Effects of Rapid Igneous Intrusion Heating on the Geochemistry, Petrography, and Microcrystalline Structure of Coals from Huainan, China.

Igneous intrusion into coal-bearing strata may change the geochemical, petrographic, and microcrystalline structural characteristics of coal. Here, a series of coal samples affected by igneous intrusion were analyzed by petrography, geochemistry, and X-ray diffraction. In addition, the trend observed in altered coal with normal burial maturity is compared to evaluate whether the intrusive coal follows another maturity path. A petrographic analysis shows that the R 0 value increased rapidly and lost the ability to distinguish liptinite. Pyrolytic carbon and isotropic and anisotropic coke with a fine-grained circular mosaic structure are formed at the intrusion. Moreover, the degree of structural order of coal samples increases in an approach to the intrusion. There are transition phases with different structural orders due to different degrees of metamorphism. Petrographic and geochemical data indicate that intrusive coals may follow a maturation pathway other than that from normal burial maturation, which may be related to the rapid geological thermal event related to the intrusion. However, the results of XRD data suggest that the microcrystalline structure of igneous intrusion coals is consistent with a growth in the trend of normal burial. This study of geochemical petrography and microcrystalline structure of surrounding coal seams by rapid intrusive heating of igneous intrusions not only greatly improves the natural coke industrial utilization but also provides an important theoretical basis for the generation and enrichment of coalbed methane in igneous thermal abnormal coal reservoirs.

Introduction

Igneous intrusions into coal seams are relatively widely distributed and may significantly alter the geochemical and microcrystalline structure characteristics of coal[1−9] and change the mineral composition within the heating aureole around the intrusions.[6,10,11] In addition, they may possibly enhance the coalbed methane production and improve reservoir characteristics.[9,12,13] The alteration of the physical and chemical effects of igneous intrusion on coal is mainly associated with the intrusion temperature, which has been described around the world.[14,15] However, the changes in the alteration zone may be significantly differently affected by many factors such as temperature and heating time on coal kinetics.[16−18] The influence of igneous intrusions on coal seam properties and chemical composition have been extensively studied by many scholars.[1,3,10,13,14,19−24] However, it has rarely been mentioned in previous papers that the magma intrudes into low-rank coal seam and that there is a difference in the thermal maturity pathway of coal under the influence of magma and normal burial.[3,20,25,26] Thus, the maturation path difference between the two has not yet been fully investigated, possibly due to the intrusions by magma of a series of coal samples in a low-rank coal seam being difficult to obtain. A large igneous intrusion scale was discovered during commercial mining in Zhuxianzhuang Coal Mine in Huainan, Anhui Province, China, and especially the intrusion of a coal seam was at the stage of low rank. Therefore, this is an ideal location to investigate the differences in maturity pathways. The study of properties of the surrounding coal seams by rapid intrusive heating of igneous intrusions not only greatly improves the industrial utilization value of natural coke but also provides an important theoretical basis for the generation and enrichment of coalbed methane in an igneous thermal abnormal coal reservoir.

Microcrystalline graphite formed in the metamorphic zone is related to the igneous heat source around graphite, semigraphite, meta-anthracite, anthracite, and low-rank coal zones.[27] Generally, high-rank coal is composed of polyaromatic layers produced by cross-linking of aliphatic or ether groups.[28] This is accompanied by the generation and release of light hydrocarbons, aromatic ring condensation, and an increase in the aromatic layer stacking in the process of coal maturation.[29] The chemical structure and macerals of coal are extremely sensitive to changes in temperature and pressure. High temperature and pressure may promote the development of coal to coking and graphitization. Especially high temperature and pressure geological activities, such as igneous intrusion or tectonic activities, can provide coal with sufficiently high activation energy in comparison to normal geothermal gradients. Laboratory tests have shown that natural coke can be formed in the presence of stress at temperatures greater than 300 °C.

Coal macerals affected by igneous intrusion usually clearly show an increase in vitrinite reflectance, carbon content, and fixed carbon, and thermally metamorphic structures such as devolatilization vacuoles, coke texture, and pyrolytic carbon are formed in the alteration aureole. While the volatile hydrogen and nitrogen contents are reduced, the ability to recognize liptinite is also reduced.[2−5,12,14,21,30] There has been a great deal of previous research on the natural coke influenced by igneous intrusion. However, it is rare to find a systematic description of coal body changes and the geochemical variations of coal from low-rank coal to coke in the literature. To help improve the understanding of coal seam changes caused by intrusion events, in this study the changes in coke chemical composition and the evolution of coal macromolecular structure under magmatism are mainly studied through optical microscopy and proximate, elemental, and geochemical analyses.

Procedures

Sampling

Freshly exposed and unweathered coal samples in the underground mine were found just days after the discovery of igneous intrusions during mining activities. The sampling position started from the contact position of the intrusion and coal. Samples were taken every 0.5 m within the first 6.3 m, and the sample numbers were denoted TRJ-1–TRJ-7 and 44-1–44-8 according to the distance from the igneous intrusion. Among them, a few samples such as 44-1 and 44-2 were not sampled at an exact interval of 0.5 m because the location of coals was not easy to obtain. After that, samples were taken every 1 m between 6.3 and 9.3 m, with sampling numbers 44-9–44-11 respectively. When the distance was greater than 9.3 m, coals were sampled every 2 m, and the sampling numbers are 44-12–44-14 (Figure ). In addition, MD-1 and MD-5 samples unaffected by contact metamorphism were collected to gather background information. The magmatic intrusion thickness cannot be measured in an underground mine, but on the basis of data from the mine, the total sampling distance is approximately 1.5 times the thickness of the igneous intrusion.

Figure 1

(a) Location of igneous intrusion and sampled coals in Huainan. (b) Schematic diagram of samples distribution near the intrusion. R0 values in coal are indicated for some samples.

Analytical Procedures

Proximate experiments and an ultimate analysis have been demonstrated to be the basic methods to investigate the chemical analysis for coals. A proximate analysis (volatile matter (Vdaf),fixed carbon (FCdaf), ash (A) and moisture (Mad)), an ultimate analysis (C, H, O, and N), and total sulfur content analyses were performed on the coal samples at the Jiangsu Geology and Mineral Design and Research Institute in China that were both unaffected and affected by igneous intrusions according to standard ASTM methodologies (ASTM standard D3172-13, 2013; ASTM standard D4239-14, 2014; ASTM standard D5373-14, 2014). Samples were crushed to <80 mesh for proximate analyses and to <200 mesh for ultimate and total sulfur content analyses.

Mean random reflectance (Rr) analyses were conducted on vitrinite or metamorphic vitrinite using a Leica DM2500P reflected-light microscope and J&M MSP200 vitrinite reflectance hardware and software at the Key Laboratory of Coalbed Methane Resource and Reservoir Formation on Process, China University of Mining and Technology. A minimum of 100 vitrinite measurement points was collected through a 360° rotating microscope stage for all samples, and the average value was obtained. Yttrium–aluminum–garnet and strontium–titanate were used for the standard values of reflectivity measurements, and the R2 values of each standardized operation of coal samples were >0.98. The petrographic composition of samples, including vitrinite, liptinite, isotropic coke, and anisotropic coke, were determined by counting 500 measurement points on each of two pellets using a Zeiss Universal reflected light microscope and a Swift stage point counter (under nonpolarized light). Photomicrographs were taken with a Zeiss Universal reflected light microscope equipped with an antiflex 50× objective lens and a 20× objective lens. All petrographic analyses were conducted at the Key Laboratory of Coalbed Methane Resource and Reservoir Formation on Process, China University of Mining and Technology.

Samples were pulverized to a fine powder of <200 mesh using a mortar and pestle and then sprinkled on a quartz background sample holder for XRD experiments. The diffractogram information was obtained on a D8 ADVANCE (Bruker) powder diffractometer. The Cu Kα X-ray source was radiation produced at I = 30 mA and U = 40 kV. Diffraction data were collected with a step length of 0.02° 2θ with a scan step size of 100 s from 3 to 70° 2θ. Peakfit software was used to deconvolute the diffractograms of the (002) peak in the range of 15–32° and the (100) peak in the range of 40–50°. The asymmetric (002) band can be divided into two parts, with sample 44-10 in Figure as an example. The broad band in the range of 20° on the left can be attributed to a highly disordered material of amorphous carbon.[29] The narrow band in the range of 26° on the right is crystallite carbon.[30,31] Here, the mean interlayer spacings (d002) were determined from the (002) peak position by applying the Bragg equation. The crystallite size parameters of crystallite height (Lc) and crystallite width (La) were calculated by measuring the fwhm values of the (002) and (100) peaks, respectively, using the Scherrer equation, with values of K = 0.9 and 1.84 for Lc and La, respectively.[33] Moreover, the average stacking number of aromatic layers was calculated by ⟨N⟩ = Lc/d002, from the procedure by Laggoun-Defarge et al.[34] The relationship DOG = (3.440 – d002)/(3.440 – 3.354) was used to determine the graphitization degree.[35] The probability p of random orientation between any two adjacent layers is given by d002 = 3.354 + 0.086p, which is based on the assumption that the nongraphite carbon interlayer spacing is 3.440 Å and 0.086 Å is the difference between the interlayer spacing values of graphite carbon (3.354 Å) and nongraphite carbon (3.440 Å).[36] The broadening of diffraction peaks due to instrumental factors was corrected by a silicon standard. Furthermore, the XRD patterns exhibit wide asymmetric (002) and (100) bands.

Figure 2

Schematic diagram of sample 44-10 peak fitting.

Results and Discussion

Geochemistry of Coked Coal

Due to the coal seam being altered to a certain extent by intrusion, the properties of coal were related to the distance from the igneous intrusion. The coal rank for the Huaibei (No. 8) coalbed is highly volatile bituminous. The original, unaltered, background reflectance levels for coals range between 0.66% and 0.72% (average 0.69%; Table ). However, the vitrinite reflectance of sample 44-14 at the farthest distance location from the intrusion in the transect is 1.60% (Table and Figure ). Closer to the intrusion, the value of Rm increases dramatically from 1.60% for the sample 44-14 to 5.73% in contact with the intrusion, corresponding to anthracite. These R0 data also indicate that the alteration halo is confined to 15.3 m from the intrusion. Figure shows a sharp decrease in vitrinite reflectance with the distance from the dike intrusion and displays an obvious concave-up pattern. The dike contact point vitrinite reflectance values vary with each contact point, probably due to local variations in the degree of convective heat dissipation.

Table 1

Vitrinite Reflectance and Proximate Analyses for Coal Samples Affected and Unaffected by Magma Collected from the Zhuxianzhuang Coal Mine

sample	dist (cm)	R0 (%)	Ad (wt %)	Mad (wt %)	Vdaf (wt %)	FCd (wt %)
TRJ-1	50	5.73	18.36	0.45	4.98	77.57
TRJ-2	100	5.40	30.35	0.84	12.54	60.91
TRJ-3	150	5.29	24.42	1.30	7.41	69.98
TRJ-4	200	5.20	21.26	1.49	7.25	73.04
TRJ-5	250	4.60	29.37	2.49	9.13	64.18
TRJ-6	300	4.15	21.42	3.28	5.76	74.05
TRJ-7	350	4.22	35.43	1.91	8.97	58.78
44-1	370	4.01	20.50	2.14	6.79	74.11
44-2	390	3.86	17.47	2.00	6.47	77.19
44-3	410	3.56	17.94	1.56	6.29	76.90
44-4	430	3.40	18.03	1.41	6.74	76.44
44-5	490	3.16	17.22	2.30	6.75	77.20
44-6	530	3.15	14.51	2.58	5.40	80.87
44-7	580	3.10	20.45	1.48	9.34	72.12
44-8	630	2.91	28.21	1.64	8.48	65.70
44-9	730	2.91	30.48	1.78	9.75	62.74
44-10	830	2.91	17.32	1.60	6.81	77.05
44-11	930	2.56	23.98	1.58	8.83	69.31
44-12	1130	2.18	19.39	1.20	7.01	74.96
44-13	1330	1.80	33.37	1.64	10.12	59.89
44-14	1530	1.60	19.71	1.32	16.24	67.25
MD-1	unaltered coal	0.66	7.76	1.60	28.78	65.69
MD-5	unaltered coal	0.72	4.43	1.70	30.74	66.20

Figure 3

Variation of Rm with distance from the magmatic intrusion.

The proximate analyses (Mad, Ad, Vdaf, and FCd) of the coal samples approaching the intrusion are given in Table . Some of these parameters show the expected variations. The most obvious change in volatile matter content overall decreases from 16.24% to 4.98% close to the intrusion (Table and Figure ). In comparison with the unaltered coal with an average volatile matter content of 29.76%, the decrease is large. The fixed carbon (dry) content ranges from approximately 59% to over 80% (Table and Figure ), but it increased significantly in comparison to the unaltered coal sample with an average fixed carbon (dry) content of 65.96%. In addition, the moisture content overall initially increases from 1.32% to 3.28% and then decreases to 0.45% as the intrusion is approached (Table ). However, the variation in ash (dry) is less pronounced, ranging from <15% up to 35% in this transect (Table ). This is a significant increase in compariosn to the average of 6% for the unaltered coal sample. The reason for the increase in ash yield percent is that the carbonate formed by the gradual cooling of the hydrothermal fluid after the igneous intrusion fills the cell cavity.

Figure 4

Variation in proximate analyses (Mad, Ad, Vdaf, and FCd) with the distance from the intrusive contact for coal samples. The red scatter diagram represents the relationship between Mad and distance, the orange scatter diagram represents the relationship between Ad and distance, the blue scatter diagram represents the relationship between Vdaf and distance, and the green scatter diagram represents the relationship between FCd and distance.

Petrography of Coked Coal

Coals in this transect show an increase in carbon content and a decrease in hydrogen, nitrogen, oxygen, and sulfur content near the intrusion. The carbon content increases from 86% to 94%, the hydrogen content decreases from 4.22% to 1.23%, the nitrogen content decreases slightly overall from 1.72% to 0.9%, the oxygen content decreases from about 7% to about 2%, and the sulfur content shows a minor decrease from 0.36% to 0.06%. Here, there is an abnormally high sulfur point in sample 44-12 (Table and Figure ). The decrease in sulfur content close to the intrusion is related to thermal elimination of the organic sulfur and possibly sulfate decomposition by the thermochemical reduction of sulfate.[9] Similarly, the atomic ratios decrease near the intrusion in the transect (Figure ). However, sample 44-7 shows abnormally low element and atomic values, but it has higher carbon and sulfur values in comparison to the other samples.

Table 2

Ultimate Analyses and Atomic Ratio Coal Samples Affected and Unaffected by Magma Collected from the Zhuxianzhuang Coal Mine

sample	Cdaf (%)	Hdaf (%)	Ndaf (%)	St,d (%)	Odaf (%)	O/C	H/C	N/C
TRJ-1	94.23	1.23	0.91	0.06	3.56	0.028	0.157	0.008
TRJ-2	87.81	1.23	0.96	0.12	9.25	0.079	0.168	0.009
TRJ-3	92.57	1.26	1.15	0.22	4.72	0.038	0.163	0.011
TRJ-4	93.58	1.25	1.16	0.25	3.67	0.029	0.160	0.011
TRJ-5	88.31	1.48	1.48	0.26	7.97	0.068	0.201	0.014
TRJ-6	93.70	1.63	1.68	0.26	2.66	0.021	0.209	0.015
TRJ-7	90.80	1.83	1.52	0.32	5.37	0.044	0.242	0.014
44-1	94.22	1.95	1.41	0.20	2.16	0.017	0.248	0.013
44-2	93.50	2.04	1.35	0.24	2.82	0.023	0.262	0.012
44-3	93.41	2.43	1.39	0.30	2.40	0.019	0.312	0.013
44-4	91.52	2.61	1.56	0.32	3.91	0.032	0.342	0.015
44-5	91.59	2.08	1.48	0.35	4.43	0.036	0.273	0.014
44-6	93.78	1.87	1.46	0.38	2.44	0.020	0.239	0.013
44-7	95.44	2.36	1.58	0.29	0.24	0.002	0.297	0.014
44-8	88.91	2.59	1.74	0.26	6.39	0.054	0.350	0.017
44-9	89.79	2.64	1.79	0.26	5.40	0.045	0.353	0.017
44-10	90.98	2.54	1.45	0.36	4.60	0.038	0.335	0.014
44-11	89.75	2.78	1.60	0.33	5.44	0.045	0.372	0.015
44-12	90.21	2.79	1.38	0.54	4.96	0.041	0.371	0.013
44-13	86.79	2.72	1.74	0.37	8.21	0.071	0.376	0.017
44-14	86.43	4.22	1.72	0.36	7.18	0.062	0.586	0.017
MD-1	84.11	4.56	1.54	0.41	9.34	0.083	0.651	0.016
MD-5	82.78	4.70	1.64	0.26	10.60	0.096	0.681	0.017

Figure 5

Variation in composition of C, H, O, N, and S with the distance from the intrusive contact for coal samples. The red line chart represents the relationship between C and distance, the orange line chart represents the relationship between H and distance, the blue line chart represents the relationship between O and distance, the green line chart represents the relationship between N and distance, and the brown line chart represents the relationship between S and distance.

Figure 6

Variation in atomic ratios with the distance from the intrusion.

Unaltered coal samples MD-1 and MD-5 were collected from the No. 8 coal seam in addition to the altered coal samples, both of which are at the stage of low rank with R0 values of 0.66–0.72% (Table ). Petrographically, the unaltered coal samples of No. 8 are mainly inertinite (mostly semifusinite and fusinite) macerals (51–51.4%), followed by vitrinite (collotelinite and collodeternite) ranging from 37.9% to 40%, and a small amount of macerals part of liptinite ranging between 8.4% and 10.1%. Liptinite is represented as being dominated by sporinite and rarely by cutinite. As shown in Figure a, the photomicrographs of MD-1 coal sample indicate a relatively pure sample without any thermal alteration structure. However, coal samples in close vicinity to the igneous intrusion show obvious thermal alteration characteristics: that is, the liptinite macerals disappear and vitrinite gradually becomes coked. Due to the larger range of igneous intrusion, the vitrinite reflectance of the 44-14 (R0 = 1.6%) coal sample 15 m away from the igneous intrusion is higher those that of unaltered coal samples MD-1and MD-5 (R0 = 0.66% and R0 = 0.72%, respectively), suggesting that the sample collected at the farthest distance from the intrusion is also within the range of the alteration halo. Since the vitrinite reflectance of the 44-14 coal sample has exceeded the rank at which liptinite is visible and liptinite macerals cannot be distinguished within the distance of the whole alteration halo, the fluorescence disappeared.[37] Near the intrusion, vitrinite appears to be increasingly coked and isotropic coke can be seen in sample 44-8 with the vitrinite reflectance R0 = 2.91% (Figure b). Most of the coke may be described as a fine-grained circular mosaic structure (Figure b), on the basis of a comparison with commercial coke textures described by Crelling.[38] However, the distorted maceral component arrangement of sample 44-14 (Figure c) may be caused by local strain near the boundary. Degradofusinite and pyrofusinite are the main forms of the inertinite macerals in this type of sample. Degradofusinite can be easily identified in the least metamorphosed sample, 44-12 (Figure d), but with an increase in metamorphism, the degradofusinite becomes more difficult to identify. Striped pyrofusinite (Figure e) and the well-preserved regular rectangular cell structure pyrofusinite appeared in sample 44-2. Close to the intrusion, the structure of pyrofusinite may be modified, changing from a striped shape to circular cell shapes of varying sizes and regular rectangles. In addition, devolatilization vacuoles occur in sample 44-14 with a distance of 16 m. On approach to the intrusion, the devolatilization vacuoles become more prevalent and obvious, as shown in Figure i of sample TRJ-1. Another distinct difference in petrography is the formation of pyrolytic carbon. The morphology and relative abundance of pyrolytic carbon depend on the distance from the dike intrusion. Here, pyrolytic carbon merely appears in samples from TRJ-1 to TRJ-4 within a distance of 2 m from the intrusion. The isotropic pyrolytic carbon appears in sample TRJ-4, as shown in Figure g. However, large flakes of laminated anisotropic pyrolytic carbon appeared in sample TRJ-1, indicating a higher formation temperature (Figure h). Notable concentrations of pyrolytic carbon increased on approach to the intrusion.

Figure 7

Micrographs of petrography compositions of an unaltered sample (a) and thermal samples (b–i) near the intrusion.

Petrographic and Geochemical Variations Associated with the Intrusion

Zhuxianzhuang coal mine No. 8 coal in Huainan with igneous intrusion petrographic data show that vitrinite is gradually coked, the liptinite disappeared in coal samples with R0 > 1.35%, and degradofusinite and pyrofusinite are the main forms of the inertinite. New components formed during the coking stage lead to strong heterogeneity of the coal samples. In comparison with the structure formed by commercial coke, a typical fine-grained circular mosaic structure and devolatilization vacuoles appear near the igneous intrusion, indicating that the temperature is close to 500 °C.[3,21] In addition to differences in macerals composition, the reflectance values of the coal samples near the intrusion increase significantly and show a concave upward trend as reported for the Illinois Basin[39] and the Raton Basin.[12] The influence of an intrusion on the surrounding coal was determined by the intrusion type, temperature, and thickness. The background coal seam is highly volatile, and the Vdaf value decreases significantly adjacent to the intrusion. Similarly, the appearance of devolatilization vacuoles indicates that the volatiles are released and formed by pyrolytic carbon, which is the condensation stage of aromatization residual products.[21]

The relationship between volatile matter and R0 (Figure ) shows that no matter whether magma intrudes into the coal seam or there is normal burial coal the coal seam Vdaf value is affected, and Vdaf decreases with an increase in coal rank. This indicates the loss of volatile products, which may be the result of residual product condensation caused by aromatization.[37] Most importantly, Figure shows a significantly different relationship between the intrusision samples and normally buried coal in comparison with the data for German coals,[40] Illinois Basin coals,[32] and graphitized coal of Hunan, China.[39] It can be clearly observed from Figure that the volatile content of intrusive coals in the range of R0 < 2.5% is within or below the normal burial coal range, while when R0 > 2.5% the volatile content is significantly higher than that for the normal burial coals. It has been suggested that a volatile content of coal being higher than that for the normal burial process may be caused by the volatile content being trapped in the coal alteration zone.[3,14] In summary, this shows that igneous intrusion may affect the volatile content of coal samples in the range of R0 > 2.5% to the largest extent but has no obvious effect on coal samples with R0 < 2.5%. Rimmer et al.[3] believed that the maturation pathway of coal with a rapid high-temperature igneous intrusion may be different from that of normal buried coal. Similarly, the relationship between C content and R0 for coal samples (Figure ) shows that the C content data of most of the coal samples in this study are concentrated in the normal burial coalification curve of Teichmüller et al.,[40] and the contents of only a few of the coal samples are higher or lower than that of the normal burial. Figure also shows two other suites of igneous intrusion data in the Tanjung Enim area of Sumatra[41] and Illinois Basin[21] (Rahman et al.). The data of Sumatra coals are within or above the normal trend, while the data of Illinois Basin are within or below the normal trend. This result is consistent with the conclusion drawn by Amijaya and Littke[41] that the intrusive coal samples follow maturation trends, which is similar to the trend observed in normal burial diagenesis.

Figure 8

Variation in the value of VM (%) with R0. The burial samples of Teichmüller and the intrusion samples studied by Presswood et al. are adapted with permission from 10.1016/j.coal.2016.08.022, the samples of Rimmer et al. are dapted with permission from 10.1016/j.coal.2009.06.002, and the samples of Li et al. are adapted with permission from 10.1016/j.coal.2018.06.009 and are shown for comparison.

Figure 9

Variation in the value of C content (%) with R0. The burial coalification trends of Teichmüller et al. is adapted with permission from 10.1016/S0070-4571(08)71074-4, those of intrusive samples studied by Rahman et al. aredapted with permission from 10.1016/j.coal.2014.06.020, and those Amijaya, Littke et al. are adapted with permission from 10.1016/j.coal.2005.07.008 and are shown for comparison.

The evolution trend diagram of elemental composition (Figure ) shows that coal samples affected by intrusion may have maturation pathways different from those of the normal buried coals.[42] In the Seylor plot, basically all coal sample data points deviate from those of the normal burial coals. For a given C content, all coals seem to have a lower H content, which may make it easier to remove the hydrogen element under the condition of rapid heating of the igneous intrusion. For the Illinois Basin, the data points of coal samples are basically within the range of normal burial coal at low ranks, which deviate from those of normal burial coal at high ranks. However, most of the Sumatra data are concentrated on normal burial coal, which implies a different maturation pathway for these coals. Rahman and Rimmer[21] believe that the maturation pathway is controlled by conditions related to the intrusive events, such as the size of the intrusive body, heating rate, continuous heating time, and other conditions.

Figure 10

Distribution diagram of percentages of H and C for coal samples. The intrusion coals for our samples are compared with those Rahman et al. and are adapted with permission from 10.1016/j.coal.2014.06.020 and those of Rimmer et al. and are adapted with permission from 10.1016/j.coal.2009.06.002. The normal coalification trend of Van Krevelen et al. is shown in gray.

Microstructure Changes Determined by X-ray Diffraction Analysis

An XRD experiment is one of the commonly used ways to study the structural order and graphitization degree of carbonaceous materials.[43−47] Although XRD patterns of coal samples at different locations from the magma are affected by other minerals, the XRD patterns of samples all have high intensity, as shown in Figure . XRD patterns can characterize the ordered evolution of coal samples structure close to the intrusion in detail. The XRD patterns of the samples are gradually ordered as the distance from the intrusion position decreases, and the structural evolution during the carbonization process can be easily observed (Figure ). In addition, the patterns also show the crystalline carbon of the (002) band at around 25° and of the (100) band at around 45°. The samples MD-1 and MD-5 of normal coal seams unaffected by intrusion are located at a distance greater than 15 m away from the igneous intrusion and belong to a low coal rank. Thus, their XRD patterns exhibit wide asymmetric (002) and (100) bands (Figure ). According to the XRD patterns, a high metamorphism profile such as that for the TRJ-1 sample close to the intrusion gradually became sharp and symmetrical.

Figure 11

XRD patterns of 21 coal samples affected by magma and 2 coal samples (MD-1, MD-5) unaffected by magma.

The progressive changes in XRD parameters close to igneous intrusion coal samples are shown in Table . The d002 parameters of igneous intrusion coal samples show an obvious decreasing trend, as shown in Table . The value of the 44-1 sample approaches that of Sri Lanka graphite. In addition, d002 values of the TRJ-1–TRJ-7 samples are less than 3.354 Å, indicating that the crystallization order is high and has entered the graphite stage. The average crystallite width (La) and crystallite height (Lc) of these samples are 8.17–14.89 Å and 21.89–42.01 Å, respectively (Table ). Lc increases significantly with the degree of deterioration. For the 44-12–44-14, MD-1, and MD-5 samples in Table , carbon atoms did not show a graphite order, but the 44-11–TRJ-1 samples show DOG increase on a approach to the intrusion. Furthermore, the random orientation parameters p between two adjacent aromatic layers decrease with an approach to the intrusion.

Table 3

Crystallite Structure Parameters of 21 Coal Samples Affected by Magma and 2 Coal Samples (MD-1, MD-5) Unaffected by Magmaa

sample	d002 (Å)	DOG	fwhm (002) (deg)	La (Å)	Lc (Å)	⟨N⟩	p	AI
TRJ-1	3.2972	1.660	2.320	8.17	35.22	11		0.81
TRJ-2	3.2998	1.630	1.945	8.52	42.01	13		0.78
TRJ-3	3.3148	1.456	3.090	8.23	26.44	8		0.75
TRJ-4	3.3181	1.418	2.475	9.72	33.01	10		0.74
TRJ-5	3.3219	1.374	3.422	9.94	23.87	7		0.66
TRJ-6	3.3464	1.088	2.321	7.98	35.18	11		0.67
TRJ-7	3.3467	1.085	3.031	7.86	26.93	8		0.65
44-1	3.3547	0.992	2.125	7.77	38.41	11	0.0081	0.61
44-2	3.3632	0.893	2.575	9.51	31.70	9	0.1070	0.60
44-3	3.3693	0.822	3.425	9.74	23.83	7	0.1779	0.65
44-4	3.3724	0.786	3.563	10.71	22.91	7	0.2140	0.59
44-5	3.3739	0.769	3.351	14.89	24.35	7	0.2314	0.41
44-6	3.3949	0.525	2.842	11.54	28.70	8	0.4756	0.39
44-7	3.3985	0.482	3.234	11.45	25.22	7	0.5174	0.38
44-8	3.4065	0.389	3.651	8.17	22.34	7	0.6105	0.38
44-9	3.4126	0.318	3.726	8.94	21.89	6	0.6814	0.35
44-10	3.4361	0.046	3.441	9.84	23.69	7	0.9547	0.40
44-11	3.4394	0.007	3.312	11.67	24.62	7	0.9930	0.25
44-12	3.4736		3.622	10.68	22.49	6	1.3907	0.16
44-13	3.4514		3.194	8.56	25.52	7	1.1326	0.21
44-14	3.4514		2.829	8.95	28.81	8	1.1326	0.20
MD-1	3.4574		3.468	9.56	23.50	7	1.2023	0.26
MD-5	3.5226		3.622	10.35	22.48	6	1.9605	0.26

Definitions: DOG, graphitization degree; fwhm, full width at half-maximum; Lc and La, microcrystalline structure size; ⟨N⟩, aromatic layer stacking number; p, random orientation; AI, asymmetry index.

It was previously reported that R0 was used to judge whether carbonaceous materials have entered the graphite stage. Moreover, several authors have proved that d002 has a good correlation with the metamorphism degree.[48−51] Therefore, it can also classify graphitized carbonaceous materials. There is a negative linear correlation between d002 values and R0 and R2 = 0.93, as shown in Figure . In conclusion, both parameters can be used as the measure to judge whether carbonaceous materials have entered the graphite stage. The classification of anthracite to graphite is based on the International Committee for Coal and Organic Petrology.[52] The anthracite is classified by a layer spacing of d002 > 0.34 nm, the d002 the spacing range of meta-anthracite is between 0.338 and 0.340 nm, the semigraphite d002 spacing range is 0.337–0.338 nm, and the d002 spacing of <0.337 nm belongs to the graphite zone. Several authors[45−47,51] have used the layer spacing parameter d002 as a standard representing the graphite crystal structure, which is equally applicable to highly ordered graphite coal samples formed under contact metamorphism. According to the experimental values of vitrinite reflectance in Table , MD-1 and MD-5 are both in the stage of low rank. The samples from 44-9 to 44-14 are within the anthracite range. Moreover, samples from 44-6 to 44-8 belong to the meta-anthracite category, samples 44-4 and 44-5 are in the range of semigraphite stage, and the samples from TRJ-1 to 44-3 belong to the graphite zone (Figure ).

Figure 12

Grouping of graphitized coal samples based on d002 and R0 values.

Previous studies[45−47,51] classified highly ordered graphite coal samples formed under contact metamorphism using d002 < 0.336 nm. The d002 value in the range of 0.336–0.337 nm represent a transitional stage between semigraphite and highly ordered graphite coal samples. In this study, samples 44-2 and 44-3 in the transition stage belong to the prophase of high-graphite coal. The d002 value of samples TRJ-1–44-1 is less than 0.336 nm, which belongs to highly ordered graphite coal. Moreover, the d002 values of samples 44-4–44-14 are all greater than 0.337 nm, and their 002 reflection bands are more asymmetrical (Figure ), which are classified as ordered graphite coal. However, the sample MD-5 obviously deviates from the correlation (Figure b), which is far from the graphite stage, probably caused by the highly inhomogeneous crystal structure. The asymmetry of the 002 reflection band in the XRD pattern in Figure is precisely caused by structural defects in the turbine layer structure and crystal structure. Previous research defined this asymmetry as the AI value, that is, the ratio of the left fwhm and the right fwhm of the 002 reflection band, as shown in Figure a. The relationship between AI and d002 shows a good negative correlation in the coal samples affected by igneous intrusion (R2 = 0.87) except for sample MD-5. However, in the highly ordered graphite coal samples TRJ-1–44-3, the AI values have not yet fully reached 1, suggesting that the 002 reflection bands is still slightly asymmetric. Probably there is a dislocation along the parallel stacking of the aromatic layers as previously reported by Zhang et al.[53] The evolution of graphitized crystallite of coal samples under magmatic contact metamorphism may be consistent with the four-stage evolution process of graphitization proposed by Oberlin.[54] In the first stage, only a single basic structural unit exists in the microstructure of carbonaceous materials. BSU refers to a minimum structural unit consisting of five to six pairs of stacking layers. In the second stage, the BSUs are interconnected to form distorted columns. Subsequently, in the third stage, adjacent columns merge into distorted wrinkled layers. The distorted layers harden and become flat and perfect in the final fourth stage. In addition, the growth trend of the other XRD parameters La, Lc, N, and p affected by the igneous intrusion agrees well with the growth trend of graphite crystallite size during the normal buried graphitization process.

Figure 13

(a) Illustration of AI calculations using the fwhms of the (002) reflection, where r1 and r2 are the right fwhm and left fwhm, respectively. (b) Correlation between AI values and d002 parameters of XRD (data from Table ).

However, apart from structural changes, there is a lack of good correlation between crystallographic parameters (XRD) and chemical parameters in these coal samples, as shown in Figure . In fact, if samples with an H/C atomic ratio larger than 0.35 are not included in the correlation, a good correlation between d002 and H/C can be observed (R2 = 0.83, Figure ). This lack of correlation was also reported by Marques et al.,[55] Li et al.[39] and Marques et al.[56] It was considered that some coal samples have low H/C but have almost no graphite order. This was also different from the results reported in this study and may be related to the environment of the sample or the nature of the sample itself.

Figure 14

Correlation between d002 and H/C of coal samples affected by igneous intrusion.

Conclusions

Geochemical and microcrystalline structures of coal in direct contact with the intrusions in the Zhuxianzhuang mining area of Huainan undergo unique specific changes on approach to the intrusion. On approach to the intrusion, R0 values showed a significant upward concave-up growth trend, increasing from the background reflectance levels for coals ranging between 0.66% and about 5.73%. The liptinite disappears when the R0 value increases more than 1.35%. In addition, the contents of FC and C increase and the percentages of H, O, N, and S decrease, but the change trend of geochemical data is not obvious in comparison with the normal buried coalification trend.

The relationship trend between VM (%) and R0 indicates that an igneous intrusion has a greater effect on the volatile content with R0 > 2.5% in comparison to normal burial coal. The distribution of carbon content with R0 is basically consistent with that of normal burial coal. However, the element distribution diagram shows that the evolution of elements is different from that of normal burial coal.

The XRD parameter d002 can clearly classify the degree of ordering of graphite coal samples formed under contact metamorphism. The degree of structural order of coal samples increases on approach to the intrusion. In addition, there are transitional stages with different structural orders. The AI values of the XRD pattern (002) reflections have a good correlation with d002, and asymmetry still exists even in highly ordered coal samples. Moreover, the growth trend of the other XRD parameters La, Lc, N, and p is consistent with the growth trend of normal graphite crystallite size, indicating that the magmatic rapid heating coal crystallite structure is consistent with the normal trend of normal burial coal. The study of the properties of surrounding coal by rapid intrusive heating of igneous intrusions not only greatly improves the natural coke industrial utilization but also provides an important theoretical basis for the generation and enrichment of coalbed methane in an igneous thermal abnormal coal reservoir.