1 Introduction
The world has 60 Mt of microcrystalline graphite reserves, of which China holds 55 Mt (
Robinson et al., 2017). The coal-derived natural graphite (CDNG) in China belongs to the microcrystalline graphite and is 60% of the domestic reserves (
Li, 2019). Owing to its high carbon content, availability, and easy exploration, there are considerable commercial opportunities due to the rapid depletion of flake graphite reserves (
Wang et al., 2019b). However, as the CDNG coexists with anthracite, which lacks effective methods to distinguish, it is often burned like coal, causing a huge waste of resources. Additionally, the graphitization process of different macerals is different, but related research remains elusive. As graphite plays an important role in the new energy revolution, getting a better understanding of the coal natural graphitization process will enhance the CDNG utilization value.
Graphitization needs graphitizable carbons (
Beyssac and Rumble, 2014;
Harris, 2005), sufficient heat (
Buseck and Beyssac, 2014;
Oberlin, 1984), adequate pressure (
Bonijoly et al., 1982), and perhaps some catalysts (
Buseck and Huang, 1985;
Duber et al., 1993). The CDNG is usually formed in a metamorphic belt associated with igneous intrusion and tectonic stress. Igneous intrusions and tectonic movements made the coal metamorphic, forming metamorphic aureole transforming coal from anthracite to CDNG. The coal closer to the magma intrusion will have been exposed to higher temperatures and experience a longer heating duration (
Yuan et al., 2021). Tectonic stress can also facilitate graphitization. Strain stress can change the coals’ molecular structure directly. The local stress concentration promotes the preferred orientations of the local basic structural units (BSUs) and removes structural defects, leading to the gradual expansion of locally ordered structures (
Wang et al., 2019a). The kinetic energy of tectonic stress can be converted into heat through friction (
Kuo et al., 2014;
Wang et al., 2014). Thus, anthracite is thought to be the main precursor of the CDNG due to its well-aligned aromatic structure. Anthracite has a better graphitizability than other graphitizing carbons when the temperature is > 2000°C in synthetic graphitization (
Oberlin and Terriere, 1975). However, the temperature required for natural graphitization is only 300°C–500°C (
Buseck and Huang, 1985;
Bustin et al., 1995), which is much lower than that of synthetic graphitization. The ability of anthracites to graphitize has been related to the characteristics, such as the initial spatial arrangement of BSUs. Some anthracites give rise to carbon that have mainly a graphitic structure, whereas others produce porous and turbostratic carbons (
Blanche et al., 1995). The maceral groups also influence the graphitization: vitrinite is more graphitizable than inertinite (
Wang et al., 2019b). Besides these factors, the amount, type, and distribution of mineral matter have been shown to facilitate (
González et al., 2003) or inhibit (
Nyathi et al., 2013) graphitization.
In graphitization, the structural ordering evolution in the microscale has been detailed through direct observation from HRTEM micrographs (
Buseck and Huang, 1985;
Oberlin and Terriere, 1975;
Zheng et al., 1996). These can be summarized as anthracite having poorly organized aromatic rings stacked within five layers and extended within 2 nm width, forming a BSU. In the meta-anthracite, these BSUs aligned along a preferential direction to form local molecular orientation domains and the local molecular orientation domain tends to coalescence forming an oriented domain (
Li et al., 2020a;
Li et al., 2021). With the metamorphic degree increasing, the curvature and structure defects of coal molecules were removed, thus forming highly ordered graphite (
Li et al., 2020a). Our previous work quantified this process (
Yuan et al., 2021), what remains elusive is the transitions within the microscopically distinguishable components (MDC).
Hower et al. (
2021a) found that the metamorphism of anthracites (typically the Cretaceous Cerrillos anthracite, Butte anthracites, and the Pennsylvania anthracites) differs from the metamorphism of breccia (typically the Ragged Edge breccia). The Cretaceous Cerrillos and Crested Butte anthracites were both metamorphosed by igneous intrusions, but not in close enough proximity to producing natural coke. For Pennsylvania anthracites, the hydrothermal mineralizing fluids at >375°C and >22 MPa driven up the rank of coal from high volatile C or B bituminous-rank level to anthracite.
Hower et al. (2021b) also pointed out that the duration of the heating, melting, and resolidification event was about 1 h based on the known behavior vitrinite at high temperatures and, to a lesser extent, at high pressures.
X-ray diffraction (XRD) provides average structure information (i.e., crystalline diameter (
La), crystalline height (
Lc), average layers (
Nave)). Petrographic analyses give information of reflectance, morphological features, and interference colors, aided in distinguishing different MDC types. The micro-Raman spectroscopy uses a microscope permitting analyses of specific components, besides micro-Raman spectroscopy of coal allows obtaining the Raman spectrum of a selective volume of material, with a lateral resolution > 1 μm. Thus, it enables the analysis of specific macerals/MDC. Previously, micro-Raman spectroscopy has been performed in various macerals up to anthracite rank (
Guedes et al., 2010 and
2012).
Rantitsch et al. (
2016) characterized graphite and semi-graphite samples from several mines worldwide by the D1/(D1 + D2 + G) area ratio (R2) and the full-width at half maxima of G band (G-FWHM), the obtained microstructural data suggest a continuous transformation of semi-graphite enclosed in lower greenschist-facies rocks to graphite from amphibolite- to granulite-facies rocks.
Kwiecinska et al. (
2010) use micro-Raman to quantify the crystallographic structure and the differences between the meta-anthracite, semi-graphite, and graphite.
The harsh CDNG deposit setting makes the CDNG a unique graphite resource worldwide. The currently mining CDNG deposit in China mainly occurs in Xinhua and Lutang, Hunan Province, and Panshi, Jilin Province. Previous studies have focused on anthracite/graphite from the same metamorphic aureole with different maturation (
Li et al., 2018 and
2019). However, samples collected from the single area do not cover all the metamorphic phases. Here, we selected samples from three typical CDNG deposits: Xinhua, Panshi, and Lutang. The CDNG in these three regions accounts for 94% of total China’s CDNG reserve (
Li, 2019) and covers each transitional phase throughout the metamorphic process, thus capturing the CDNG processes.
2 Geological settings
Fig.1 shows the sampling locations. The Xinhua mining areas are located in the Lianyuan Basin in the central Hunan Province. The coal-bearing strata are within the lower Carboniferous Ceshui Formation (earlier than 290 Ma). The samples CM, SL, BC, and SXL were collected in the Choumu, Shenli, Baicong, and Shixiangli mines, toward the Tianlongshan granitic pluton with distances of 2.0, 1.1, 0.6, and 0.3 km. The age of the granite is ~202–208 Ma. All of these samples are collected in the No. 3 coal seam. Coal strata in the area closest to the pluton are deformed and seams are nearly vertical.
The Lutang mining areas are located in the southwest of Hunan Province, which lies between the Cathaysia and the Yangtze Blocks. The tectonic and magmatic activities were most intense during the Indosinian-Yanshanian Period (135–233 Ma) (
Huang, 2005). The Qitianling granitic pluton intruded east of the research area during the Yanshanian period (146–163 Ma) (
Zhu et al., 2009), covering an area of 530 km
2. The coal-bearing strata occurred in the upper Permian Longtan Formation. The metamorphism gradually decreases with increasing distance from the pluton (
Chao et al., 2017). The study area is divided into three zones near the intrusion: the chlorite zone (700–1300 m, 300°C–450°C), the biotite zone (400–700 m, 400°C–500°C), and the hornfels zone (0–400 m, 450°C–600°C) (
Wang et al., 2019a;
Wu et al., 2021). The main tectonic structure in the area is the NNE-striking Lutang–Shatian syncline. The western flank manifests as broad and gentle folds, while structures at the eastern flank are characterized by tight folds. The samples were collected in Nanfang No. 6 mine (NF63, NF68, and NF617). Their distance to the Qitianling granitic pluton was 0.5, 1.5, and 3.0 km, respectively.
The Panshi mining areas are located in the central part of Jilin Province, between the Dunmi fault zone and the Lamulun fault zone, covering an area of 25 km
2 and the coal seam was highly altered during the Yanshanian Period (
Yuan et al., 2021). The PSZK and PSXRD were sampled in the Zhang, and Xianrendong mines, respectively, where coal-bearing strata occurred in the Triassic Dajianggang Formation. The distances to the intrusion body were 0.6 and 0.9 km.
3 Samples and methods
3.1 Sample preparation
The powdered samples (−200 mesh) were used for proximate analyses. A three-step HCl-HF demineralization process was performed, using 5 g of the sample dispersed in 30 mL of HCl solution (37 wt%) and 20 mL of HF solution (40 wt%); with stirred for 12 h. This process was performed sequentially three times. The samples were filtered and washed with deionized water until the pH value was 7. Samples were filtered and dried in a muffle furnace at 60°C for 12 h. The acid-treat samples were used for XRD.
The samples were crushed to −20 mesh and pulverized to −200 mesh. The crushed samples (−20 mesh) were mixed with epoxy resin to make pellets for petrographic analyses and micro-Raman spectroscopic analysis. The pellets were allowed to cure and were polished using a Buehler Autonet 250.
3.2 Analytical methods
3.2.1 Proximate and ultimate analyses
The proximate and ultimate analyses were conducted per ASTM standard D3172-13 and ASTM standard D5373-14, respectively. These analyses were conducted at the Shanxi Institute of Geology and Mineral Resources.
3.2.2 XRD
The acid-treat powder samples were analyzed using a Rigaku D/max-2500 PC diffractometer equipped with Cu Ka radiation. The X-ray generator voltage and current were constant at 40 kV and 100 mA. The sample was scanned using the continuous sweep method from 2.5° to 80° (in the 2θ range) at a rate of 2°/min. The XRD test was conducted at the China University of Mining & Technology, Beijing.
Three detected peaks were the
-band (002), γ-band (left
-band), and the (100) band (around 43°) (
Lu et al., 2001). To quantify the structural evolution, peak-fitting using the pseudo-Voigt function in the Jade 5 package was applied. For the crystal structure parameters: interlayer spacing (
d002), crystallite height (
Lc), crystallite diameter (
La), and average stacked aromatic layers (
Nave) were calculated using the Scherrer equations:
La=1.84
λ/
βacos(
θa);
Lc=0.89
λ/
βccos(
θc);
Nave=
Lc/
d002+1.
Here, λ is the wavelength (0.154056 nm) of the radiation; βa and βc are the FWHM of the (002) and (10) peaks, respectively, and θa and θc are their corresponding Bragg angles.
For better characterizing the structural ordering evolution after entering the meta-anthracite stage, the
AI (Asymmetric index of the (002) band) was adopted per
Zhang et al. (
2020). As the turbostratic structure and structural defects cause the
γ band, resulting in the asymmetry of the (002) band. Therefore, the
AI value can evaluate the structural ordering. The
AI can be detailed as follows (Fig.2).
AI =
γ1 /
γ2.
3.2.3 Petrographic analyses
The Rmax value was measured using a CRAIC Coal-Pro micro-reflectometer coupled with a Leica DM 2500P reflected-light microscope as per the ASTM D2798-11a. For the highly ordered graphite (SXL, NF63, NF68, NF617, PSXRD, and PSZK), Graphite grains were selected for testing reflectance (Rmax, %) as vitrinite was altered in the CDNG samples. Photomicrographs were taken under white light, oil immersion.
3.2.4 Micro-Raman spectroscopy
The micro-Raman spectrum of samples was collected by a confocal Raman Microscope alpha 300R at WITec GmbH (Ulm, Germany). This system is equipped with a solid-state continuous-wave laser emitting at 532 nm. A piece of single-crystal silicon was used to calibrate the wavenumbers of the shifts. A 50× or 100× (NA = 0.9) ZEISS objective was selected for excitation and detection, and the Rayleigh light was rejected by an edge filter. A 300 grooves/mm grating was used to disperse the light with the spectral resolution of 4.8 cm−1. The laser with a power of 5 mW was focused on the sample, and the acquisition time was 10 s with an accumulation of 2. The micro-Raman tests were conducted in the Institute of Geology and Geophysics, Chinese Academy of Sciences.
The spectra were analyzed using the Lorentzian curve-fitting procedure in OriginLab 2021 software. In the first-order region, five distinguishing bands were fitted. The G band (~1580 cm
−1) is attributed to the E
2g vibration mode of carbon rings in graphite sheets, and the D1 band (~1350 cm
−1), D2 band (~1620 cm
−1), D3 band (~1500 cm
−1), and D4 band (~1200 cm
−1) are all described as defect-induced bands (
Kwiecinska et al., 2010). The D1 band is attributed to the in-plane defects, while the D2 band at the shoulder of the G band is not yet well understood until recently (
Schwan et al., 1996). The D3 band and D4 band are separately related to the defects outside the plane and hexagonal diamond or sp
3-rich phase (
Hinrichs et al., 2014;
Potgieter-Vermaak et al., 2011). For the raw Raman peaks in the second peak region, the 2D
R (~2450 cm
−1), 2D peak (~2700 cm
−1), D1+G peak (~2900 cm
−1), and 2D
2 peak (~3240 cm
−1) are often present, they were treated as the overtones and combinations of the disorder-induced bands (
Nemanich and Solin, 1979).
Carbonaceous matter can be sensitive to the polishing process, which leads to erroneous Raman spectra of these materials. The samples investigated have been processed by crushing and pulverization. Because mechanical stress from shearing or polishing may influence the Raman spectra, it is necessary to evaluate this potential contributing factor. A highly ordered graphite sample from Sri Lanka was polished and analyzed to determine whether polishing had any effect on the Raman spectra. No defect-induced band was detected in the Raman spectra of the polished Sri Lanka graphite (see the supplementary material), suggesting no adverse effects from the polishing.
3.2.5 Scanning electron microscope (SEM)
The demineralized samples were used for SEM analysis. First, Rotary-pump sputtering was used to coat the pump samples (heights < 0.5 cm and diameters < 1 cm) with an ultra-thin coating of gold to afford a thickness range of 1–2 nm to enhance their electrical conductivity. Then, surface morphology of series samples was analyzed using a Hitachi S-4800 SEM.
4 Results and discussion
4.1 Conventional analyses
Basic coal data are shown in Tab.1. All the samples exhibit a high metamorphic degree with a high C content value (> 90%) and low volatile yielding, H (< 0.5%), O (< 5%) content value. The data provided by ultimate analyses varies subtly between samples. In coalification, the loss of the aliphatic side chain will lead to the decrease of H, O content value, while in graphitization, the major structural change is the rearrangement of the C atoms network rather than the loss of aliphatic side chains. The
Rmax value was high for the CM sample and SL sample. However, for the other samples, the
Rmax remains stable at 4%–6%, this is consistent with the previous reported values for naturally graphitized coals (
Li et al., 2018). The changes in the size and orientation of the BSU have a contribution to the increase in the vitrinite reflectance, while in graphitization the BSU rearrangement terminated this increase. The BC sample is in graphitization with a low
Rmax value.
4.2 Average crystalline structure
The XRD patterns of the samples are shown in Fig.3. The CM is classified as coal samples with a board (002) peak within 20°–30° (2θ). From the XRD pattern, with the distance from the coal seam to the igneous rock decrease in the same region, the (002) peak gradually becomes narrower with greater intensity and a smaller FWHM, indicating a better structural ordering is achieved. The Raman spectra exhibited a continued evident ordering from the reduction in the D bands and the emergence of higher intensity, narrower G bands which is consistent with the XRD data.
Detailed crystalline parameters are listed in Tab.2. For the structural ordering parameters, the International Committee for Coal and Organic Petrology (ICCP) uses
d002 value, H/C, and
Rmax% to classify the metamorphic degree above anthracite rank (
Kwiecińska and Petersen, 2004). While the vitrinite reflectance is not a valid parameter for highly graphitized coals due to the structural rearrangement of the BSUs (
Rantitsch et al., 2016). Unfortunately, as the H content is very low, the H/C ratio cannot be used as a reliable parameter for the CDNG samples, therefore, the only parameter to distinguish the transitional phase is
d002.
Here, we use the d002 interlayer distance of samples as an ICCP-defined reference. The sample CM is classified as anthracites with their d002 values > 0.34 nm. The SL sample with a d002 value between 0.338 and 0.34 nm is classified as meta-anthracite, and BC with a d002 value between 0.337 and 0.338 nm is classified as semi-graphite. The other samples with d002 values ≤ 0.337 nm are classified as CDNG. The La, Lc, and Nave of samples in the Xinhua region increased as the d002 interlayer distance decreased. However, for the CDNG samples in Lutang and Panshi, the La, Lc, and Nave do not increase with the decreasing d002 values as expected. The NF68 sample has the lowest Lc, and Nave in comparison with NF617 and NF63 CDNG samples. The local molecular orientation domains may slip and splice and finally connect under shear stress. As we can see from Fig.1. The NF68 is sampled near a reverse fault, where direct shear stress made the sample’s BSU diameter longer but a short height along with less stacking.
As is shown in Fig.4. The
AI has a linear relationship with
d002. The samples NF63, NF68, NF617, PSXRD, and PSZK are all classified as highly ordered graphite with their
AI values > 0.63. As is shown in Tab.2, the G-FWHM of the CDNG samples varies slightly. As the predominantly triperiodical structural order is attained (Tab.1), a further decrease of the
d002 parameter results only in a minor decrease of the G-FWHM. The structural defects of graphene and anthracite shared many similarities in Raman spectra (
Han et al., 2016). In graphene, the D1/D2 intensity ratio can be used to distinguish the type of structure defects (
Eckmann et al., 2012). While the R1 (D1/G intensity ratio), D1/G area ratio, G-FWHM, these parameters do not consider the D2 peak in comparison with R2 make R2 a better parameter to evaluate the structural order. Fig.3 shows the relationship between the R2 and
d002; the R2 value has a better linear relationship with
d002 after graphitization.
4.3 Morphological features of MDC (via optical microscopy and SEM)
Although the macerals had been altered significantly by igneous intrusion and tectonic stress, some maceral precursors could be identified through microscope. For the less altered anthracite sample CM (
Rmax = 9.49%). The thermoplastic vitrinite (Fig.5(a)–Fig.5(b)), the devolatilized vitrinite (Fig.5(c)), and some “normal” macerals were present. The “normal” macerals were composed of desmocollinite (Fig.5(d)). The thermoplastic vitrinite exhibits a weak optical anisotropy as it shows no difference when the microscope platform rotated 90° (Fig.5(a)–Fig.5(b)). No liptinite was found and no preservation of maceral textures of bituminous precursors. The SEM images show that the edge side and frontage side of the graphite sheet with length < 3 μm and heights < 40 nm. These graphite sheets are stacked randomly. Here, the CM anthracite shows a significant difference compared with Pennsylvania anthracite reported before (
Hower et al., 2019,
2021a, and
2021b).
Hower et al. (2019) suggests that the thermoplastic vitrinite is not an igneous magma and heat alone is not sufficient to keep it in a fluid state based on comparisons between laboratory studies of thermoplasticity of vitrinite and the natural conditions.
For the representative meta-anthracite sample SL (
Rmax = 9.96%), the major of the MDC are the thermoplastic vitrinite. Besides, some newly formed components were present include pyrolytic carbon, mosaic structure, and crystalline tar. The pyrolytic carbon occurs along the edges of the brecciated fragments (Fig.6(a)), which are the deposits formed on a hot substrate by dehydrogenation of a gaseous hydrocarbon (
Kwiecińska and Pusz, 2016;
Oberlin, 2002). The presence of pyrolytic carbon, both in vitrinite cavities, at the edge of the melted zone, and in fractures suggests a gas phase associated with the melting event (
Goodarzi et al., 1992). The mosaic structure (Fig.6(b)) lacks a clear outline between other macerals or MDC as a result of the local softening and melting of the “normal” macerals. The mosaic structure has been identified similar features as microcrystalline graphite (
Li et al., 2018). The crystalline tar exhibits a lamellar structure which is also called needle-like graphite or silk-like graphite in some articles (
Li et al., 2018 and
2019;
Wang et al., 2019b). It usually formed in the void cavities such as cells (Fig.6(b)–Fig.6(c)) or fractures (Fig.6(d)). Per the SEM, the pyrolytic carbon (Fig.6(e) –Fig.6(f)) presents some microspheres (20–40 nm) agglomerate. These microspheres are much smaller in size than the pyrolytic carbon reported previously in synthetic graphitization (1–10 μm) (
Oberlin, 2002) while the crystalline tar (Fig.6(g)–Fig.6(h)) adhered to the grains’ surfaces of TV.
For the representative semi-graphite sample BC (Rmax = 5.39%). The newly formed MDC now includes matrix graphite, flake graphite, and crystalline aggregates. In the semi-graphite, the pyrolytic carbon deposits on the edge of matrix graphite, forming a graphite shell (Fig.7(a)). The shells’ void cavities are filled with crystalline tar (Fig.7(b)). The flake graphite is distributed in the matrix graphite fractures along with the crystalline aggregates (Fig.7(c)–Fig.7(d)). The flake graphite was formed as a result of the further crystallization of the crystalline tar. In comparison with the meta-anthracite, the matrix graphite takes up the biggest share of all MDC while more thermoplastic vitrinite is no longer visible.
In the representative CDNG sample (PSZK), most of the MDCs are similar to that of semi-graphite. However, the flake graphite (Fig.7(e)–Fig.7(f)) takes the place of crystalline tar with further crystallization. The crystalline aggregates also take a larger share than that in semi-graphite. Fig.7(g) shows the SEM image of the surface of matrix graphite, the matrix graphite particles were present on the surface of lump graphite. Fig.7(h) shows the graphite basal plane (GBP) of matrix graphite. Some graphite particles show a rough hexagon shape, as a highly ordered graphite structure is attained, it represents the property of self-confinement. Fig.7(e) –Fig.7(f) exhibit the graphite edge plane, the graphite particles represent a thin sheet with heights of about 50 nm, which is consistent with the Lc value calculated by XRD (Tab.2). However, these graphite sheets do not stack parallelly in a long range because of the existence of structural defects.
4.4 Micro-Raman analyses
4.4.1 Microstructural structural evolution as detected by micro-Raman spectroscopy
The representative micro-Raman spectra and the peak-fitting pattern were shown in Fig.8 and the quantized data of Raman parameters were shown in Tab.3. For the anthracite sample CM (Fig.8(a)), the Raman pattern of the “normal” vitrinite shows a typical coal Raman pattern with well-separated D1, D3, and D4 peaks, also with two broad peaks in the second peak region (around 2700 cm
−1 and 3240 cm
−1). The I
D1/I
D2 (R1) and R2 values were used to evaluate the structural ordering degree, lower R1 and R2 values mean a higher structural ordering degree for carbonaceous materials (
Li et al., 2020b;
Potgieter-Vermaak et al., 2011;
Tuinstra and Koenig, 1970;
Xu et al., 2017). While for the “normal” macerals, the R1 and R2 values are both smaller than altered macerals or MDCs which is contrary to expectations. The thermoplastic vitrinite (Fig.8(b)) shows a different pattern to the “normal” macerals with board D1, D4, and G peaks but the D3 peak was not present. The disappearance of the D3 peak indicated that the aliphatic chain was reduced. The G peak is also asymmetrical. In the second peak region, the 2D1, D1 + G, and 2D2 peaks were present. For the D-V (Fig.8(c)), the D4 peak disappears, while the well-separated D2 peak occurs, indicating that a higher metamorphic degree is achieved.
For the meta-anthracite sample SL, the micro-Raman pattern of crystalline tar was shown in Fig.8(d). The D1, G, and D1 + G peaks are present along with a D2 band that occurs in the right shoulder of the G band in the first-order region. The 2D
R, 2D1, D1 + G, and 2D2 are observed as well. The mosaic structure (Fig.8(e)) usually occurs in the meta-anthracite. The low R1 (0.59) and R2 (0.43) values indicated that the mosaic structure has a high structural ordering value. The 2D
R peak occurs indicated a large aromatic ring system is formed (
Xu et al., 2017).
For the semi-graphite and CDNG sample, the matrix graphite, flake graphite, and crystalline aggregates were shown in Fig.8(f)–Fig.8(h), respectively. The flake graphite (Fig.8(g)) shows a quite similar Raman pattern with the crystalline tar. Per the Fig.7(b)–Fig.7(d), it can be inferred that the tar gradually fills and crystallizes in the void cavities, ultimately forming flake graphite. The flake graphite is less ordered with R2 value > 0.4, R1 value > 0.5 than the crystalline aggregates. The Raman pattern also shows that these flake graphite particles are equivalent to the GEP of natural graphite per previous research (
Rodrigues et al., 2013). The crystalline aggregates exhibit the highest structural ordering degree with R2 = 0.22 and G-FWHM = 21 cm
−1. Additionally, the D1 + G peak began to split and the 2D2 peak is much weaker, which also indicated the triperiodical graphite structure is achieved. These Raman characteristics also correspond to the GBP per the previous report on natural graphite (
Rodrigues et al., 2013), which revealed that the crystalline aggregates are GBP indeed. For the matrix graphite, the peak shape shows no significant difference with flake graphite except for the intensity. A quantity of data are needed to distinguish them.
Here, the crystalline aggregates and crystalline tar coexist within the same MDC particle void cavities in the same MDC (Fig.9), suggesting that crystalline tar and crystalline aggregates formed under the similar temperature and stress condition. Their differences in optical and micro-Raman spectra may be due to the orientation of the crystal plane. As crystalline tar is the main precursor of flake graphite, flake graphite is GEP. The crystalline aggregate is GBP.
Graphitization in coal begins at the semi-anthracite and continues through anthracite and meta-anthracite ranks, ultimately (with the right conditions) leading to semi-graphite and graphite (
Li et al., 2018). While graphitization
sensu stricto consists mostly of polymerizations and structural rearrangement of the aromatic skeleton toward the thermodynamically stable ABAB layered sequence of graphite (
Buseck and Beyssac, 2014); the transformation from semi-anthracite to semi-graphite is also called carbonization. An empirical plot of the D1-FWHM versus R1 (Fig.10) was used to discriminate the carbonization and graphitization. The components in graphitization usually have an R1 value < 1.5 and D1-FWHM value < 50 cm
−1. The crystalline tar, mosaic thermoplastic maceral, flake graphite, and crystalline aggregates are all in graphitization (Fig.9).
4.5 Coal natural graphitization
Based on the bulk characterization, morphological feature, and micro-Raman data. It can be inferred there may be three kinds of natural graphitization phases, namely in situ solid-state transformation (solid-phase), gas-phase (pyrolytic carbon conversion), and liquid-phase (crystalline tar transformation). The in situ solid-state transformation refers to the macerals transformation without migration; the macerals were heated by magma and massive gas precipitation (volatile) thus presents a mosaic structure (Fig.6(b)), similar to that of natural coke. These mosaic structures are usually found in the meta-anthracite. With graphitization, the granular structure within the maceral becomes dense thus forming the matrix graphite. The pyrolytic carbon are the deposits formed on a hot substrate by the dehydrogenation of a gaseous hydrocarbon. Pyrolytic carbon was formed due to the chemical cracking of volatiles produced by the thermally altered macerals. In comparison with the pyrolytic carbon in laboratories, this pyrolytic carbon in natural graphitization also shows small spheres but have a much smaller size (Fig.6(e)–Fig.6(f)) (20–40 nm) than the previous reports (1–10 μm) under SEM. Under the optical microscope, the pyrolytic carbon lies in the edge of macerals (Fig.7(a)), matrix graphite or fill in the cell cavities, exhibiting a shell shape. The crystallin tar were found in the meta-anthracite and semi-graphite. They usually filled in fracture and void cavities (Fig.6(b)–Fig.6(d), and Fig.7(b)). The macerals were altered thus present a flow structure. Coal tar precipitates and crystallizes in the cavities.
5 Conclusions
Based on the bulk characterization (geochemistry, XRD, and powder Raman spectroscopy), in Xinhua, the d002 decreases while the La, Lc, and Nave increase as the distance to granitic pluton decrease. However, in a region with a big granitic mass that can provide sufficient heat, the distance impacts slightly. A further decrease of the distance results only in a minor decrease of the d002 value. However, the d002 decrease is not continuous due to shear stress. Per the d002 data, the metamorphic degree of the CDNG sample in the Panshi region is the highest. After entering graphitization, R2 and AI have a better correlation with d002.
The thermoplastic vitrinite, devolatilized vitrinite, and some “normal” macerals were present on the anthracite. While the crystalline tar, pyrolytic carbon, and mosaic structure were present on the meta-anthracite. In semi-graphite and CDNG, the matrix graphite, flake graphite, and crystalline aggregates are the newly formed MDCs. The thermoplastic vitrinite exhibit a weak optical anisotropy while the devolatilized shows a reticular structure, the “normal” macerals were composed of some desmocollinite. The crystalline tar shows a fluid state in some void cavities, the pyrolytic carbon lies in the edges of the brecciated fragments, while the mosaic structure lacks a clear outline between other macerals or MDCs. The matrix graphite was present on the surface of lump graphite and takes the dominant position of all MDCs. The flake graphite is distributed in the matrix graphite fractures along with the crystalline aggregates. Based on the micro-Raman pattern and quantified data. In anthracite, the D1, D3, D4, and G peaks were present, while the D3 peak disappears in thermoplastic vitrinite. In the devolatilized vitrinite, the D4 peak disappear while the D2 peak occurs in the shoulder of G peak. The pyrolytic carbons, and crystalline tar present a similar Raman pattern but different Raman parameters in the meta-anthracite sample. Per the Raman parameter, the pyrolytic carbon shows the highest structural ordering degree among the three kinds of particles. The crystalline aggregates, matrix graphite, flake graphite along with some Pyrolytic carbon, and CT were found in CDNG. The crystalline aggregates show the highest structural ordering degree with G-FWHM of about 20 cm−1, R2 < 0.5. The flake graphite is less ordered with its G-FWHM of about 28 cm−1, 0.5 < R2 < 1, but a larger size up to 50 µm. Per the micro-Raman data and direct observation, the flake graphite and crystalline are GEP and GBP, respectively.
Based on these features, three kinds of graphitization approaches can be summarized: the in situ solid-state transformation, pyrolytic carbon conversion, and crystalline tar transformation. The in situ solid state transformation refers to the thermoplastic mosaic macerals to the matrix graphite, the pyrolytic carbon and crystalline tar conversion refer to the gas-phase and liquid-phase transformation.
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