Raman spectroscopy of intruded coals from the Illinois Basin: Correlation with rank and estimated alteration temperature

International Journal of Coal Geology 219 (2020) 103369

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Raman spectroscopy of intruded coals from the Illinois Basin: Correlation with rank and estimated alteration temperature

T

⁎

Kuo Lia,b, Susan M. Rimmerb, , Severin M. Presswoodb, Qinfu Liua a b

College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, PR China School of Earth Systems and Sustainability, Southern Illinois University Carbondale, Carbondale, IL 62901, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Intruded coals Contact metamorphism Temperature Raman spectroscopy Geothermometer

Raman spectroscopy was used to evaluate rank for a series of coals collected adjacent to an igneous intrusion. For these intruded coals, Raman parameters for collotelinite and its coked equivalent show complex changes during the transformation from low to very high levels of maturity (with vitrinite reflectance increasing from 0.55% to 5.0%). With increasing coal rank up to anthracite-level reflectance, the Raman spectra show a decrease in both G band full width at half maximum (FWHM) and D band intensity. The D band shape becomes increasingly asymmetric during the bituminization and de-bituminization stages, whereas in the anthracitization stage, the width of the D and G bands continues to decrease while D band intensity increases. In meta-anthracites (as seen in the ancillary Hunan samples), the G band has a low intensity and undergoes an apparent broadening due to the presence of the D2 band that occurs as a shoulder on the G band. Differences in Raman spectral parameters for vitrinite and inertinite macerals suggest differences in molecular structure between the macerals; as such, the heterogeneous nature of coal may affect Raman spectral results significantly if different maceral types are not considered. Within individual samples, Raman spectra indicate a high level of structural homogeneity between and within vitrinite particles, allowing assessment of differences between samples of different ranks. For vitrinite, the Raman spectral parameters G FWHM and DAs/GA (Area(1100-s)/Area(s-1650), s is the saddle point between 1100 cm−1 and 1650 cm−1) are highly correlated (R2 > 0.9) with vitrinite reflectance (Rr) and maximum alteration temperature (Tpeak), and allow establishment of geothermometers over the temperature range of ~70–300 °C that can be used to evaluate temperatures attained in intruded coals.

1. Introduction Raman spectroscopy has been used widely for structural characterization of carbonaceous materials (CMs) (e.g., graphene, carbon nanotubes (CNTs), carbon black, and amorphous carbons such as coal, char, and kerogen), as it can detect subtle changes in lattice vibrations of CMs (Cuesta et al., 1998; Kwiecinska et al., 2010; Wopenka and Pasteris, 1993). In recent decades, Raman spectroscopy has been applied in a variety of geological research fields (Henry et al., 2019a). For example, Schopf and Kudryavtsev (2009) used Raman spectroscopy for in situ characterization of the molecular structural composition and geochemical maturity of organic fossils. Wopenka and Pasteris (1993) and Yui et al. (1996) used Raman spectra of CMs as an indicator of metamorphic grade in sedimentary rocks. Raman spectral parameters of CMs in metasediments were correlated with maximum metamorphic temperature by Beyssac et al. (2002), who established a

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geothermometer reliable over the range 330–650 °C ( ± 50 °C). Subsequently, Lahfid et al. (2010) proposed Raman parameters as proxies for thermal metamorphism between advanced diagenesis (~200 °C) and low-grade metamorphism (~350 °C). More recently, Lünsdorf et al. (2017) developed a Raman geothermometer that covers the range 160–600 °C for dispersed organic matter. The structural evolution of kerogens and coals during catagenesis and metagenesis as revealed by Raman spectroscopy has the potential to indicate maturity levels of organic matter for petroleum exploration (Jubb et al., 2018; Kelemen and Fang, 2001; Liu et al., 2013; Schito et al., 2017). Changes in Raman spectral parameters with maturity for solid bitumen, graptolites, dispersed vitrinite, and phytoclasts have been reported previously (Khatibi et al., 2019; Lünsdorf, 2016; Morga and Pawlyta, 2018; Mumm and İnan, 2016; Schmidt et al., 2017; Zhou et al., 2014). Raman spectroscopy has also been applied extensively in the analysis of organic matter in meteorites (Charon et al., 2014; Quirico et al., 2003), has potential in

Corresponding author. E-mail address: [email protected] (S.M. Rimmer).

https://doi.org/10.1016/j.coal.2019.103369 Received 9 August 2019; Received in revised form 12 December 2019; Accepted 12 December 2019 Available online 12 December 2019 0166-5162/ © 2019 Elsevier B.V. All rights reserved.

International Journal of Coal Geology 219 (2020) 103369

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Mine location Pennsylvanian strata within the Illinois Basin

80 km

New Era Mine

Fig. 1. Map showing sampling location site, New Era Mine near Galatia, IL. Modified from Presswood et al. (2016).

treatment temperature (Guedes et al., 2012; Kelemen and Fang, 2001; Zeng and Wu, 2007), burial depth (Henry et al., 2019b; Schito et al., 2017), maximum temperature of metamorphism (Lahfid et al., 2010; Wopenka and Pasteris, 1993), and degree of deformation of coal (Bustin et al., 1995; Pan et al., 2017). Despite the growing use of Raman spectroscopy as a maturity indicator for coals through anthracite level (e.g., Guedes et al., 2010; Hinrichs et al., 2014) as well as for solid bitumen with reflectance up to 4.8% (Zhou et al., 2014), there have been few studies directed at rapidly heated coals associated with igneous intrusion. Based on borehole samples from the Midland Valley (Scotland) and the southern Pennine Basin (England), Henry et al. (2019b) calibrated Raman parameters with rank for coal and shale in a sequence influenced by dolerite sills, and recommended the Raman parameter G FWHM to evaluate maturity levels of organic matter within the oil and gas windows (Ro = 0–3.0%). Chen et al. (2017) established empirical relationships between Raman parameters and temperature for contact metamorphosed coals. However, these authors did not consider the heterogeneity introduced by maceral types (e.g., Henry et al., 2018; Rantitsch et al., 2004; Wilkins et al., 2018), and the effects of different spectral processing techniques on Raman results. Coals altered by the intrusion of a single seam provide a rank suite of samples that avoid the additional unknowns of different diagenetic origins and burial conditions (pressure and geothermal gradient). In the current study, Raman spectroscopy was used to analyze a suite of intruded coals from the Illinois Basin at a maceral scale, following the earlier molecular structural study by Presswood et al. (2016). The objectives of the current study were to: 1) evaluate changes in Raman spectral parameters of vitrinite (specifically, collotelinite) with increased rank of intruded coals; 2) establish relationships between Raman parameters and estimated maximum temperature experienced by the intruded coals; and 3) to use selected Raman parameters to establish geothermometers for intruded coals.

archaeological studies (Deldicque et al., 2016). Coal macromolecular structure shows a gradual increase in aromaticity and loss of heteroatoms such as H, N, S, and O during coalification (van Krevelen, 1993). The structural evolution of coal is mainly controlled by the maximum paleotemperature attained (Murchison, 2006; Taylor et al., 1998), although time is also thought to be an important factor (e.g., Hood et al., 1975; Waples, 1980). In addition, other authors have suggested structural changes may be affected by the nature of the precursor materials, pressure (Goodarzi and Murchison, 1977; Quirico et al., 2005), and even catalysis by inorganics (Khatibi et al., 2019; Vandenbroucke and Largeau, 2007). Conventional optical methods, such as vitrinite reflectance, liptinite fluorescence, and thermal alteration index, have been used to assess coal rank and thermal maturity of dispersed organic matter in sediments (Schito et al., 2017; Taylor et al., 1998). Among these optical methods, reflectance values measured on vitrinite, graptolite, and solid bitumen have been used extensively. However, reflectance analysis has potential errors, including those associated with inadequate polish (Valentine et al., 2019), misidentification of macerals, difficulties in measuring very small components (e.g., in the case of some solid bitumen), and decreased reflectance caused by the presence of abundant small pores or other voids (Li et al., 2018, 2019; Sanei et al., 2015). Raman spectral parameters of CMs are intrinsic properties and are mainly dependent on molecular vibrations; thus, Raman analysis can supplement or even replace traditional methods of rank assessment. Several authors have used Raman spectroscopy of vitrinite and other organic components to evaluate maturity (e.g., Guedes et al., 2010; Hinrichs et al., 2014; Liu et al., 2013; Mumm and İnan, 2016; Quirico et al., 2005; Wilkins et al., 2014), and Potgieter-Vermaak et al. (2010) provided a review of Raman spectroscopy of coals. Hinrichs et al. (2014) showed that the Raman parameters G band width, D band position, and D band width correlate well with coal rank. The ratio of the D and G band areas also shows good correlation with rank (Guedes et al., 2010). Hackley and Lünsdorf (2018) reported that the G and D band separation (RBS) can be used an indicator of maturation of organic matter in shale. Relationships have also been reported between Raman spectral parameters (band position, FWHM, intensity, and area ratio, etc.) and reflectance (Guedes et al., 2010; Hinrichs et al., 2014; Lünsdorf, 2016; Morga and Pawlyta, 2018; Zhou et al., 2014), maturity of organic matter (Schito et al., 2017; Wilkins et al., 2014), heat-

2. Methods 2.1. Samples and preparation A suite of 18 samples of intruded Herrin (No. 6) Coal (Pennsylvanian) were collected from an underground mine in northern 2

International Journal of Coal Geology 219 (2020) 103369

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Table 1 Petrographic and micro-FTIR data for intruded coals. D = distance from the intrusion, m; Rr = mean random vitrinite reflectance (%, in oil); IC = isotropic coke, and AC = anisotropic coke, all petrographic data are on a volume % basis; Tpeak = estimated maximum temperature (°C) where Tpeak = (ln(Rr) + 1.19)/0.00782 (Barker and Pawlewicz, 1994); note: this relationship is valid up to 300 °C. 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. CH2/CH3 = area ratio of peak centered at 2920 cm−1 versus peak centered at 2960 cm−1. DOS = area ratio of 870 cm−1 peak versus 750 cm−1 peak. (Petrographic and FTIR data from Presswood et al., 2016). Sample

NEM-1 NEM-2 NEM-3 NEM-4 NEM-5 NEM-7 NEM-8 NEM-9 NEM-10 NEM-11 NEM-12 NEM-13 NEM-14 NEM-15 NEM-16 NEM-17 NEM-19 NEM-27

Dist.

Rr

Tpeak

Vitrinite

Liptinite

Inertinite

IC

AC

m

%

°C

%

%

%

%

%

0.04 0.23 0.38 0.58 0.70 1.17 1.43 1.65 2.02 2.25 2.55 2.78 2.98 3.30 3.62 3.83 4.43 7.73

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

358 298 294 288 283 272 267 250 229 227 216 215 197 191 177 153 91 76

75.2 77.2 78.4 76.4 79.2 76.4 74.4 72.8 80.2 81.6 82.8 79.2 81.4 85.0 83.4 82.8 80.6 81.6

0 0 0 0 0 0 0 0 0 0 0 0 0 0 2.4 2.2 3.2 2.4

24.8 22.8 21.6 23.6 20.8 23.6 25.6 27.2 19.8 18.4 17.2 20.8 18.6 15.0 14.2 15.0 16.2 16.0

32.4 74.8 76.4 76.4 79.2 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

AR1

AR2

CH2/CH3

DOS

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

9.39 10.95 9.47 8.94 7.96 6.73 3.36 2.65 2.54 2.3 1.77 1.57 1.44 1.14 0.73 0.82 0.65 0.54

0.52 0.59 0.49 0.55 0.70 0.64 0.86 0.90 0.90 0.85 0.78 0.81 0.86 0.81 1.36 1.37 1.44 1.49

1.33 0.88 0.97 1.05 0.95 0.62 0.89 0.77 0.93 1.54 1.49 1.44 1.40 1.45 1.51 2.11 2.23 2.90

Fig. 2. Representative photomicrographs of coal samples. All images taken under white light, 40× antiflex objective, and oil immersion; e and f, crossed polarizers. Scale bar length is 20 μm. a) collotelinite (Ct) and collodetrinite (Cd), NEM-27 (Rr = 0.55%); b) fusinite (Fus) and cutinite (Cut), NEM-27 (Rr = 0.55%); c) vacuoles developed in vitrinite, NEM-16 (Rr = 1.21%); (d) vacuoles developed in vitrinite NEM-9 (Rr = 2.15%) – note increased reflectance and continued development of vacuoles; e) isotropic coke (IC), NEM-5 (Rr = 2.79%); f) isotropic coke (IC) and anisotropic coke (AC), the latter showing fine-grained circular mosaic, NEM-1 (Rr = 5.00%).

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Rr

400

Temp,°C

350 5.0 300 4.0 250 200

3.0

150 2.0 100 1.0

Estimated peak paleotemperature (°C)

Mean random vitrinite reflectance, Rr (%, in oil)

6.0

50 0

0.0 0

1

2

3

4

5

6

7

8

9

Distance from dike contact (m) Fig. 3. Mean random vitrinite reflectance (Rr %, in oil) and estimated peak paleotemperature (°C) versus distance from dike contact. Paleotemperature estimated according to Barker and Pawlewicz (1994).

Saline County, Illinois (Fig. 1). Details on the geological setting and sampling methods are provided in Presswood et al. (2016), but briefly these coals represent samples collected at varying distances (0.04 to 7.73 m) from a Permian-age dike as shown in Table 1. Coals were crushed to minus 20-mesh and embedded in epoxy resin; after curing, the pellets were ground and polished. All polishing/ grinding was performed using a Buehler Automet 250; distilled water was used throughout. Each step ran for 2 mins using the following settings: 150 rpm speed (base), 60 rpm speed (head), 24 lbs. pressure, counter-clockwise rotation, and with a slow drip of distilled water. After each step, samples were washed thoroughly and (except for the final polishing step) cleaned in an ultrasonic bath. The two grinding steps utilized 400 grit and 600 grit SiC papers (Buehler Carbimet). The two polishing steps employed water-based Buehler 1 μm alumina suspension and 0.06 μm colloidal silica suspension, both dispersed on Buehler TriDent cloths; following each polishing step, samples were run for an additional 2 min under a strong stream of distilled water to aid in removal of the polishing compound. Samples were then air dried with compressed air and capped to protect the pellet surface. Two pellets were prepared for each sample, as required for ASTM standard procedures (e.g., ASTM, 2011). Freshly polished pellets were used for both petrographic and Raman spectroscopic analysis.

G band

Relative intensity

D band NEM-27

Fluorescence background

NEM-19

Fluorescence background

NEM-16

NEM-12

NEM-5 2.2. Optical microscopy Photomicrographs of the coal samples were taken using a Leica DFC 7000 T camera mounted on a Zeiss Universal reflected-light microscope. Crossed polarizers and a 40/0.85 Pol oil immersion antiflex objective were used to distinguish coke textures. Point counts of organic components that included vitrinite, liptinite, inertinite, isotropic coke, and anisotropic coke, were based on 500 points on each of two pellets. Mean random vitrinite reflectance (Rr %, in oil) was determined using a Leica DM 2500P petrographic microscope equipped with J&M Analytic TIDAS hardware and software, using a 3-point standardization (1.672%, 3.170%, and 5.460%, for higher rank samples, and 0.426%, 0.940%, and 1.717% for lower rank samples; all calibrations had R2 > 0.99%). Following ASTM D2798-11a procedures (ASTM, 2011), fifty reflectance measurements on collotelinite or its coked equivalent

NEM-2

NEM-1 900

1100

1300

1500

1700

1900

Raman shift (cm-1) Fig. 4. Representative Raman spectra of vitrinite in the intruded coal series. Fluorescence backgrounds of samples NEM-27 and NEM-19 are indicated.

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International Journal of Coal Geology 219 (2020) 103369

D FWHM (cm-1)

y = -6.9288x + 1375.8 R² = 0.82

1610

b)

280 260 240 220 200

100

1

2

3

4

5

6

0

0.70

d)

80

1

2

3

4

5

y = -15.86ln(x) + 75.962 R² = 0.98

1595

0 300

e)

1

2

3

4

5

6

f)

280

0.60 0.55

20

1600

G-D (cm-1)

40

1605

6

0.65

60

c)

1590

180 0

G FWHM (cm-1)

300

a)

G Position (cm-1)

1400 1390 1380 1370 1360 1350 1340 1330 1320

ID/IG

D Position (cm-1)

K. Li, et al.

260 240 220

y = 7.1235x + 227.2 R² = 0.80

200 180

0

160

0.50 0

1.40

1

2

3

4

5

6

0

2.00

g)

1

2

3

4

5

0

6

2.00

h)

1.80

DAs/GA

DA1/GA

RIP

1.60 1.40 1.20

y = 0.0556x + 1.2834 R² = 0.83

0.80 1.00 0

1

2

3

4

5

6

3

4

5

6

i)

1.60 1.40 y = 0.1595ln(x) + 1.4119 R² = 0.92

1.20 1.00

0.80

0.60

2

1.80

1.20 1.00

1

0.80 0

1

2

3

4

5

6

0

1

2

3

4

5

6

Mean random vitrinite reflectance (Rr%, in oil) Fig. 5. Raman parameters versus rank as shown by mean random vitrinite reflectance (Rr, % in oil). Error bars show one standard deviation.

were collected on each of two pellets per sample (for a total of 100 points) and then averaged. Based on the assumption that vitrinite reflectance (Rr) can be used as a paleogeothermometer, the peak temperatures (Tpeak) to which the coals had been exposed was estimated using the relationship, Tpeak = (ln(Rr) + 1.19)/0.00782; this relationship was developed for hydrothermally metamorphosed vitrinite by Barker and Pawlewicz (1994) and is thought to be valid up to ~300 °C.

structure had been altered by the laser power. Spectra were collected on collotelinite (or its coked equivalent) for all samples to allow evaluation of changes in Raman spectra with increased rank; in each sample, 3–5 spectra were collected on vitrinite. In addition, repeat spectra were collected to assess variability within and between collotelinite particles. Several spectra were also collected on inertinite macerals, fusinite and semifusinite, to compare with the collotelinite spectra for the same samples.

2.3. Raman spectroscopy

2.4. Raman spectra processing

Raman microspectrometry was performed using a Renishaw inVia Laser Raman spectrometer equipped with a Leica microscope. A semiconductor laser with a 532 nm exciting wavelength was chosen as the light source, Raman scattering signals were dispersed by a holographic diffraction grating (2400 lines per mm, resolution up to ~1 cm−1), and the reflected-light signal was detected by a CCD camera. The Raman analysis system was calibrated using a standard silicon wafer at the 520.4 cm−1 line before measurement, and this line was measured again after all the measurements to check for system drift. The laser was focused on macerals using a 50× long-working-distance objective (N.A. = 0.50), and Raman spectra were collected over the range 900–2000 cm−1, covering the first-order region of Raman spectra for carbonaceous materials. Laser power was set to < 0.2 mW during measurement to avoid photodecomposition. Each measured spot was checked visually after a spectrum had been collected; if there was evidence of alteration at the point of analysis, the laser power was lowered and the measurement was re-run; in addition, consecutive spectra on the same point were compared to check whether the

Smoothing and baseline corrections were performed following the methods of Henry et al. (2018). Each spectrum was subjected to Savitzky-Golay smoothing using a 21-point quadratic polynomial algorithm, and a 3rd-order polynomial baseline correction. During deconvolution, the fitting of more bands provides a better fit with the original spectrum (Henry et al., 2018; Lünsdorf et al., 2014), but this may result in unrealistic band positions, heights, and widths (Deldicque et al., 2016; Henry et al., 2018; Pan et al., 2017), and may reduce the area and width of the primary bands of interest (G and D bands) (Guedes et al., 2010; Kelemen and Fang, 2001). As a result, Raman parameters following deconvolution may vary significantly due to the peak-fitting bias of different researchers (Lünsdorf et al., 2014). Only two Voigt bands (D and G bands) were used in the deconvolution after baseline correction following the method of Hinrichs et al. (2014). Spectral deconvolution was accomplished using Origin Lab software. Raman parameters band positions (D position and G position), band intensities (ID and IG), and area ratios were obtained from un-deconvolution spectra to avoid shifts in the maximum peak position and 5

International Journal of Coal Geology 219 (2020) 103369

K. Li, et al.

Table 2 Raman parameters for vitrinite in intruded coal suite. n = number of spectra collected per sample; R1 = ID/IG, where ID and IG are the intensities of the D and G bands; DA1/GA = Area(1100–1400)/Area(1550–1650); RIP = Area(1100–1300)/Area(1300–1370); and DAs/GA = Area(1100-s)/Area(s-1650), where s is the position with lowest intensity between the D and G bands. Sample

Rr

n

% NEM-1-V s.d. NEM-2-V s.d. NEM-3-V s.d. NEM-4-V s.d. NEM-5-V s.d. NEM-7-V s.d. NEM-8-V s.d. NEM-9-V s.d. NEM-10-V s.d. NEM-11-V s.d. NEM-12-V s.d. NEM-13-V s.d. NEM-14-V s.d. NEM-15-V s.d. NEM-16-V s.d. NEM-17-V s.d. NEM-19-V s.d. NEM-27-V s.d.

D Position cm

5.00

5

3.12

3

3.03

3

2.90

3

2.79

3

2.55

3

2.46

4

2.15

3

1.83

5

1.79

4

1.65

4

1.63

5

1.42

5

1.36

5

1.21

5

1.01

4

0.62

5

0.55

5

−1

1333 3.00 1358 0.58 1358 0.58 1360 1.53 1360 0.58 1360 1.53 1359 0.58 1361 1.00 1368 2.00 1365 0.00 1364 0.82 1360 4.43 1366 1.48 1363 1.79 1367 2.17 1364 2.65 1370 2.59 1372 2.22

D FWHM cm

−1

197 4.19 244 3.54 244 2.00 265 2.52 261 2.31 264 0.58 262 2.52 260 1.15 261 1.00 262 2.00 260 1.91 254 5.15 263 3.36 263 3.03 261 3.74 230 2.89 270 4.28 266 1.73

G Position cm

G FWHM

−1

cm

1605 0.00 1604 0.00 1604 0.58 1603 0.58 1603 0.58 1604 0.58 1604 1.00 1603 1.15 1603 0.58 1603 1.00 1603 1.15 1604 0.58 1601 0.89 1602 0.45 1603 1.22 1603 0.58 1604 1.48 1607 0.58

−1

48 1.15 58 1.89 58 0.00 61 1.00 62 0.00 61 0.58 62 0.00 65 0.58 66 0.58 68 1.00 67 0.00 65 1.32 70 1.41 71 0.84 74 1.14 74 0.58 84 1.29 86 1.77

G-D cm

ID/IG

DA1/GA

RIP

DAs/GA

0.62 0.03 0.57 0.01 0.57 0.00 0.57 0.01 0.56 0.01 0.56 0.01 0.56 0.01 0.56 0.01 0.57 0.00 0.58 0.01 0.57 0.00 0.58 0.01 0.58 0.01 0.59 0.00 0.59 0.01 0.59 0.01 0.65 0.01 0.63 0.03

1.60 0.03 1.45 0.03 1.45 0.01 1.44 0.03 1.42 0.03 1.43 0.03 1.42 0.04 1.36 0.02 1.36 0.01 1.38 0.01 1.36 0.02 1.41 0.02 1.35 0.02 1.35 0.02 1.31 0.03 1.34 0.02 1.38 0.03 1.35 0.03

0.99 0.04 1.12 0.03 1.12 0.02 1.15 0.05 1.13 0.02 1.16 0.01 1.13 0.03 1.12 0.02 1.12 0.01 1.14 0.02 1.12 0.02 1.12 0.03 1.11 0.02 1.11 0.01 1.06 0.04 1.11 0.02 1.14 0.03 1.11 0.06

1.66 0.03 1.58 0.03 1.58 0.04 1.61 0.04 1.57 0.04 1.61 0.05 1.56 0.06 1.49 0.05 1.53 0.01 1.49 0.03 1.55 0.04 1.48 0.05 1.46 0.05 1.43 0.04 1.42 0.05 1.40 0.02 1.36 0.03 1.32 0.06

−1

272 3.00 246 0.58 246 1.00 243 2.00 243 0.00 244 2.00 245 1.53 242 0.58 235 1.53 238 1.00 239 0.82 244 4.97 235 1.92 239 1.82 236 2.59 239 2.31 234 3.36 234 2.63

incomplete peak-fitting during the deconvolution process. The deconvolution spectra were used to obtain the Raman parameters D FWHM and G FWHM. Other Raman parameters were calculated as follows: R1 = ID/IG, where ID and IG are the intensities of the D and G bands, respectively; RBS = G-D, referred to as the Raman band separation, or the distance between the G and D band positions; DA1/ GA = Area(1100–1400)/Area(1550–1650), where area is the integrated intensity; RIP = Area(1100−1300)/Area(1300–1370), known as the Raman index of preservation; and DAs/GA = Area(1100-s)/Area(s-1650), where s is the position with lowest intensity between the D and G bands.

Inertinite macerals, including fusinite and semifusinite, are also fairly common (Fig. 2b), constituting 14–27% by volume (Table 1). Liptinite macerals, including sporinite, cutinite (Fig. 2b), and resinite, make up 2–4% of the lower rank samples, exhibit yellow to dark orange fluorescence color with rank, and are not visible in samples with Rr > 1.36%. Based on the relationship of Barker and Pawlewicz (1994), paleotemperature estimates for the intruded coal suite range between 76 °C and 358 °C directly adjacent to the dike (Fig. 3, Table 1).

3. Results

3.2.1. Raman spectra of vitrinite in intruded coals The Raman spectra of lower rank samples have a strong fluorescence background; the relative fluorescence intensity decreases with increasing rank of the coals (Fig. 4). All spectra show a disordered (D) band in the range of ~1330–1370 cm−1 and a graphitic (G) band at ~1600 cm−1. The D band position decreases from 1372 cm−1 to 1333 cm−1 with increased coal rank, showing a linear relationship (R2 = 0.82, Fig. 5a, Table 2); D FWHM varies between 270 cm−1 and 197 cm−1 and does not show a well-defined trend with increased rank (Fig. 5b). The G band position ranges between 1607 cm−1 and 1601 cm−1, and shows no clear relationship with rank (Fig. 5c). However, the G band FWHM shows a clear logarithmic relationship (R2 = 0.98) with increasing rank (Fig. 5d). The intensity ratio of the D and G bands shows a “u” shape trend with respect to coal rank (Fig. 5e). The separation between the G and D bands increases slightly with coal rank, from 234 cm−1 to 272 cm−1 (linear, R2 = 0.80, Fig. 5f). The RIP

3.2. Raman spectroscopic analysis

3.1. Petrographic analysis Vitrinite group macerals, including homogenous collotelinite and matrix collodetrinite, are readily identified based on reflectance and morphology (Fig. 2a), and make up 73–85% of the total organic matter by volume in the coals of the sampling transect (Table 1). Devolatilization vacuoles are observed in altered vitrinite in samples within ~3.8 m of the dike (Fig. 2c, d), beginning with sample NEM-17. Mean random vitrinite reflectance (Rr, %) increases progressively from a background level of 0.55% up to 5.0% approaching the contact with the dike (Fig. 3, Table 1). Altered vitrinite in samples within ~1 m of the dike (Rr > 2.9%) exhibits isotropic coke (Fig. 2e); samples closest to the dike (within 0.4 m, Rr > 3.03%) also include anisotropic coke, with areas of incipient and fine-grained circular mosaic (Fig. 2f). 6

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Fig. 6. Comparison of Raman spectral profiles a) within an individual vitrinite particle (“intra vitrinite”) (NEM-11; Rr = 1.79%); b) between different vitrinite particles (“inter vitrinite”) (NEM-8; Rr = 2.46%); c) exact locations for the Raman spectra shown in a), as seen through the Raman microscope.

Fig. 7. Representative Raman spectra of inertinite in intruded coals. a) fusinite (NEM-19; Rr = 0.62%); b) semifusinite (NEM-17; Rr = 1.01%); exact measured spots indicated by red box as seen through the Raman microscope. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

show a high level of similarity (Fig. 6a; the specific spots measured for each spectrum in Fig. 6a are shown in Fig. 6c) as do the spectra collected on different vitrinite particles in the same sample (Fig. 6b). The highly degree of similarity in Raman spectra of these intruded coals suggests a high level of structural homogeneity within and between vitrinite particles in the samples.

(Area(1100–1300)/Area(1300–1370)) that has been used to study organic matter preservation in fossils shows no clear relationship with reflectance (Fig. 5g). The ratio DA1/GA shows a linear increase with coal rank (R2 = 0.83) (Fig. 5h), and DAs/GA (Fig. 5i) shows a logarithmic correlation with rank (R2 = 0.92). Of all the Raman parameters, the G band FWHM and the ratio DAs/GA show the highest correlations with rank for these intruded coals. To evaluate potential variation in Raman spectra within and between vitrinite, spectral profiles were measured on different locations within a single vitrinite particle as well as on different vitrinite particles. The multiple spectra collected within a single vitrinite particle

3.2.2. Raman spectra of inertinite in intruded coals Inertinite in the lower rank samples is readily identifiable under the Raman microscope because of the high contrast between vitrinite and inertinite gray levels due to different reflectance. Compared to the 7

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Table 3 Raman parameters for inertinite in intruded coal suite. R1 = ID/IG, where ID and IG are the intensities of the D and G bands; DA1/GA = Area(1100–1400)/ Area(1550–1650); RIP = Area(1100–1300)/Area(1300–1370); and DAs/GA = Area(1100-s)/Area(s-1650), where s is the position with lowest intensity between the D and G bands. Sample

D Position cm b

NEM-12 NEM-17b NEM-19a NEM-19b NEM-27a NEM-27b a b

D FWHM

−1

cm

1358 1345 1342 1363 1348 1368

G Position

−1

cm

258 219 179 239 220 246

G FWHM

−1

cm

1602 1603 1602 1601 1605 1601

G-D

−1

cm

62 56 56 62 60 66

ID/IG

DA1/GA

RIP

DAs/GA

0.56 0.60 0.68 0.59 0.64 0.58

1.41 1.49 1.54 1.38 1.52 1.38

1.16 1.05 0.87 1.11 1.05 1.11

1.55 1.59 1.50 1.53 1.63 1.37

−1

244 258 260 238 257 233

Fusinite. Semifusinite.

2.5

a)

10

2.0

8

1.5

AR2

AR1

b)

12

1.0

6 4

0.5

2

0.0

0 90

80

70

60

50

90

40

80

70

60

50

40

50

40

G FWHM (cm-1)

G FWHM (cm-1) 3.0

1.6

2.5 2.0

DOS

CH2/CH3

1.2 0.8

1.5 1.0

0.4 0.5

c)

d)

0.0

0.0 90

80

70

60

50

90

40

G FWHM (cm-1)

80

70

60

G FWHM (cm-1)

Fig. 8. Micro-FTIR parameters versus G FWHM for intruded coals. a) AR1 = ratio of peak areas at 3100–3000 cm−1 and 3000–2800 cm−1; b) AR2 = ratio of peak areas at 900–700 cm−1 and 3000–2800 cm−1; c) CH2/CH3 = 2920 cm−1/2960 cm−1 peak; d) DOS = 870 cm−1/750 cm−1 peak. Micro-FTIR data from Presswood et al. (2016).

bond stretching of polyaromatic carbon atoms (Ammar et al., 2015; Malard et al., 2009). Both the D and G bands include several overlapping sub-bands (D1, D2, D3, and D4 bands) (Li et al., 2019), especially in highly disordered carbonaceous materials (Li, 2007). Each of these sub-bands is attributed to a unique structural unit of the carbon materials, but the assignments of some sub-bands are still in dispute (Hackley and Lünsdorf, 2018; Li, 2007; Schito et al., 2017). In the current study, the D and G bands are used to relate Raman parameters to coal rank and estimated paleotemperature; however, as the contributions of the sub-bands are not assessed, our approach does not allow detailed interpretation of structural characteristics of the coals approaching the intrusion (Hinrichs et al., 2014; Wilkins et al., 2018). Raman spectra for multiple positions within a single collotelinite particle and for several different collotelinite particles within a sample in these intruded coals show a high level of similarity (Fig. 6), suggesting structural homogeneity. This contrasts with previous studies on dispersed organic matter (Jubb et al., 2018; Khatibi et al., 2019), which show high levels of variability in the Raman spectra at a microscopic scale, indicating heterogeneity of the organic matter. The heterogeneous nature of organic matter indicated in those studies is probably

Raman spectra obtained on vitrinite in the same samples (Fig. 4), inertinite macerals have relatively narrower D and G bands; in addition, there is no fluorescence background for fusinite (Fig. 7a) and only minor fluorescence background for semifusinite (Fig. 7b). Raman parameters derived from the spectra of inertinite differ from those of vitrinite in the same sample (compare Tables 2 and 3): inertinite has lower D position and G FWHM and a higher DA1/GA and DAs/GA than vitrinite in the same sample. Raman parameters for semifusinite from sample NEM-12 are closer to those of vitrinite (Table 3). 4. Discussion 4.1. Raman spectral variations within coal samples The disordered (D) band in the range of ~1330–1370 cm−1 and the graphitic (G) band at ~1600 cm−1 seen in the vitrinite spectra are typical Raman spectral features of organic matter that contains polycyclic hydrocarbons (Schmidt et al., 2017). The D band is usually related to defects in the polyaromatic layers (Ammar et al., 2015; Deldicque et al., 2016), whereas the G band corresponds to the in-plane 8

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a specific coal sample in the current study allows confidence in the assessment of maturity differences seen between samples of different ranks. There are clear differences in Raman spectral parameters between vitrinite and inertinite macerals in the same coal (Tables 2 and 3) indicating differences in molecular structure for these two macerals. This has been reported previously (Guedes et al., 2010), and the molecular structural differences indicated agree with results of micro-FTIR studies that show functional groups in vitrinite, inertinite, and liptinite macerals vary both in type and amount (Chen et al., 2012; Mastalerz and Bustin, 1996; Mastalerz and Bustin, 1995). Note that for sample NEM-12, there are some similarities in Raman parameters between semifusinite and vitrinite (Table 3), suggesting some semifusinite has similar structural characteristics to vitrinite, which may be why reactive inertinites can exhibit coking characteristics in coking coals (Morga, 2011). Previous studies have demonstrated that maceral heterogeneity in sedimentary rocks has a significant effect on Raman spectral results (Jubb et al., 2018; Lünsdorf et al., 2014). Variations in Raman parameters of different macerals may have been an unrecognized factor in previous studies (e.g., Ammar et al., 2015; Henry et al., 2018; Lünsdorf, 2016; Rantitsch et al., 2004). Henry et al. (2018) reported an intraparticle variation in organic matter in shale of up to 9% RSD (relative standard deviation), but noted that this could be reduced to 1–3% RSD when three measurements were averaged. They attributed this variation to heterogeneity in the organic matter or to sample processing or polishing. Additionally, Katagiri et al. (1988) suggested that a polish can impact the Raman spectrum significantly. However, polished pellets allow clear identification of maceral groups and sub-groups, and have been used in a number of Raman studies of coal and solid bitumen (Dai et al., 2017; Hackley and Lünsdorf, 2018; Hinrichs et al., 2014; Li et al., 2019; Potgieter-Vermaak et al., 2010; Zhou et al., 2014). Recently, Wang et al. (2019) demonstrated that Raman spectra collected on polished graptolites from an Upper Ordovician marine shale (Sichuan Basin, China) had no significant differences to those obtained on a sample of unpolished shale. These results indicate that Raman spectroscopic analysis of polished pellets can provide detailed information on different maceral groups and sub-groups in coal.

Fig. 9. Raman spectral profile changes from sub-bituminous coal to meta-anthracite. Spectra a and b are for metamorphosed coal associated with a large granitic intrusion (Li et al., 2019). Spectra c, d, e and f are for intruded coals in this study.

400

a)

Tpeak (oC)

350 300 250 y = -7.8829x + 752.59 R² = 0.98

200 150 100 50 0 100

80

60

40

20

G FWHM (cm-1) 400

4.2. Raman spectral evolution with coal rank

b)

The most pronounced change in the Raman spectra between high volatile bituminous coal and coked coal (of anthracite reflectance levels) is the progressive narrowing of the width of the G band (G FWHM) (Fig. 4 and 5d). This is related to the enhanced condensation of aromatic rings and the shortening of alkyl chains with coal rank increase (Ferrari and Robertson, 2000; Li, 2007), and is supported by microFTIR results (Chen et al., 2012; Presswood et al., 2016). The band area ratio (AR1) of the aromatic CH infrared stretching mode at ~3100–3000 cm−1 versus the aliphatic CHx stretching modes at 3000–2800 cm−1 increases with decreasing G FWHM (Fig. 8a, Table 1). A similar trend is observed between AR2 and G FWHM, where AR2 is the ratio of aromatic out-of-plane deformation modes between ~900–700 cm−1 and the aliphatic CHx modes (Fig. 8b, Table 1, FTIR data collected on collotelinite; data from Presswood et al., 2016). The CH2/CH3 ratio, which reflects the average chain length of aliphatic hydrocarbon, decreases as G FWHM decreases (Fig. 8c). The degree of substitution (DOS) of aromatic sites with alkyl groups also shows a decrease as G FWHM decreases (Fig. 8d). The de-alkylation and enhanced aromatization of vitrinite with rank also influence the position and width of the D peak (Guedes et al., 2012; Lünsdorf, 2016), although the correlation between the D band width (D FWHM) and rank in this study (Fig. 5a) is not as clear as that reported previously (Guedes et al., 2010; Wopenka and Pasteris, 1993); a similar lack of correlation between D FWHM and maturity was reported by Henry et al. (2019b). With increased rank, the intensity ratio of the D and G bands (ID/IG) initially decreases (Fig. 5e), as reported previously (Chabalala et al.,

350

Tpeak (oC)

300 250 200 y = 739.82x - 886.68 R² = 0.92

150 100 50 0 1.20

1.30

1.40

1.50

1.60

1.70

1.80

DAs/GA Fig. 10. Peak temperature (°C) versus a) G FWHM, and b) DAs/GA, showing linear regression relationships. Temperature is based on the relationship established by Barker and Pawlewicz (1994).

related to different origins of the source rocks, maturation history, or mineral matter (Wilkins et al., 2018). To trace thermal maturity, the relatively high degree of spectral variability shown for dispersed organic matter can be decreased by accurate maceral identification and averaging of spectra. The consistency in spectra for collotelinite within

9

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Fig. 11. Comparison of peak-fitting methods used to determine Raman spectral parameters: a) current study; b) Kouketsu et al. (2014).

meaningful to assess selected Raman parameters across specific maturation stages.

100 G FWHM, current study D2 FWHM, Kouketsu et al. (2014)

Band FWHM (cm-1)

80

4.3. Temperature determination for intruded coals using Raman data The Raman parameter with the highest correlation with rank in the intruded coals is G FWHM; this relationship was observed in previous studies as well (Guedes et al., 2010; Hinrichs et al., 2014; Henry et al., 2019b; Marques et al., 2009; Quirico et al., 2005; Schito et al., 2017). The parameter (DAs/GA) proposed in this study is similar to the previously reported Raman parameter RA2 that was based on deconvoluted spectra (Lahfid et al., 2010; Schito et al., 2017); both indicate the area ratio of the D and G bands, but the DAs/GA proposed here avoids bias introduced by peak-fitting. Based on relationships between Raman parameters and coal rank (Fig. 5), G FWHM and DAs/GA were selected to build paleotemperature equations. Using the relationship between reflectance and temperature developed by Barker and Pawlewicz (1994), the relationships between maximum temperatures that the intruded coals had experienced (Tpeak, °C) and the two Raman parameters are shown in Fig. 10. Assuming similar techniques are used (in terms of sample preparation and spectral processing), temperatures for intruded coals can be estimated using Raman parameters according to the following formulae:

60

40

20

0 0

100

200

300

400

500

600

700

T (oC)

Fig. 12. Comparison of band width versus temperature (°C) for current study (G FWHM) and that of Kouketsu et al. (2014) (D2 FWHM).

2011; Quirico et al., 2005). According to Levine (1993), coal along this trend is in the bituminization and de-bituminization stages. The polycyclic aromatic hydrocarbons in vitrinite of coal are randomly oriented during the bituminization stage but rearrange to form local molecularoriented domains following devolatilization (Oberlin et al., 1999; Lünsdorf, 2016). At the highest ranks, an increase in the ratio is seen, as observed by Deldicque et al. (2016) for carbonized pine wood and by Li et al. (2019) for natural graphitized coals. The ratio increases due to an increase in the defect density on the boundary between polyaromatic units during anthracitization (Bogdanov et al., 2014; Deldicque et al., 2016). Representative Raman spectral profiles for samples from the current study, bituminous (Rr = 0.55%) through meta-anthracite rank (Rr = 7.48%), are summarized in Fig. 9 (c through f), along with spectra for two additional samples collected adjacent to a large pluton in Hunan Province, China (a and b, Li et al., 2019). During the bituminization and de-bituminization stages (Fig. 9, f, e, d), Raman spectra show a progressive narrowing of G FWHM, a decrease in ID, and increased asymmetry of the D band with increased rank. A continued decrease in ID and G FWHM, along with the disappearance of minor bands (like D3 and D4 bands) is observed early in the anthracitization stage. Meanwhile, increases in ID are clearly seen in the coked coal with reflectance levels equivalent to anthracite through meta-anthracite rank (e.g., Fig. 9b, a). The D2 band becomes much more pronounced in the highest rank meta-anthracite; this shoulder on the G band results in an apparent broadening of G band (Fig. 9a). During graphitization, the intensity of all the defect bands decreases and G FWHM decreases with increased graphitization (Li et al., 2019). It appears that there is no single Raman parameter that shows a steady change throughout the transformation from low maturity materials through maturation to meta-anthracite and graphite (Lünsdorf et al., 2017); thus, it is more

Tpeak (°C) = −7.8829 (G FWHM) + 752.59 (R2 = 0.98)

(1)

Tpeak (°C) = 739.82 (DAs/GA)–886.68 (R2 = 0.92)

(2)

The effective temperature range for these two geothermometers is restricted to the range 70 to ~300 °C based on samples in this study (or 0.55% to 3.17% Rr). For samples with much higher reflectance, these two thermometers would likely underestimate the maximum temperature; this is because the relationship between Rr and Tpeak proposed by Barker and Pawlewicz (1994) is only valid up to ~300 °C. The most altered sample in this study (sample NEM-1) may have experienced higher peak temperatures, perhaps as much as 450–500 °C based on the observed coke textures (Presswood et al., 2016; Rimmer et al., 2009), rather than ~360 °C calculated using the equations proposed by Barker and Pawlewicz (1994). However, that higher temperature estimate assumes similar controls on the development of coke textures in nature as would be experienced in commercial coke ovens. Obviously, heating rates and duration of heating would be different in nature, and thus there is some question about the temperature at which a specific coke texture would be developed (see Murchison, 2006, for further discussion of rate of heating on coke texture). In addition, maturation rates under normal burial conditions, hydrothermal metamorphism, and contact metamorphism may differ (e.g., Barker and Pawlewicz, 1994; Rimmer et al., 2009), thus the temperature estimates made here should be considered a first approximation. Chen et al. (2017) suggested that the formula developed by Barker and Pawlewicz (1994) is only suitable for hydrothermal metamorphism, not contact metamorphism, and they recommended a formula derived from laboratory hydrothermal “bomb” data of Bostick and 10

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obtained in this paper minimize peak-fitting bias as much as possible. Two geothermometers for natural rapidly heated coals are proposed based on the Raman parameters G FWHM and DAs/GA. These geothermometers are suitable for the temperature range ~70–300 °C, and for hydrothermally metamorphosed or intruded coals, rather than coals matured under normal geothermal gradients, provided the sample preparation protocols are followed.

Pawlewicz (1984) instead. However, the Bostick and Pawlewicz's (1984) relationship does not consider the reaction between organic matter and supercritical fluids or water vapor that may decrease vitrinite reflectance (Barker et al., 1998; Wang et al., 2008). Based on a comparison with fluid inclusion data, Barker et al. (1998) determined that the hydrothermal equation used in the present study could be used for contact metamorphism up to temperature ≤ 300 °C, but probably not at higher temperatures. It is important to note that, in several examples of intruded coals and shales, a sharp decrease in vitrinite reflectance is observed directly adjacent to a sill (e.g., Bishop and Abbott, 1995; Henry et al., 2019b; Khorasani et al., 1990; Raymond and Murchison, 1989), which would also complicate calculation of temperature based on vitrinite reflectance alone; such a decrease is not observed in the current study. Raman spectral parameters were first used by Beyssac et al. (2002) to calibrate a geothermometer for regional metamorphic belts for greenschist metamorphic grades and above. However, Lahfid et al. (2010) pointed out that geothermometry suitable for medium- to highgrade metamorphism cannot simply be extrapolated to lower maturity samples, due to the complexities in the Raman spectra at vastly different metamorphic levels. Kouketsu et al. (2014) proposed two new Raman geothermometers suitable for low-grade and medium-grade metamorphism (over the temperature ranges 150–280 °C and 280–400 °C, respectively). These were based on the peak-fitted Raman parameters D1 FWHM and D2 FWHM. The D2 band in their expression is comparable to the G band in the current paper (Fig. 11, see also Fig. 2f in their paper). Over the temperature range 150–300 °C, G FWHM in the current study is always higher than their D2 FWHM (Fig. 12), because the authors used two sub-bands to fit the band at ~1600 cm−1 (Fig. 11b, Kouketsu et al., 2014). Thus, different peakfitting methods have a significant impact on the final Raman parameters, and therefore it is important to only compare Raman parameters obtained by the same spectral processing protocol. The Raman parameters used to build the geothermometer in the current study have the advantages of being easy to obtain and of minimizing peak-fitting bias introduced by different researchers. Additionally, it is questionable whether geothermometry developed from the Raman spectra of dispersed organic matter can be applied directly to coals. Dispersed organic matter may be graphitized in metamorphic belts at lower temperatures (~300 °C), in part due to additional deformation stress (Bustin et al., 1995), or to interaction between organic matter and inorganic elements (Charon et al., 2014; Jubb et al., 2018).

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported, in part, by the National Natural Science Foundation of China (41672150), the Spackman Award from The Society for Organic Petrology (to K.L.), and the Medlin Scholarship from the Energy Division, Geological Society of America (to K.L. and S.M.P.), and a scholarship from the Graduate and Professional Research Council, Southern Illinois University Carbondale (SIUC) (to S.M.P.). Kuo Li thanks the China Scholarship Council (CSC) for financial support (No. 201606430030) during his visit to the Department of Geology (now part of the School of Earth Systems and Sustainability), SIUC. Dr. Alian Wang of the Department of Earth and Planetary Sciences, Washington University in St. Louis is acknowledged for access to her Laser Raman Imaging Laboratory. The authors thank Justin Filiberto (formerly of SIUC) for helpful discussions on Raman spectroscopy; Yilin Lu of Horiba (China) for discussions on Raman fluorescence; and Dandan Hou at The Pennsylvania State University for assistance with Raman spectral processing. We thank two anonymous reviewers for their helpful comments that improved the manuscript. Finally, we thank Dr. John C. Crelling for the many insightful discussions on intruded coals over the course of this and our previous studies. References Ammar, M.R., Galy, N., Rouzaud, J.N., Toulhoat, N., Vaudey, C.E., Simon, P., Moncoffre, N., 2015. Characterizing various types of defects in nuclear graphite using Raman scattering: heat treatment, ion irradiation and polishing. Carbon 95, 364–373. ASTM, 2011. D2798-11a, Standard Test Method for Microscopical Determination of the Vitrinite Reflectance of Coal. ASTM International, West Conshohocken, PA. www. astm.org. Barker, C.E., Pawlewicz, M.J., 1994. Calculation of vitrinite reflectance from thermal histories and peak temperatures: a comparison of methods. In: Mukhopadhyay, P.K., Dow, W.G. (Eds.), Vitrinite Reflectance as a Maturity Parameter: Amer. Chem. Soc. Symp. Series 570, pp. 216–229. Barker, C.E., Bone, Y., Lewan, M.D., 1998. Fluid inclusion and vitrinite-reflectance geothermometry compared to heat-flow models of maximum paleotemperature next to dikes, western onshore Gippsland Basin. Australia. Int. J. Coal Geol. 37, 73–111. Beyssac, O., Goffé, B., Chopin, C., Rouzaud, J.N., 2002. Raman spectra of carbonaceous material in metasediments: a new geothermometer. J. Metamorph. Geol. 20, 859–871. Bishop, A.N., Abbott, G.D., 1995. Vitrinite reflectance and molecular geochemistry of Jurassic sediments: the influence of heating by Tertiary dykes (Northwest Scotland). Org. Geochem. 22, 165–177. Bogdanov, K., Fedorov, A., Osipov, V., Enoki, T., Takai, K., Hayashi, T., Ermakov, V., Moshkalev, S., Baranov, A., 2014. Annealing-induced structural changes of carbon onions: high-resolution transmission electron microscopy and Raman studies. Carbon 73, 78–86. Bostick, N.H., Pawlewicz, M.J., 1984. Paleotemperatures based on vitrinite reflectance of shales and limestones in igneous dike aureoles in the upper Cretaceous Pierre Shale, Walsenburg, Colorado. In: Woodward, J., Meisner, F.F., Clayton, J.L. (Eds.), Hydrocarbon Source Rocks of the Greater RockyMountain Region. RockyMountain Association of Geologists Denver, pp. 387–392. Bustin, R.M., Rouzaud, J.N., Ross, J.V., 1995. Natural graphitization of anthracite: experimental considerations. Carbon 33, 679–691. Chabalala, V.P., Wagner, N., Potgieter-Vermaak, S., 2011. Investigation into the evolution of char structure using Raman spectroscopy in conjunction with coal petrography; part 1. Fuel Process. Technol. 92, 750–756. Charon, E., Rouzaud, J.-N., Aléon, J., 2014. Graphitization at low temperatures (600–1200°C) in the presence of iron implications in planetology. Carbon 66, 178–190. Chen, Y., Mastalerz, M., Schimmelmann, A., 2012. Characterization of chemical functional groups in macerals across different coal ranks via micro-FTIR spectroscopy. Int.

5. Conclusions From high volatile bituminous to low volatile bituminous rank, Raman data for polished pellets of intruded coals show a decrease in G FWHM, a progressive decrease in the D band intensity, and an increase in the asymmetry of the D band. During anthracitization, the width of the D and G bands continues to decrease, and with increasing rank minor D3 and D4 bands disappear but the D band intensity increases. The structural evolution of coal at different maturation levels is responsible for the evolution of Raman spectral profiles. The enhanced condensation of aromatic rings and shortening alkyl chains with coal rank, as demonstrated by Raman spectroscopy, are consistent with the results obtained by micro-FTIR techniques (data from Presswood et al., 2016). The heterogeneous nature of coal affects Raman results if maceral differences are not considered; however, within individual samples there is a very high level of consistency of Raman spectra, and hence inferred structural homogeneity, both within and between vitrinite particles. Differences in Raman data processing methods may also impact Raman parameters significantly; thus, only Raman parameters obtained by the same protocol are comparable. The Raman spectral parameters G FWHM and DAs/GA correlate well with vitrinite reflectance of intruded coals. The parameters 11

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