.22 3.99 4.62 3.90 3.00 3.74 3.20 3.15 3.01 2.90 2.24 1.46 1.57 1.20 0.79 1.10 1.02 0.78 0.90 0.72 0.84 0.95 0.98 0.76 0.74 0.70 0.68 0.70 0.65 0.60 0.94 0.85

2.35 1.61 1.91 2.26 2.22 2.32 2.69 4.83 5.07 7.35 6.95 8.94 9.99 10.71 11.59 19.13 16.54 13.38 15.32 21.08 20.27 18.88 18.20 19.38 17.58 18.95 19.60 17.41 18.72 16.29 17.25 18.20 17.49 17.12 18.89 18.34

22.02 17.65 18.13 20.25 17.66 16.46 18.12 16.20 13.42 18.67 16.00 15.84 15.68 15.44 13.20 14.02 13.52 12.23 9.92 10.77 10.74 8.68 9.91 9.41 10.33 10.40 10.63 9.20 10.18 9.90 9.37 9.71 9.84 8.93 10.86 9.32

1.79 2.35 2.13 1.68 1.90 1.72 1.72 0.81 0.59 0.51 0.46 0.35 0.30 0.27 0.19 0.08 0.09 0.09 0.05 0.05 0.05 0.04 0.05 0.04 0.05 0.05 0.05 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.05 0.05

9.39 10.95 9.47 8.94 7.96 7.11 6.73 3.36 2.65 2.54 2.30 1.77 1.57 1.44 1.14 0.73 0.82 0.91 0.65 0.51 0.53 0.46 0.54 0.49 0.59 0.55 0.54 0.53 0.54 0.61 0.54 0.53 0.56 0.52 0.57 0.51

0.42 0.40 0.41 0.39 0.34 0.36 0.35 0.26 0.26 0.26 0.29 0.31 0.32 0.30 0.33 0.27 0.28 0.30 0.28 0.27 0.25 0.24 0.25 0.26 0.25 0.23 0.25 0.26 0.26 0.24 0.24 0.25 0.26 0.27 0.28 0.26

0.22 0.24 0.20 0.22 0.24 0.25 0.22 0.22 0.23 0.23 0.25 0.24 0.26 0.25 0.27 0.37 0.39 0.39 0.40 0.40 0.39 0.37 0.37 0.37 0.36 0.36 0.37 0.39 0.40 0.39 0.39 0.40 0.39 0.38 0.39 0.39

0.52 0.59 0.49 0.55 0.70 0.69 0.64 0.86 0.90 0.90 0.85 0.78 0.81 0.86 0.81 1.36 1.37 1.32 1.44 1.47 1.54 1.58 1.46 1.44 1.43 1.54 1.49 1.49 1.53 1.61 1.63 1.55 1.52 1.42 1.39 1.54

0.34 0.27 0.30 0.28 0.27 0.31 0.22 0.28 0.23 0.27 0.32 0.30 0.30 0.30 0.32 0.26 0.34 0.33 0.33 0.44 0.43 0.43 0.42 0.41 0.44 0.44 0.40 0.38 0.41 0.39 0.40 0.44 0.41 0.38 0.45 0.39

0.26 0.31 0.30 0.27 0.29 0.30 0.39 0.32 0.30 0.29 0.20 0.20 0.21 0.21 0.22 0.17 0.16 0.17 0.15 0.14 0.15 0.14 0.13 0.13 0.15 0.13 0.14 0.12 0.13 0.12 0.13 0.14 0.13 0.12 0.14 0.12

1.33 0.88 0.97 1.05 0.95 1.03 0.62 0.89 0.77 0.93 1.54 1.49 1.44 1.40 1.45 1.51 2.11 1.95 2.23 3.15 2.83 3.05 3.19 3.12 2.87 3.29 2.90 3.15 3.20 3.14 3.09 3.13 3.11 3.03 3.18 3.33

Fig. 6. Micro-FTIR spectra for vitrinite macerals from three intruded coals of different ranks. Spectra are baseline corrected. Spectra are normalized to a maximum absorbance intensity of 1.00.

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a)

251

b)

Fig. 7. a) Ratio of peak areas at 3100–3000 cm−1 and 3000–2800 cm−1 (AR1), and b) ratio of peak areas at 900–700 cm−1 and 3000–2800 cm−1 (AR2) versus mean random reflectance (Rr, % in oil).

Rr = 2.55%, at which point the lowest contribution of both the 870 cm−1 and 815 cm− 1 peaks and the highest contribution of the 750 cm−1 peak are seen. Beyond this maturity, the relative contributions of the 870 cm−1 and 815 cm−1 peaks begin to increase, whereas the contribution of the 750 cm− 1 peak decreases slightly (Table 4, Fig. 11). 4. Discussion 4.1. Coke textures There is some debate about the relative importance of temperature and time on coal maturation. Heating duration may have a significant effect on vitrinite reflectance (Bostick, 1979; Gretener and Curtis, 1982; Hood et al., 1975). For example, Hood et al. (1975) suggested that the length of time a coal experiences temperatures within 15 °C of its maximum temperature may be important in determining its final rank. Gretener and Curtis (1982) emphasized the importance of

the time during which a coal is exposed to temperatures between 70 °C and 100 °C. However, other studies suggest vitrinite reflectance is not time dependent, and therefore it can be calibrated as an indicator of maximum temperature experienced by a coal (Barker and Pawlewicz, 1994). The proposed relationship between random vitrinite reflectance (Rr) and maximum temperature (Tmax) is Ln(Rr) = 0.0078Tmax − 1.2 (Barker and Pawlewicz, 1994), but this relationship is thought to be valid only up to ~300 °C (Barker et al., 1998). Only the coal at the contact of the intrusion in the current study had an estimated paleotemperature above 300 °C (NEM-1 = 360 °C) (Table 2). However, this is likely an underestimation of the maximum temperature experienced by the coal adjacent to the dike (NEM-1) based on the presence of anisotropic fine-grained circular mosaic and incipient coking structures. The only available analogue for natural coke formation is industrial coking. The coke textures of intruded coals are similar to textures displayed in industrial coke products (c.f., Crelling and Rimmer, 2015) (Fig. 12). Coking coals form anisotropic mosaics (circular, lenticular, and ribbon) through the formation of a fluid

Fig. 8. Deconvolution and assignment of the 3000–2800 cm−1 region. The two main peaks of interest are the asymmetric CH3 and asymmetric CH2. Green lines are deconvoluted peaks; the red line is the result of deconvolution (the original curve); the yellow line is the second derivative of the original curve that was used to assign peaks.

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coke production, the heating rate, duration, maximum temperature and pressure are all controlled. In natural settings, these conditions can be variable. Very rapid and variable heating rates could significantly increase the anisotropic nature of intruded coals (Murchison, 2006). The heating duration and pressure could also increase anisotropic textures in coals (Goodarzi and Murchison, 1977; Murchison, 2006). Coals coked in a laboratory setting show increased anisotropy with high heating rates (Murchison, 2006). Prolonged heating in natural settings versus industrial processes could also increase the anisotropic nature of the coke (Goodarzi and Murchison, 1977). The vast number of variables in natural coke formation clearly makes it problematic to associate coke texture directly with pre-intrusion rank. 4.2. Geochemistry of intruded versus non-intruded coals Fig. 9. Ratio of CH2 (2920 cm−1) to CH3 (2960 cm−1) peak areas versus Rr (%, in oil) values.

stage at 350 °C to 450 °C followed by coke solidification between 450 °C and 550 °C. A nearby intrusion into the Springfield (No. 5) coal seam, which produced similar coke textures, was estimated to have a coal/ contact temperature of 600 °C (Stewart et al., 2005) or around 500 °C (Rimmer et al., 2009). Argon dating suggests the age of the dikes that intrude the Herrin and Springfield coals is around 270 Ma (Fifarek et al., 2001). Basin modeling suggests the rank of the Herrin Coal at the time of intrusion was approximately 0.5% Rr (Rowan et al., 2002). During the industrial production of coke, only coals with maximum vitrinite reflectances (Ro) above 0.9% produce anisotropic fine-grained circular coke; isotropic coke is expected to form from coals with a rank of 0.6–0.79% maximum reflectance (Crelling, 2008). Thus, based on industrial coke textures, the textures formed in the vicinity of this dike are not consistent with the estimated coal rank at the time of intrusion. Rather, the coke textures are indicative of higher rank coals. The textures observed here are consistent with those in another recent study of intruded coals in the Illinois Basin, in which anisotropic coke textures were observed in heat-altered coals from the Springfield (No. 5) Coal seam that has background Rr values between 0.55% and 0.62% (Rahman and Rimmer, 2014). There are several possible explanations for the coke textures. Coals may experience a preheating stage just before emplacement of the intrusion, in which rank is raised enough to produce the observed coke textures (Rahman and Rimmer, 2014). There may have also been more than one episode of intrusive heating along the same pathway, which could help explain the different anisotropic textures (Amijaya and Littke, 2006). Alternatively, the observed coke textures may reflect the different heating rates associated with an intrusion. In industrial

The decrease in VM as a function of rank for the intruded coal is significantly different from trends observed in coals that have been exposed to normal burial temperatures (Fig. 13). The intruded coals have higher-than-expected VM contents at high rank, even after carbonate removal with HCl. It has been proposed that heating rates are the primary factor that leads to different VM–Rr relationships in intruded coals (Murchison, 2006; Pearson and Murchison, 1999; Rimmer et al., 2009). Rapid heating by the intrusion may have caused the Herrin (No. 6) samples to follow a different coalification trend compared to normally matured coals (Rimmer et al., 2009). High pressure associated with the Permian intrusive events could have also trapped VM within intruded coals (Crelling and Dutcher, 1968). Post-intrusion hydrocarbon entrapment could have also contributed to elevated VM readings within the alteration zone, especially in samples nearest the intrusion (Rimmer et al., 2009). Unlike some previous reports (Barker et al., 1998; Raymond and Murchison, 1989), there is no increase in VM adjacent to the intrusion. Previous studies (Rimmer et al., 2009) have shown there are often fluctuations in VM within dike contact zones, even after carbonate removal. Such deviations are likely due to complex dike fluid migration through non-uniform and complex fracture zones, causing small and localized anomalies. The smooth and consistent decrease observed in VM content within the contact zone in this study is contrary to these observations, which further illustrates the complexity of dike alterations halos. Evolution of the elemental composition of the coals investigated in this study also suggested that alteration of the coal due to rapid heating by the intrusion did not follow a typical coalification pathway. The relationship between Rr and C (daf) for the intruded rank series follow a slightly different trend than the normal burial maturation trend (Fig. 14). Similar carbon versus Rr relationships were reported within

Fig. 10. Deconvolution and assignment of the 900–700 cm−1 peaks. Green lines represent individual peaks. Blue line is the summary curve of the individual peaks.

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Fig. 11. DOS (870 cm−1/750 cm−1 peak) versus Rr (%, in oil) for intruded coals.

the alteration zone of dikes in the Springfield (No. 5) Coal (Rahman and Rimmer, 2014; Rimmer et al., 2009). The lower C values across rank in the intruded coals could be explained by the presence of higher than expected VM contents, which contributed to lower relative levels of C. H/C versus O/C data for the intruded coals are clearly distinct from comparable data for normally matured coals (van Krevelen, 1950, 1993) (Fig. 15). The relationship between H/C and O/C reported here is comparable to that shown by Rimmer et al. (2009) and Rahman and Rimmer (2014) for other intruded Illinois Basin coals. This provides compelling evidence for different coalification trends in intruded coals compared to those that have undergone normal burial maturation. It should be stressed that complete removal of carbonate in intruded coals is essential to delineate this trend (Rimmer et al., 2009). Incomplete removal of carbonate minerals significantly increases oxygen content in intruded coals, causing scatter in the data. Plotting the data on a Seyler chart (Seyler, 1931) provides further evidence for a unique coalification pathway in these intruded coals

253

(Fig. 16). There is clearly a different relationship between H and C (%, dmmf) in the normal coalification trend (Seyler, 1931) compared to the intruded coals from this study and others in this area (Rahman and Rimmer, 2014; Rimmer et al., 2009). (Note: The linear fit for the data suggests effective carbonate removal). At a given C content (%, dmmf), H is lower than expected, and VM is higher than expected, across much of the alteration zone (Figs. 13 and 16). Furthermore, at a given H/C value, the O/C ratio is higher than expected (Fig. 15). These combined observations suggest the intruded coals described herein follow unique trends in geochemistry across rank when compared to coals altered by normal diagenesis. This may reflect the inability of volatile products to escape effectively from the coals during the intrusive event(s), contributing to the unique trends in geochemistry. The changes in elemental abundances are related to increased coal rank associated with the intrusion, rather than variations in the abundance of specific macerals. There is no correlation between geochemical data and the abundance of specific maceral groups. Liptinite loss certainly contributes to the overall increased rate of H loss up to the medium to low volatile bituminous stage, but samples with considerably different H contents were found to have similar vitrinite and inertinite contents. 4.3. FTIR analysis of vitrinites in intruded coals Results obtained via FTIR analysis provide insights on the types of changes that have occurred in the intruded coals relative to the unaltered coals. Within the aliphatic CHx stretching modes, there is an increase in methyl asymmetric stretching adsorption (CH3, centered at ~2960 cm−1) relative to methylene asymmetric stretching absorption (CH2, centered at 2920 cm− 1), suggesting an increase in the relative abundance of methyl groups in residual aliphatic structures in samples progressively approaching the intrusion. Decreasing H content suggests

Fig. 12. Coke textures in NEM samples and industrial coals. a) NEM-1 (Rr = 5.00%) displaying isotropic coke (IC) and anisotropic coke (AC) textures; b) NEM-5 (Rr = 2.79%) displaying isotropic coke textures; c) industrial coking coals displaying circular anisotropic coke texture, from a high volatile bituminous coal of V-type 9 (Ro = 0.90–0.99); d) industrial coking coal with isotropic coke texture from a high volatile bituminous coal of V-type 7 (Ro = 0.70–0.79). c) and d) from Crelling and Rimmer (2015), used with permission.

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Fig. 15. van Krevelen plot of H/C and O/C atomic ratios (HCl-treated, dmmf basis) for intruded coals from this study and for a nearby intruded site (Rimmer et al., 2009). Normal coalification trend of van Krevelen (1993) shown in gray.

Fig. 13. VM (%, daf) versus Rr (%, in oil) for HCl-treated samples for the current study and for a nearby intruded site (Rimmer et al., 2009). Burial (normal) coalification trend of Teichmüller and Teichmüller (1979) shown (in gray) for comparison, along with data fields for non-intruded Illinois coals based on data from the Illinois State Geological Survey (ISGS) and the University of Kentucky Center for Applied Energy Research (CAER). (Modified from Rimmer et al., 2009).

an overall decrease in total aliphatic structures in coals close to the intrusion. The ratio of the CH2 to CH3 aliphatic structures reflects the average aliphatic chain length, and can also be related to oil-proneness verses gas-proneness in a source rock or kerogen (Lin and Ritz, 1993). Shorter alkyl chains contain more carbon atoms bound to three, rather than two hydrogen atoms relative to longer alkyl chains. Only terminal carbon atoms can be bound to three hydrogen atoms. It is possible that the decrease in the CH2/CH3 ratio reflects the overall shortening of aliphatic chains as coals become progressively more altered. The CH2/ CH3 absorption ratio can also indicate the degree of branching in aliphatic chains (Lin and Ritz, 1993). Highly branched aliphatic isomers will contain relatively more CH and CH3 bonds, and relatively fewer CH2 bonds compared to their parent n-alkane. However, it is unlikely that rapid heating from the intrusion would have resulted in either formation or preferential preservation of branched aliphatic structures with a decrease in total hydrogen content. The decrease in the CH2/ CH3 ratio is most likely attributed to the overall shortening of aliphatic chain length within the alteration halo of the intrusion. Lower rank coals in the transect have strong bands at 870 cm−1 and 815 cm−1, indicative of isolated and 2 adjacent aromatic H, respectively. This indicates the presence of a relatively high abundance of highly substituted aromatic rings (Chen et al., 2012; Iglesias et al., 1995. As

Fig. 14. C (%, daf) versus Rr (%, in oil) for HCl-treated samples for the current study and for a nearby intruded site (Rimmer et al., 2009). Normal coalification track (shown in gray) from Teichmüller and Teichmüller (1979).

the coals become progressively higher in rank approaching the intrusion, the relative contribution of the 750 cm−1 peak (indicative of 4 adjacent aromatic H) becomes more prominent. This reflects the formation of aromatic rings with four adjacent non-substituted sites as volatiles are driven off, decreasing alkyl substitution. Chen et al. (2012) recently reported a significant increase in the 870 cm−1/ 750 cm− 1 ratio above 2.5% Rr for non-intruded coals. This is not observed in the present study (Fig. 17). At higher rank, only a slight increase in the 870 cm−1/750 cm−1 ratio is observed. Chen et al. (2012) postulate that the increase in this ratio at high rank is associated with the increased condensation of aromatic structures, and the concomitant increase in bridge head-type aromatic structures. One explanation for the less dramatic rise in the 870 cm−1/750 cm−1 ratio for the intruded coals in this study is that large and condensed aromatic sheets either break down, or do not form to the extent that they do during burial metamorphism. Although the high reflectance intruded coals are considered very aromatic based on AR1 and AR2 measurements, they may not be as condensed as coals altered through burial metamorphism. This would suggest almost all C is present in aromatic rings, but not all aromatic rings share multiple bonding sites with other aromatic rings, allowing for more rings with four adjacent H atoms. 5. Conclusions A section of the Herrin (No. 6) Coal was altered by the intrusion of a Permian ultramafic lamprophyre dike. Mean random vitrinite reflectance (Rr, % in oil) increases approaching the intrusion, from background levels of 0.5% up to 5.0% at the contact with the dike. Volatile matter and moisture are lost approaching the intrusion, however VM (daf) yields are higher than expected at high Rr values compared to coals altered by normal coalification. Hydrogen and oxygen (%, daf)

Fig. 16. Seyler plot of data for HCl-treated intruded coals from the current study and from Rimmer et al. (2009) compared to normal coalification trend of van Krevelen (1993). The R2 value is calculated on the HCl-treated coals from this study.

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Acknowledgements The authors would like to thank Dr. M. Wahid Rahman for his help with sample collection, along with Gary Vancil, Anthony Moorehead, and the American Coal Company for access to the sampling site. Thanks go to the Kentucky Geological Society and Jason Backus for assisting with proximate and sulfur analysis, and to ALS Environmental for ultimate analysis. The Antoinette Lierman Medlin Scholarship from the Energy Geology Division of the Geological Society of America, and a scholarship from the Graduate and Professional Research Council, Southern Illinois University funded this research. The authors appreciate the helpful comments from two anonymous reviewers. References Fig. 17. DOS (870 cm−1/750 cm−1 peak) versus Rr (%, in oil) for intruded coals from this study and non-intruded coals from Chen et al. (2012).

are lost, and carbon content (%, daf) increases approaching the dike. Intruded coals show different O/C – H/C and H – C (%, dmmf) relationships compared to those that result from normal burial coalification (Seyler, 1931; van Krevelen, 1950, 1993). The different coalification trends seen in this transect of intruded coals are consistent with data collected on intruded coals from the Springfield (No. 5) Coal (Rahman and Rimmer, 2014; Rimmer et al., 2009) at nearby locations. This suggests intruded coals follow different coalification trends compared to nonintruded coals (Rimmer et al., 2009). Sulfur content does not seem to be influenced by heat from the intrusion. Analysis of unique coalification trends is only possible after complete removal of carbonate with acid, as carbonate minerals such as calcite and ankerite influence elemental ratios and volatile matter contents. Coke textures are consistent with a higher coal rank at the time of intrusion than is suggested by the basin modeling by Rowan et al. (2002). The presence of anisotropic coke formed by intrusion of coals of this rank was also discussed in Rahman and Rimmer (2014). Different coke textures may represent pre-heating of coals prior to the main intrusive event (Rahman and Rimmer, 2014; Rimmer et al., 2009), may be due to more than one injection of hydrothermal fluid along the same pathway (Amijaya and Littke, 2006), or may relate to differences in heating rates and pressure compared to industrial coking. Aromaticity is estimated from the ratio of aromatic CH to total aliphatic CHx band absorption area (AR1), and the ratio of aromatic outof-plane deformation modes versus the total aliphatic CH absorption area (AR2). Vitrinite in the intruded coals shows increased aromaticity with increased Rr approaching the dike/coal contact. Deconvolution of the aliphatic CHx stretching absorption shows the peak centered at 2960 cm−1 (methyl, CH3, asymmetric stretching) increases in abundance relative to the peak centered at 2920 cm− 1 (methylene, CH2, asymmetric stretching). The CH2/CH3 ratio reflects the average aliphatic chain length (Lin and Ritz, 1993). Intruded coals show an overall decrease in this ratio, reflecting an overall shortening of aliphatic chains, consistent with preferential loss of longer aliphatic chains or preferential preservation of methyl substituted aromatic structures. Deconvolution of the 900–700 cm−1 region reveals three unique peaks at 870 cm−1, 815 cm−1, and 750 cm−1, which represent aromatic rings with one isolated, two to three adjacent, and four adjacent hydrogen atoms, respectively (Yen et al., 1984). The relative abundance of the 750 cm−1 peak increases up to a rank of ~2.5% Rr in intruded coals, likely due to decreased alkyl substitution on aromatic sites (Chen et al., 2012). Unlike trends observed in normally matured coals above this rank (Chen et al., 2012), the 750 cm−1 peak remains relatively constant, suggesting less polycondensation of aromatic structures in these intruded coals relative to normally matured coals.

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