Mineralogical and Geochemical Characteristics of Trace Elements in the Yongdingzhuang Mine , Datong Coalfield , Shanxi Province , China

Fifteen samples of No. 4 coal from the Yongdingzhuang Mine in Datong Coalfield were tested for their elemental compositions, modes of occurrence, and mineralogical compositions, using X-ray powder diffraction, X-ray fluorescence spectrometry, inductively coupled plasma mass spectrometry, and scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer. The samples have low sulfur content (0.63%). The major minerals are kaolinite and quartz, followed by pyrite and anatase. Compared with averages for the Chinese coals, the percentages of SiO2 (15.11%), TiO2 (0.7%), and Al2O3 (10.39%) are much higher. In No. 4 coals, Li (62.81 μg/g), Be (6.94 μg/g), Zr (235 μg/g), Ga (17.04 μg/g), F (165.53 μg/g), Tl (1.93 μg/g), and Hg (0.34 μg/g) are some potentially valuable and toxic trace elements with higher concentrations than Chinese coals and World hard coals. Lithium and F mainly have kaolinite associations. With the exception of kaolinite, Li, and F also partly occur in anatase, gorceixite and goyazite. Beryllium largely occurs in anatase; gallium is mainly associated with kaolinite and to a lesser extent, with gorceixite and goyazite; zirconium is associated with kaolinite, gorceixite and goyazite; and thallium and Hg occur in in pyrite. Potentially valuable elements (including Al, Li, Ga, and Zr) might be recovered as value-added byproducts from coal ash. Toxic elements (e.g., Be, F, Tl, and Hg) might have potential adverse effects to the environment and human health during coal processing. In addition, the distribution patterns of rare earth elements and yttrium (REY) indicate that the REY in No. 4 coals originated from the granite of Yinshan Oldland, and natural waters or hydrothermal solutions that may circulate in coal basins.


Introduction
China is the major consumer of coal worldwide, accounting for 50% of the total annual world coal consumption [1] and coal utilization will continue to play a leading role in energy consumption in the future [2].Due to potentially hazardous compositions of coal, both the environment and human health are tremendously threatened by the pollution generated through both coal production and utilization [3,4].In recent decades, an increasing number of studies have focused on relevant studies [5][6][7], detailing the abundances, elemental compositions, and occurrence modes of various trace elements in coal, including toxic (e.g., F, As, Hg, and Pb) [8,9] and valuable elements (e.g., Ge, Ga, Al, Nb, Zr, and rare earth elements) [10][11][12].The trace elements can provide not only geologic information about depositional conditions of coal, coal-bearing sequences, and regional tectonic history [13][14][15], but also practical information for potential industrial utilization of rare metals (e.g., Ge, Ga, and Al) which can be industrially utilized if they are enriched to the levels exceeding economic grades [16,17].
The Datong Coalfield is one of the largest coal producers of China and is located in northern Shanxi Province, which is adjacent to Shaanxi Province, Hebei Province, and the Inner Mongolia Autonomous Region (Figure 1).Many studies have been conducted on the geochemical characteristics of the Ordos Basin and the Ningwu Basin, which are adjacent to the Datong Coalfield [18,19].However, few previous studies have been carried out about the geochemical characteristics on the coals in the Datong Coalfield.This paper primarily reports the mineralogical compositions, geochemical characteristics and origin of the trace elements from the Yongdingzhuang Mine in the Datong Coalfield.
In order to investigate the potential for industrial utilization of the valuable elements and potential threats of the toxic elements, this paper also focuses on the concentrations and modes of occurrence of these valuable and toxic elements [20].
Minerals 2018, 8, x FOR PEER REVIEW 2 of 23 Nb, Zr, and rare earth elements) [10][11][12].The trace elements can provide not only geologic information about depositional conditions of coal, coal-bearing sequences, and regional tectonic history [13][14][15], but also practical information for potential industrial utilization of rare metals (e.g., Ge, Ga, and Al) which can be industrially utilized if they are enriched to the levels exceeding economic grades [16,17].
The Datong Coalfield is one of the largest coal producers of China and is located in northern Shanxi Province, which is adjacent to Shaanxi Province, Hebei Province, and the Inner Mongolia Autonomous Region (Figure 1).Many studies have been conducted on the geochemical characteristics of the Ordos Basin and the Ningwu Basin, which are adjacent to the Datong Coalfield [18,19].However, few previous studies have been carried out about the geochemical characteristics on the coals in the Datong Coalfield.This paper primarily reports the mineralogical compositions, geochemical characteristics and origin of the trace elements from the Yongdingzhuang Mine in the Datong Coalfield.In order to investigate the potential for industrial utilization of the valuable elements and potential threats of the toxic elements, this paper also focuses on the concentrations and modes of occurrence of these valuable and toxic elements [20].

Geological Setting
The Datong Coalfield is located to the south of the Yinshan Oldland and stretches 50 km long (N-S) and 30 km wide (W-E), covering a total area of 1900 km 2 [21].It is bound to the west by the Pingwang-Emaokou fault, to the east by the Lvliang mountain syncline, and to the north by the Hongtao mountain syncline.Due to Caledonian tectonic movement, the Ordovician Majiagou Formation has been extensively weathered, leading to erosion of the Upper Ordovician, the Silurian, Devonian, and the Lower Carboniferous strata.The coals in the Datong Coalfield began to accumulate sediments during the Late Palaeozoic in part of the North China Craton.Consequently, the strata were assigned to the Benxi, Taiyuan, Shanxi, Shihezi, and Shiqianfeng Formations.The faults in the northern coalfield provided channels for magma intrusion.The lamprophyre intrusions, which occurred during the Indosinian epoch in the north of the coalfield, resulted in thermal contact metamorphism and silicification between coal seams [21].
The Shanxi Formation has a total thickness of 20-80 m and is mainly composed of conglomerate, sandstone, siltstone, mudstone, and four coal seams identified as Nos. 1, 2, 3, and 4 coal, respectively, from top to bottom.The thickness of the coal seams ranges between 0.16 and 10.63 m (Figure 2).According to previous research, the Shanxi Formation was deposited in fluvial facies [22].

Geological Setting
The Datong Coalfield is located to the south of the Yinshan Oldland and stretches 50 km long (N-S) and 30 km wide (W-E), covering a total area of 1900 km 2 [21].It is bound to the west by the Pingwang-Emaokou fault, to the east by the Lvliang mountain syncline, and to the north by the Hongtao mountain syncline.Due to Caledonian tectonic movement, the Ordovician Majiagou Formation has been extensively weathered, leading to erosion of the Upper Ordovician, the Silurian, Devonian, and the Lower Carboniferous strata.The coals in the Datong Coalfield began to accumulate sediments during the Late Palaeozoic in part of the North China Craton.Consequently, the strata were assigned to the Benxi, Taiyuan, Shanxi, Shihezi, and Shiqianfeng Formations.The faults in the northern coalfield provided channels for magma intrusion.The lamprophyre intrusions, which occurred during the Indosinian epoch in the north of the coalfield, resulted in thermal contact metamorphism and silicification between coal seams [21].
The Shanxi Formation has a total thickness of 20-80 m and is mainly composed of conglomerate, sandstone, siltstone, mudstone, and four coal seams identified as Nos. 1, 2, 3, and 4 coal, respectively, from top to bottom.The thickness of the coal seams ranges between 0.16 and 10.63 m (Figure 2).According to previous research, the Shanxi Formation was deposited in fluvial facies [22].

Samples and Methods
Fifteen samples, consisting of two partings and 13 coal benches with a total thickness of 1.15 m from top to bottom, were collected from No. 4 coal in the Yongdingzhuang Mine following the Chinese Standard Method GB 482-2008 [23].The samples were cut into sections of 10 cm width and 10 cm depth and immediately stored in plastic bags to prevent contamination and oxidation.All samples were ground to 80-mesh and 200-mesh prior to geochemical analysis.
The 80-mesh samples were used to analyze both the mineralogical composition and distribution.The mineralogical composition was determined via microscopy (Leica DM 2500P microscope by Leica Microsystems, Solms, Germany).A scanning electron microscope (HITACHI UHR FE-SEM, SU8220, HITACHI, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (SEM-EDS, HITACHI, Tokyo, Japan) was used to study the mineral distribution of the coal and the distribution patterns of several elements of interest.
Low-temperature ashes were used for the 200-mesh samples and performed on an EMITECH K1050 plasma asher (Quorum, Ashford, UK), with the temperature maintained below 150 °C .X-ray diffraction (XRD, Rigaku, Tokyo, Japan) analyses on the low-temperature ashes were performed via Ni-filtered Cu-Kα radiation and a scintillation detector.XRD patterns were recorded over a 2θ interval from 10° to 70°, at a step size of 0.01°.

Samples and Methods
Fifteen samples, consisting of two partings and 13 coal benches with a total thickness of 1.15 m from top to bottom, were collected from No. 4 coal in the Yongdingzhuang Mine following the Chinese Standard Method GB 482-2008 [23].The samples were cut into sections of 10 cm width and 10 cm depth and immediately stored in plastic bags to prevent contamination and oxidation.All samples were ground to 80-mesh and 200-mesh prior to geochemical analysis.
The 80-mesh samples were used to analyze both the mineralogical composition and distribution.The mineralogical composition was determined via microscopy (Leica DM 2500P microscope by Leica Microsystems, Solms, Germany).A scanning electron microscope (HITACHI UHR FE-SEM, SU8220, HITACHI, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (SEM-EDS, HITACHI, Tokyo, Japan) was used to study the mineral distribution of the coal and the distribution patterns of several elements of interest.
Low-temperature ashes were used for the 200-mesh samples and performed on an EMITECH K1050 plasma asher (Quorum, Ashford, UK), with the temperature maintained below 150 • C. X-ray diffraction (XRD, Rigaku, Tokyo, Japan) analyses on the low-temperature ashes were performed via Ni-filtered Cu-Kα radiation and a scintillation detector.XRD patterns were recorded over a 2θ interval from 10 • to 70 • , at a step size of 0.01 • .

Proximate Analysis
Bench thickness, forms of sulfur and proximate analysis of the No. 4 coal seam of the Yongdingzhuang Mine are listed in Table 1.The No. 4 coals from Yongdingzhuang Mine are considered as a medium ash coal, with an average ash yield value of 20.76% (ranging from 9.82% to 48.67%), according to the Chinese National Standard (GB/T15224.1-2010, 2011, 10.0-20.00%for low ash coal, 20.0-30.00%for medium ash coal, and 30.0-40.00% for high ash coal) [28].
The volatile matter content of No. 4 coal seam ranges from 16.07% to 35.94%, with a mean of 27.45% suggesting that this coal is a medium volatile coal according to the MT/T 849-2000 [30] (20.01-28.00%for medium volatile coal, 28.01-37.00%for medium-high volatile coal, 37.01-50.00%for high volatile coal, and >50.01%for super high volatile coal).
In summary, the No. 4 coals of the Yongdingzhuang Mine are characterized by medium ash, low moisture, medium volatile, and low sulfur coals.

Mineralogical Composition
The mineralogical composition of the coal samples from No. 4 coal of the Yongdingzhuang Mine is primarily characterized by kaolinite and quartz and, to a lesser extent, by pyrite and anatase, based on the SEM-EDS and XRD analyses (Figure 3).The volatile matter content of No. 4 coal seam ranges from 16.07% to 35.94%, with a mean of 27.45% suggesting that this coal is a medium volatile coal according to the MT/T 849-2000 [30] (20.01-28.00%for medium volatile coal, 28.01-37.00%for medium-high volatile coal, 37.01-50.00%for high volatile coal, and >50.01%for super high volatile coal).
In summary, the No. 4 coals of the Yongdingzhuang Mine are characterized by medium ash, low moisture, medium volatile, and low sulfur coals.

Mineralogical Composition
The mineralogical composition of the coal samples from No. 4 coal of the Yongdingzhuang Mine is primarily characterized by kaolinite and quartz and, to a lesser extent, by pyrite and anatase, based on the SEM-EDS and XRD analyses (Figure 3).Clay minerals are commonly detected in coals, including kaolinite, illite, chlorite and montmorillonite.Based on both XRD and SEM-EDS, kaolinite was found to be the most abundant mineral and mainly occurs as lumps and infillings of macerals (Figure 4A,B).Two main occurrences of kaolinite have been found in samples, one as disseminated particles in collodetrinite and the other as cell-fillings in fusinite.Filling fusinite indicates that kaolinite may indicate formation via authigenic processes (Figure 4A) [32].Part of the kaolinite shows dissemination in collodetrinite, which is suggestive of a syngenetic origin (Figure 4B) [32,33].
Quartz is distributed as detrital grains in macerals throughout samples, along with vein quartz in coals (Figure 4C).Detrital grains with round edges have a terrigenous detrital origin, indicating transportation over a long distance from the sediment source region (Figure 4D) [34,35].Furthermore, epigenetic quartz predominately occurs as vein quartz.Clay minerals are commonly detected in coals, including kaolinite, illite, chlorite and montmorillonite.Based on both XRD and SEM-EDS, kaolinite was found to be the most abundant mineral and mainly occurs as lumps and infillings of macerals (Figure 4A,B).Two main occurrences of kaolinite have been found in samples, one as disseminated particles in collodetrinite and the other as cell-fillings in fusinite.Filling fusinite indicates that kaolinite may indicate formation via authigenic processes (Figure 4A) [32].Part of the kaolinite shows dissemination in collodetrinite, which is suggestive of a syngenetic origin (Figure 4B) [32,33].
Quartz is distributed as detrital grains in macerals throughout samples, along with vein quartz in coals (Figure 4C).Detrital grains with round edges have a terrigenous detrital origin, indicating transportation over a long distance from the sediment source region (Figure 4D) [34,35].Furthermore, epigenetic quartz predominately occurs as vein quartz.Pyrite mainly occurs as fracture-filling and discrete crystals in No. 4 coals.It can be deduced that discrete crystals are related to a syngenetic origin (Figure 5A) [36].However, fracture-filling pyrite was deposited from migrating solutions, after compaction of the peat into coals, suggesting an epigenetic origin (Figure 5B) [37].Anatase occurs as detrital grains in collodetrinite, indicating a syngenetic origin (Figure 5A,B) [36,38].Pyrite mainly occurs as fracture-filling and discrete crystals in No. 4 coals.It can be deduced that discrete crystals are related to a syngenetic origin (Figure 5A) [36].However, fracture-filling pyrite was deposited from migrating solutions, after compaction of the peat into coals, suggesting an epigenetic origin (Figure 5B) [37].Anatase occurs as detrital grains in collodetrinite, indicating a syngenetic origin (Figure 5A,B) [36,38].

Major Elements
The concentrations of major oxides in the No. 4 coals are listed in Table 2, including average values, LOI (loss on ignition), and concentration coefficients (CC = the ratio of an average elemental concentration in the investigated coal/the average concentrations for either world coals or Chinese coals) [38].The concentrations for major oxides are as follows: SiO 2 (11.14%),Al 2 O 3 (7.16%),TiO 2 (0.67%), Fe 2 O 3 (0.60%), MgO (0.03%), K 2 O (0.03%), CaO (0.10%), P 2 O 5 (0.01%), Na 2 O (0.0028%), and MnO (0.0008%).Compared with the average values for Chinese coals [20], TiO 2 is slightly enriched (2 < CC < 5) in No. 4 coals, while the Al 2 O 3 and SiO 2 concentrations are close to the average Chinese coal values (0.5 < CC < 2).The remaining major oxides are depleted (CC < 0.5).In general, Al 2 O 3 and SiO 2 are the most abundant element oxides in all samples.The ratios of the SiO 2 /Al 2 O 3 range from 1.24 to 2.00 (average 1.54), which are higher than those of average Chinese coal (1.42) [20] and the theoretical ratio of kaolinite (1.18).This is due to the relatively high concentration of free silicon in the coal, which was mainly found in the form of quartz [39,40].

Trace Elements
The concentrations of 32 trace elements determined in the No. 4 coal seam samples from the Yongdingzhuang Mine are listed in Table 3, 6B).Note that almost all the trace elements have higher concentrations in partings (YDZ4-4 and YDZ4-8) than those in coal benches.

Elemental Associations
Affinity and cluster analyses among elements are effective indirect methods to analyze elemental modes of occurrence [42,43].Correlation of the element concentrations with ash yield may provide preliminary information on their organic or inorganic affinities [44].In this paper, all elements were classified by the correlation between their ash yields and elemental concentrations, demonstrating either inorganic or organic affinity (Figure 7).All of these elements were then divided into five groups according to their correlation coefficients with their ash yields [45].
Group 1 includes Na 2 O, Al 2 O 3 , SiO 2 , P 2 O 5 , K 2 O, Li, F, Sc, Rb, Nb, In, Cs, Ba, Hf, Ta, Bi, Th, and U, all of which have a strong correlation with the ash yield (r ash = 0.7-1.0).The correlation coefficients between ash yield as well as Al 2 O 3 and SiO 2 are 0.99 and 1.00, respectively, indicating silicate and aluminosilicate associations (especially clay minerals) as major minerals in the coal samples.Additionally, Li, F, Sc, Rb, Nb, In, Cs, Ba, Hf, Ta, Bi, Th, and U also show high correlation coefficients with Al 2 O 3 and SiO 2 (Table 4), indicating an aluminosilicate affinity (mainly in kaolinite).Moreover, Li, F, Sc, Rb, Cs, In, Ba, Bi, and Th have a strong correlation (r > 0.7) with P 2 O 5 (Table 4), which is likely due to phosphate affinity [27,46].Group 2 consists of TiO2, Ga, Sr, Cd, W, Pb, and REY.These elements have a lesser but still high inorganic affinity, with correlation coefficients ranging between 0.4 and 0.69.Furthermore, all of these elements have a relatively strong affinity to Al2O3 and SiO2, suggesting high affinity to silicate and aluminosilicate associations in the coals.
Group 3 includes Be, Cr, Se, and Zr, which have correlation coefficients with ash yield ranging from 0.20 to 0.39.These elements have similar correlation coefficients to ash yield as well as Al2O3 and SiO2, which suggest aluminosilicate affinity.
Group 4 includes MgO, Cu, and Zn.Among these elements, MgO has high correlation coefficients with CaO and pyritic sulfur, indicating that MgO probably occurs in carbonate minerals and pyrite.Moreover, Cu and Zn both have inorganic and organic affinity, expressed by slightly weak correlation with ash yield.
Group 5 includes Fe2O3, CaO, MnO, Ni, Cu, Co, Ni, As, Mo, Sb, Hg, and Tl.Stibium is associated with the organic matters, which is due to its negative correlation with ash yield.In addition, MnO, Ni, Mo, Co, and As occur in carbonate minerals, demonstrated by high affinity to CaO.Furthermore, Ni, As, Mo, Co, Hg, and Tl are positively correlated with pyritic sulfur.Arsenic, Hg, Co, and Tl have high correlation coefficients with either sulfate or organic sulfur.In summary, these elements are combined with pyritic, sulfate and organic sulfur in coals.Group 2 consists of TiO 2 , Ga, Sr, Cd, W, Pb, and REY.These elements have a lesser but still high inorganic affinity, with correlation coefficients ranging between 0.4 and 0.69.Furthermore, all of these elements have a relatively strong affinity to Al 2 O 3 and SiO 2 , suggesting high affinity to silicate and aluminosilicate associations in the coals.
Group 3 includes Be, Cr, Se, and Zr, which have correlation coefficients with ash yield ranging from 0.20 to 0.39.These elements have similar correlation coefficients to ash yield as well as Al 2 O 3 and SiO 2 , which suggest aluminosilicate affinity.
Group 4 includes MgO, Cu, and Zn.Among these elements, MgO has high correlation coefficients with CaO and pyritic sulfur, indicating that MgO probably occurs in carbonate minerals and pyrite.Moreover, Cu and Zn both have inorganic and organic affinity, expressed by slightly weak correlation with ash yield.
Group 5 includes Fe 2 O 3 , CaO, MnO, Ni, Cu, Co, Ni, As, Mo, Sb, Hg, and Tl.Stibium is associated with the organic matters, which is due to its negative correlation with ash yield.In addition, MnO, Ni, Mo, Co, and As occur in carbonate minerals, demonstrated by high affinity to CaO.Furthermore, Ni, As, Mo, Co, Hg, and Tl are positively correlated with pyritic sulfur.Arsenic, Hg, Co, and Tl have high correlation coefficients with either sulfate or organic sulfur.In summary, these elements are combined with pyritic, sulfate and organic sulfur in coals.

TiO 2 and Al 2 O 3
Aluminum and Ti are regarded as essentially immobile elements due to their low solubility of oxides and hydroxides in low temperature aqueous solutions [47].Therefore, the ratios between Al 2 O 3 and TiO 2 should be close to the characteristics of their parent rocks [47,48]

Beryllium
Compared to Chinese coals (2.11 µg/g) [20] and World hard coals (2 µg/g) [41], Be (6.94 µg/g) is enriched in No. 4 coals.Many scholars have stated that beryllium has organic and clay mineral associations in most coals [50,54].In No. 4 coals, Be has positive correlation with ash yield (r = 0.31), which indicates that Be has inorganic affinity.The high correlation coefficient for Be-TiO 2 (r = 0.86), but low correlation coefficients for Be-Al 2 O 3 (0.19) and Be-SiO 2 (0.36), suggest that Be is mainly associated with anatase and to a lesser extent, with clay minerals.

Strontium and Ba
Both Sr and Ba show significantly positive correlation with the ash yield (r ash = 0.60 and 0.97, respectively), indicating an inorganic association.Strontium and Ba have high correlations with Al 2 O 3 (r = 0.52 and 0.92, respectively) and SiO 2 (r = 0.64 and 0.98, respectively), which point to aluminosilicate affinity (mainly in clay minerals) [45].Moreover, the correlation coefficients of Sr-P 2 O 5 (r = 0.73) and Ba-P 2 O 5 (r = 0.92) display that gorceixite and goyazite may also be the carrier of Sr and Ba.In addition, both of Sr and Ba have high correlation coefficients with TiO 2 (r = 0.64 and 0.78, respectively), indicating that Sr and Ba may also occur in anatase.In terms of geochemical properties, Sr/Ba can reflect the sedimentary environment of coal formation.The average Sr/Ba ratio can represent marine sediment (r > 1) and terrestrial sediment (r < 1) [15].In the studied area, the ratios of Sr and Ba range from 0.09 to 0.37 (0.21 on average), suggesting a terrestrial sedimentary environment.

Lithium
Compared to average Chinese coals [20] and world hard coals [41], Li is relatively enriched in No. 4 coal samples (52.11 µg/g on average).It should be noted that Li concentrations in the partings (132.30µg/g on average) are typically higher than those of coal benches.Li is positively correlated with ash yield (r = 0.99), Al 2 O 3 (r = 0.99), and SiO 2 (r = 0.97), suggesting that Li is associated with silicate and aluminosilicate minerals (mainly as kaolinite) [50].In previous studies, the supply of sediment to the Datong Coalfield has been determined to originate from the Yinshan Oldland, located to the north of the studied area, which is mainly composed of moyite with an enrichment of Li.This is most likely the dominant source of Li [51].In addition, the high correlation coefficients of Li-P 2 O 5 (r = 0.91), Li-Sr (r = 0.52) and Li-Ba (r = 0.93) point to that Li may partly has association with gorceixite and goyazite.Furthermore, the high correlation coefficient of Li-TiO 2 shows that anatase may also be the carrier of Li.In No. 4 coals, Li largely occurs in kaolinite, followed by anatase, gorceixite and goyazite.

Thallium and Hg
Compared to both average Chinese coal [20] and world hard coal [41], Tl is relatively enriched in the No. 4 coal (2.17 µg/g on average).Moreover, Hg (0.37 µg/g on average) has a higher concentration than that in both Chinese coals (0.16 µg/g) [20] and world hard coals (0.10 µg/g) [41].Thallium and Hg are negatively associated with ash yield, indicating that they occur in organic matter in the coal.Furthermore, the correlation coefficients of Tl-S p (r = 0.47) and Hg-S p (r = 0.60) indicate that Tl and Hg occur as sulfide minerals (pyrite) in the coal.In addition, Tl and Hg have high correlation coefficients with sulfate (r = 0.81 and 0.69, respectively) and organic sulfur (r = 0.87 and 0.80, respectively) [51,52].

Fluorine
The concentration of fluorine in No. 4 coals (123.54 µg/g on average) is higher than that in Chinese coals (130 µg/g) [20] and world hard coals (80 µg/g) [41].The high correlation coefficient between F and ash yield (r = 0.96) indicates an inorganic affinity.Fluorine is significantly positively correlated with Al 2 O 3 (r = 0.95) and SiO 2 (r = 0.95), indicating a close relationship between F and kaolinite.Additionally, the correlation coefficients of F-P 2 O 5 (r = 0.91), F-Sr (r = 0.64), and F-Ba (r = 0.94) indicate that F also occurs in gorceixite and goyazite [36,51].Moreover, F is positively correlated with TiO 2 (r = 0.58), which is suggestive that anatase is also one of the carriers of F. And thus, F largrly occurs in kaolinite and to a lesser extent, in anatase, gorceixite and goyazite.

Geochemical Characteristics of REY
The REY concentration in these coals ranges from 57.74 to 282.78 µg/g (149.09µg/g on average), much higher than both average Chinese coal (135.89µg/g) [20] and world hard coal (68.27 µg/g) [41].The parting samples YDZ4-4 (339.88 µg/g) and YDZ4-8 (144.76 µg/g) have relatively higher concentrations than the other samples.In general, the REY concentration in the coal bench samples underlying partings (YDZ4-4) is higher than other samples, which are probably due to leaching, as previously reported by Crowley et al. [55] and Dai et al. [40].However, the REY concentration of parting (YDZ4-8) is lower than that of the underlying and overlying coal benches.

Patterns of REY
In general, to investigate the fractionation of REY concentrations in coal, the REY should be normalized by standard materials that have similar origins to coal [55,56].As a result, it is advisable to use the UCC (Upper Continental Crust) as the standard reference for normalization rather than chondrite [56][57][58].
All of the coal samples are characterized by no pronounced or weak negative Ce anomalies (δCe = Ce N /Ce N * ranging from 0.78 to 1.0, average of 0.92) [58] and no pronounced and weak negative Eu anomalies (δEu = Eu N /Eu N * ranging from 0.80 to 1.04, average of 0.92) (Figure 8) [59].Three types of coal benches were identified: L-type (L-REY; La N /Lu N > 1), M-type (MREY; La N /Sm N < 1, Gd N /Lu N > 1), and H-type (H-REY; La N /Lu N < 1) [56].In general, to investigate the fractionation of REY concentrations in coal, the REY should be normalized by standard materials that have similar origins to coal [55,56].As a result, it is advisable to use the UCC (Upper Continental Crust) as the standard reference for normalization rather than chondrite [56][57][58].
All of the coal samples are characterized by no pronounced or weak negative Ce anomalies (δCe = CeN/CeN * ranging from 0.78 to 1.0, average of 0.92) [58] and no pronounced and weak negative Eu anomalies (δEu = EuN/EuN * ranging from 0.80 to 1.04, average of 0.92) (Figure 8) [59].Three types of coal benches were identified: L-type (L-REY; LaN/LuN > 1), M-type (MREY; LaN/SmN < 1, GdN/LuN > 1), and H-type (H-REY; LaN/LuN < 1) [56].The REY enrichment patterns can be characterized as H-type in most coal benches (YDZ4-2, YDZ4-3, YDZ4-12, and YDZ4-15).Coal samples with similar distribution patterns include YDZ4-5, YDZ4-7, YDZ4-10, YDZ4-13, and YDZ4-14 and can be characterized as M-H type enrichment.The remaining coal samples are characterized as L-type enrichment as well as two partings (YDZ4-4 and YDZ4-8).The L-type enrichment generally reflects a terrigenous origin [56].The H-type enrichment is probably due to the injection of hydrothermal solutions, while the M-type is possibly related to natural waters [56] In the parting sample YDZ4-8, Ce shows a slight positive Ce anomaly (δCe = 1.08) with a low concentration of REE, which is due to groundwater leaching [40,55].Cerium is the only rare earth element that can be oxidized to Ce 4+ and could have been precipitated in-situ, leading to high Ce and low REE concentration [51].
Dai showed that positive Eu anomalies could be caused by over estimation of Eu during the ICP-MS analysis due to interference from BaO or BaOH [58].If Ba/Eu is > 1000, the interfered Eu is highly elevated.This indicates that when samples contain Ba/Eu less than 1000, the interference of Ba on Eu can be ignored [60].Although No. 4 coals have high correlation coefficient between Ba and Eu (r = 0.60), low Ba/Eu value (24.50-108.77,71.16 on average) may indicate that Eu anomalies in these samples are not caused by the interference of Ba.
As previously reported, coals with input of felsic or felsic-intermediate terrigenous materials usually display distinct negative Eu anomalies [58].Furthermore, the ratio of Al2O3/TiO2 (18.96 on average) also indicates that the input of the No. 4 coals is felsic or felsic-intermediate terrigenous materials.And thus, the Eu in No. 4 coal would be expected to have negative anomalies because the sediment source region is mainly of felsic to intermediate materials [58].The partings (including YDZ4-4 and YDZ 4-8) have negative Eu anomalies with L-type enrichment, indicating that partings originated from felsic or felsic-intermediate terrigenous materials [56,58].The coal bench samples, including YDZ4-1, YDZ4-6, YDZ4-9 and YDZ4-11, have no pronounced or weak negative Eu anomalis with L-type enrichment, which is due to that the input of the coals mainly originated from felsic or felsic-intermediate terrigenous materials and to a lesser extent, from mafic terrigenous materials [56,58,61].This may be due to that the mafic terrigenous materials are characterized by positive Eu anomalies which overprinted the negative Eu negative anomalies caused by input of the felsic or felsic-intermediate terrigenous materials [56,58,61].In addition, the samples include YDZ4-2, YDZ4-3, YDZ4-5, YDZ4-7, YDZ4-10, YDZ4-12, YDZ4-13, YDZ4-14, and YDZ4-15, showing no pronounced or weak negative Eu anomalies with H-and M-H type enrichment.In these samples, Eu dispalys no pronounced or weakly negative anomalies, probably due to the injection of hydrothermal solutions that is characterized by positive Eu anomalies, which overprinted the negative Eu anomalies inherited from the terrigenous materials of the sediment source region [56,58,62].The REY enrichment patterns can be characterized as H-type in most coal benches (YDZ4-2, YDZ4-3, YDZ4-12, and YDZ4-15).Coal samples with similar distribution patterns include YDZ4-5, YDZ4-7, YDZ4-10, YDZ4-13, and YDZ4-14 and can be characterized as M-H type enrichment.The remaining coal samples are characterized as L-type enrichment as well as two partings (YDZ4-4 and YDZ4-8).The L-type enrichment generally reflects a terrigenous origin [56].The H-type enrichment is probably due to the injection of hydrothermal solutions, while the M-type is possibly related to natural waters [56] In the parting sample YDZ4-8, Ce shows a slight positive Ce anomaly (δCe = 1.08) with a low concentration of REE, which is due to groundwater leaching [40,55].Cerium is the only rare earth element that can be oxidized to Ce 4+ and could have been precipitated in-situ, leading to high Ce and low REE concentration [51].
Dai showed that positive Eu anomalies could be caused by over estimation of Eu during the ICP-MS analysis due to interference from BaO or BaOH [58].If Ba/Eu is > 1000, the interfered Eu is highly elevated.This indicates that when samples contain Ba/Eu less than 1000, the interference of Ba on Eu can be ignored [60].Although No. 4 coals have high correlation coefficient between Ba and Eu (r = 0.60), low Ba/Eu value (24.50-108.77,71.16 on average) may indicate that Eu anomalies in these samples are not caused by the interference of Ba.
As previously reported, coals with input of felsic or felsic-intermediate terrigenous materials usually display distinct negative Eu anomalies [58].Furthermore, the ratio of Al 2 O 3 /TiO 2 (18.96 on average) also indicates that the input of the No. 4 coals is felsic or felsic-intermediate terrigenous materials.And thus, the Eu in No. 4 coal would be expected to have negative anomalies because the sediment source region is mainly of felsic to intermediate materials [58].The partings (including YDZ4-4 and YDZ 4-8) have negative Eu anomalies with L-type enrichment, indicating that partings originated from felsic or felsic-intermediate terrigenous materials [56,58].The coal bench samples, including YDZ4-1, YDZ4-6, YDZ4-9 and YDZ4-11, have no pronounced or weak negative Eu anomalis with L-type enrichment, which is due to that the input of the coals mainly originated from felsic or felsic-intermediate terrigenous materials and to a lesser extent, from mafic terrigenous materials [56,58,61].This may be due to that the mafic terrigenous materials are characterized by positive Eu anomalies which overprinted the negative Eu negative anomalies caused by input of the felsic or felsic-intermediate terrigenous materials [56,58,61].In addition, the samples include YDZ4-2, YDZ4-3, YDZ4-5, YDZ4-7, YDZ4-10, YDZ4-12, YDZ4-13, YDZ4-14, and YDZ4-15, showing no pronounced or weak negative Eu anomalies with H-and M-H type enrichment.In these samples, Eu dispalys no pronounced or weakly negative anomalies, probably due to the injection of hydrothermal solutions that is characterized by positive Eu anomalies, which overprinted the negative Eu anomalies inherited from the terrigenous materials of the sediment source region [56,58,62].
In conclusion, the REY originated from the felsic or felsic-intermediate terrigenous materials (granite of Yinshan Oldland), and the natural waters or hydrothermal solutions that may circulate in coal basins [61][62][63].

Conclusions
The geochemical and mineralogical characteristics of the coal seam No. 4 of the Yongdingzhuang Mine are summarized below: (1) The No. 4 coal samples of the Yongdingzhuang Mine have a medium ash yield content (average 20.76%), a low moisture content (average 1.46%), a medium-high volatile content (average 27.45%), and a low sulfur content (average 0.70%).( 2) The mineralogical compositions are mainly kaolinite and quartz with minor amounts of pyrite and anatase.Kaolinite occurs in the form of infillings in fusinite and disseminations in collodetrinite, suggesting syngenetic and early diagenetic authigenic origin.In addition, quartz occurs primarily as detrital grains derived from the source region.(1.54), compared to average Chinese coals, with a large proportion of SiO 2 in the form of free silica within coals.With regard to valuable elements, the concentrations of Al, Li, Ga, Zr, Nb, Ta, Hf, Th, and REY were slightly higher than those of average world hard coals.In some coal seams, the concentrations of these elements reached industrial levels and are thus economical to extract.Furthermore, As, Hg, Be, F, U, Pb, Se, Cr, Cd, Ni, and Tl are the main hazardous trace elements in No. 4 coals.Moreover, the Be, Tl, Hg and Pb are slightly higher than in average world hard coals, which is a concern for human and environmental health during combustion and utilization.In terms of the mode of occurrence, Li, F, Cr, Ga, Se, Cd, Zr, Pb, Nb, Ta, and Hf occur as inorganic matter in clay minerals.Cobalt, Ni, As, Tl, and Hg have a strong correlation with sulfur, suggesting a occurrence in sulfide minerals.Lithium and F also occur in anatase, gorceixite and goyazite.Beryllium has affinity to anatase; gallium is mainly associated with kaolinite and to a lesser extent, with gorceixite and goyazite; zirconium also occurs as inorganic matter in ncluding kaolinie, gorceixite and goyazite.These elements and minerals should be further investigated in future studies.

Figure 1 .
Figure 1.Location map of both the Datong Coalfield and the Yongdingzhuang Mine [18].

Figure 1 .
Figure 1.Location map of both the Datong Coalfield and the Yongdingzhuang Mine [18].

Figure 2 .
Figure 2. General stratigraphic sequence of the Datong Coalfield.

Figure 6 .
Figure 6.Concentration coefficients of trace elements in No. 4 coal; (A) normalized by average concentrations in the world hard coals; (B) normalized by the average concentration in the Chinese coals.

Figure 7 .
Figure 7. Concentration variations of trace elements and rare earth elements and yttrium (REY) as well as ash yield of the No. 4 coals.

Figure 7 .
Figure 7. Concentration variations of trace elements and rare earth elements and yttrium (REY) as well as ash yield of the No. 4 coals.
. The different ratios of Al 2 O 3 /TiO 2 represent different types of parent rocks.Ratios of Al 2 O 3 /TiO 2 ranging from 3 to 8 indicate mafic source rocks, 8-21 for intermediate source rocks, and 21-70 for felsic igneous rocks [47-52].The ratio of the No. 4 coal seam samples ranges from 4.36 to 44.50 (18.96 on average), indicating that the sediments of the parent rocks have felsic or intermediate geochemical characteristics.

Figure 8 .
Figure 8. REY distribution patterns in the coal and partings, normalized to the upper continental crust (UCC); (A) pattern of LREE in coals; (B) pattern of MREE in coals; (C) pattern of HREE in coals; (D) pattern of LREE in partings.

( 3 )
The ratio of Al 2 O 3 and TiO 2 in coal seam No. 4 ranged from 4.36 to 44.50(18.96on average), indicating that sediments formed by weathering of the parent rocks, which mainly had felsic or intermediate geochemical characteristics.The ratio of Sr and Ba ranged from 0.09 to 0.37 (0.21 on average), clearly suggesting a terrestrial sedimentary environment.(4) Compared to both average Chinese coals and average world hard coals, REY in the No. 4 coal seam have higher concentration.The REY mainly occurs as inorganic matter in the ash yield, which is consistent with its strong correlation with ash yield.Furthermore, no pronounced or negative Ce anomalies and weak negative or positive Eu anomalies demonstrate that the REY originated from granite of Yinshan Oldland and natural waters or hydrothermal solutions that may circulate in coal basins.(5) Al 2 O 3 and SiO 2 are prevailing abundant major oxides in No. 4 coals, with higher SiO 2 /Al 2 O 3

Table 2 .
Concentration of major oxides in No. 4 coal (%) (Recalculated from the dry ash basis to the whole-coal basis).

Table 3 .
Concentrations of trace elements in the No. 4 coal from the Yongdingzhuang Mine (µg/g) (on whole-coal basis).

Table 4 .
Correlation coefficients between the concentration of each element in coal and ash yield or selected elements.