Lithium Enrichment in the No. 2 1 Coal of the Hebi No. 6 Mine, Anhe Coalﬁeld, Henan Province, China

: Lithium (Li) is an important strategic resource, and with the increasing demand for Li, there are some limitations in the exploitation and utilization of conventional deposits such as the pegmatite-type and brine-type Li deposits. Therefore, it has become imperative to search for Li from other sources. Li in coal is thought to be one of the candidates. In this study, the petrology, mineralogy, and geochemistry of No. 2 1 coal from the Hebi No. 6 mine, Anhe Coalﬁeld, China, was reported, with an emphasis on the distribution, modes of occurrence, and origin of Li. The results show that Li is enriched in the No. 2 1 coal, and its concentration coe ﬃ cient (CC) value is 6.6 on average in comparison with common world coals. Lithium in the studied coal is mainly present in aluminosilicates, mainly clay minerals, some of which contain a signiﬁcant amount of Ti. The Li enrichment in the No. 2 1 coal is mainly controlled by the terrigenous materials and sourced from the moyite of the Yinshan Upland. Furthermore, Li in the No. 2 1 coal is more enriched in coals formed in acidic and humid conditions and coals inﬂuenced by fresh water during peat accumulation.

Lithium in coal is generally considered to occur as an organic or inorganic form [11,31,34,36,43,[47][48][49][50][51]. Lithium being associated with the inorganic matter in coal, is generally suggested to be related to aluminosilicate minerals, namely clay minerals in most cases [52,53]. The adsorption/incorporation  The Taiyuan Formation can be correlated to the Kasimovian-Asselian stages in the international chronostratigraphic nomenclature [70,71]. The depositional environment of the Taiyuan Formation is characterized by tidal flat, barrier-lagoon, carbonate platform, and peat swamp facies [72]. The numbers of coal seams in the Taiyuan Formation can reach up to 11 ( Figure 2A). Due to marine influence, the coal seams are thin and the lateral distribution of the seams is limited.

Coal Characteristics and Coal Petrology
The ash yield, moisture, volatile material and sulfur contents, various forms of sulfur and macerals results are summarized in Table 2. The mean random vitrinite reflectance (1.87%, based on mixed-coal samples) and volatile matter contents (range from 15.7% to 17.8%, 16.2% on average) of coal samples with relatively low contents of CaO (samples HB-2 0.94%, HB-4 0.91%, HB-7 0.85%, HB-10 0.85%, HB-18 0.29%) in the Hebi No. 6 mine indicate a low-volatile bituminous coal according to the ASTM classification (ASTM D388-12, 2012) [84]. The ash yield of the No. 2 1 coal samples in the Hebi No. 6 mine is 7.4-37.8%, with an average content of 13.7%. The total sulfur content of the coal samples ranges from 0.2% to 0.4%, and the average is 0.3%. The contents of ash yield and total sulfur above indicate that the No. 2 1 coal is a low-ash and super-low sulfur coal according to Chinese standards GB15224.1-2010 [85] (coals with an ash yield of >10% and ≤20% are low-ash coals) and GB15224.2-2010 [86] (coals with a total sulfur content of ≤0.5% are super-low sulfur coals). In addition, organic sulfur (0.08% to 0.27%, averaging 0.19%) is the main form of sulfur in the coal, followed by pyritic sulfur (0.01% to 0.20%, averaging 0.07%).  Table 3). Liptinite was not observed, since it is difficult to identify liptinites in low volatile bituminous coals [87]. The relative proportions of maceral groups in the No. 2 1 coal is quite different from that found in other Late Paleozoic coals in the northern part of North China, which usually have slightly higher proportions of inertinite than vitrinite [88].  Figure 3A,B) and telinite (11.2 vol.% on average; Figure 3A,C), with trace amounts of vitrodetrinite (2.7 vol.% on average) and collotelinite (1.9 vol.% on average). Sometimes, the collodetrinite contains fracture-filling calcite ( Figure 3B) and clay minerals filled in deformed cells of the telinite ( Figure 3C). In most cases, the collodetrinite occurs as matrix-containing embedded ( Figure 3D) or banded ( Figure 3E) clay minerals and massive quartz ( Figure 3D).
Minerals 2020, 10, 521 7 of 30 on average) and collotelinite (1.9 vol.% on average). Sometimes, the collodetrinite contains fracturefilling calcite ( Figure 3b) and clay minerals filled in deformed cells of the telinite ( Figure 3C). In most cases, the collodetrinite occurs as matrix-containing embedded ( Figure 3D) or banded ( Figure 3E) clay minerals and massive quartz ( Figure 3D). The inertinite in the No. 21 coal is mainly composed of semifusinite ( Figure 3A,F; 15.8 vol.% on average) and, to a lesser extent, inertodetrinite (8.9 vol.% on average), with small amounts of fusinite, macrinite ( Figure 3D), and micrinite. In most cases, the cell structures of the semifusinite are not wellpreserved and have a swelled and degraded form ( Figure 3A,F), which is sometimes filled with clay minerals ( Figure 3F).  The inertinite in the No. 2 1 coal is mainly composed of semifusinite (Figure 3A,F; 15.8 vol.% on average) and, to a lesser extent, inertodetrinite (8.9 vol.% on average), with small amounts of fusinite, macrinite ( Figure 3D), and micrinite. In most cases, the cell structures of the semifusinite are not well-preserved and have a swelled and degraded form ( Figure 3A,F), which is sometimes filled with clay minerals ( Figure 3F).

Major-Element Oxides
The percentages of the major-element oxides in the No. 2 1 coal and parting samples, as well as the average values for Chinese coals, are listed in Table 4. The major-element oxides in the studied No. 2 1 coal samples are mainly dominated by SiO 2 and Al 2 O 3 , ranging from 2.8% to 18.1% (6.3% on average on a whole-coal basis) and 2.4% to 13.5% (5.3% on average on a whole-coal basis), respectively ( Table 4). Compared with the average values for average Chinese coals reported by Dai et al. [12], the percentages of CaO and Na 2 O are slightly higher, while other major oxides (SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, MnO, and K 2 O) are either close to or lower than those of average Chinese coals (Table 4). Notes: LOI, loss on ignition; bdl, below detection limit; a , major-element oxides in Chinese coals. Data from Dai et al. [12].
The SiO 2 /Al 2 O 3 ratio of the studied coal samples in Hebi No. 6 mine (1.19) is lower than that of the common Chinese coals (1.42) [12] but slightly higher than the theoretical SiO 2 /Al 2 O 3 ratio of kaolinite (1.18), probably because quartz is also present along with abundant kaolinite ( Figure 3D).

Li Concentrations
The concentrations of trace elements in all the coal and non-coal samples are listed in Table 5. The concentration of Li in the No. 2 1 coal samples varies from 40.2 µg/g to 183 µg/g, with an average value of 79.0 µg/g. This is much higher than that of common Chinese coals (31.8 µg/g) [12] and world hard coals (14 µg/g) [69]. A high Li content has been also reported in other coals from northern China, such as coals from the Haerwusu mine of the Junger Coalfield (116 µg/g on average) [11], the Guanbanwusu mine of the Junger Coalfield (175 µg/g on average [26]; 264 µg/g on average [43]), the Heidaigou mine of the Junger Coalfield (143 µg/g on average) [89], the Buertaohai-Tianjiashipan mine of the Junger Coalfield (70-79 µg/g on average) [52], the Dongpo mine of the Weibei Coalfield (84.98 µg/g on average) [35], and the Wangtaipu mine of the Jincheng Coalfield (136.5 µg/g on average) [53].
The concentration of Li in the partings and floor from the Hebi No. 6 mine are two to three times higher than that of the coal samples. Specifically, the concentration of Li in the partings decrease gradually from top to bottom, ranging from 171 to 260 µg/g with an average of 203.5 µg/g, while the concentration of Li in the floor increases again up to 244 µg/g ( Figure 4A).   The concentration coefficient (CC) proposed by Dai et al. [9,25] was used to evaluate the enrichment degree of the minor and trace elements in coal and organic-rich rocks. In this paper, the concentration coefficient value is the ratio of Li concentration in the coal samples correlated with the mean value of Li concentration in common world coal [69] and the ratio of Li concentration in the parting and floor mudstone samples compared to the mean Li concentration in common world clays [90]. Lithium is enriched in the No. 2 1 coal and the CC value of Li in coal is 6.6 on average. Lithium is slightly enriched in the parting and floor mudstones of the No. 2 1 coal seam and the relevant CC value is 3.9 on average.
Note that Li is more enriched in the upper part of the No. 2 1 coal seam and the CC value of Li in the upper part (including HB-1 to HB-12) of the No. 2 1 coal is 7.3. The CC value of Li in the lower part of the No. 2 1 coal and in the partings and the floor is between 2 and 5, and the Li is slightly enriched ( Figure 4B).  [91]; Sm N = 0.5 * (Nd N + Eu N ); Dy = Dy N * the concentration of Dy in the upper continental crust (UCC) [91]; Dy N = 0.5 * (Tb N + Y N ); Nd N , Eu N , Tb N , and Y N , the ratios of Nd, Eu, Tb, and Y values (tested data) to those in the upper continental crust, respectively.

Rare Earth Elements and Yttrium
In the literature, the term rare earth elements (REE) or rare earth elements and yttrium (REY) has been used somewhat inconsistently [92], but recently in Dai et al.'s [93] review study, REY (or REE if yttrium is not included) is used to specifically represent the elemental suite La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu. The ionic radius of Y 3+ is very similar to that of Ho 3+ , and thus Y 3+ can be placed between isovalent Dy 3+ and Ho 3+ in normalized REY distribution patterns [93,94]. In the present study, a three-fold geochemical classification of REY, which was classified by Seredin and Dai. [94], was used, including light REY (LREY: La, Ce, Pr, Nd, and Sm), medium REY (MREY: Eu, Gd, Tb, Dy, and Y), and heavy REY (HREY: Ho, Er, Tm, Yb, and Lu). Correspondingly, in comparison with the upper continental crust concentrations (UCC) [91], three enrichment patterns were also identified as L-type (light-REY; La N /Lu N > 1), M-type (medium-REY; La N /Sm N < 1, Gd N /Lu N > 1), and H-type (heavy REY; La N /Lu N < 1) [94].
Previous studies have shown that the measured content of Eu and the identification of positive Eu anomalies by quadrupole-based ICP-MS in coals should be used with great caution, because the Eu content of the samples may be interfered with by Ba [93,95]. If the Ba/Eu value is higher than 1000, the Eu content of the samples is thought to be interfered with by element Ba [93,95]. In this study, the relationship between the Ba and Eu concentrations (r = 0.29; Figure 5) is weak, and almost all the Ba/Eu values of the studied coal samples are lower than 1000 (604 on average), except the sample HB-1 with a Ba/Eu value of 1132. However, the Eu N /Eu N * in the sample HB-1 is 0.78 (Table 6), which is lower than 1, indicating no positive Eu anomalies in this sample. Furthermore, the Ba contents in samples with a relatively high Eu or Eu N /Eu N * content are relatively lower than those in samples with a relatively low Eu or Eu N /Eu N * content, such as the samples HB-23 (Eu = 1.47 µg/g, Eu N /Eu N * =1.18, Ba = 288 µg/g) and HB-1 (Eu = 0.28 µg/g, Eu N /Eu N * = 0.78, Ba = 317 µg/g). Therefore, the high Eu or Eu N /Eu N * contents in this study were generally not interfered with by the Ba concentration.

Rare Earth Elements and Yttrium
In the literature, the term rare earth elements (REE) or rare earth elements and yttrium (REY) has been used somewhat inconsistently [92], but recently in Dai et al.'s [93] review study, REY (or REE if yttrium is not included) is used to specifically represent the elemental suite La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu. The ionic radius of Y 3+ is very similar to that of Ho 3+ , and thus Y 3+ can be placed between isovalent Dy 3+ and Ho 3+ in normalized REY distribution patterns [93,94]. In the present study, a three-fold geochemical classification of REY, which was classified by Seredin and Dai. [94], was used, including light REY (LREY: La, Ce, Pr, Nd, and Sm), medium REY (MREY: Eu, Gd, Tb, Dy, and Y), and heavy REY (HREY: Ho, Er, Tm, Yb, and Lu). Correspondingly, in comparison with the upper continental crust concentrations (UCC) [91], three enrichment patterns were also identified as L-type (light-REY; LaN/LuN > 1), M-type (medium-REY; LaN/SmN < 1, GdN/LuN > 1), and H-type (heavy REY; LaN/LuN < 1) [94].
Previous studies have shown that the measured content of Eu and the identification of positive Eu anomalies by quadrupole-based ICP-MS in coals should be used with great caution, because the Eu content of the samples may be interfered with by Ba [93,95]. If the Ba/Eu value is higher than 1000, the Eu content of the samples is thought to be interfered with by element Ba [93,95]. In this study, the relationship between the Ba and Eu concentrations (r = 0.29; Figure 5) is weak, and almost all the Ba/Eu values of the studied coal samples are lower than 1000 (604 on average), except the sample HB-1 with a Ba/Eu value of 1132. However, the EuN/EuN* in the sample HB-1 is 0.78 (Table 6), which is lower than 1, indicating no positive Eu anomalies in this sample. Furthermore, the Ba contents in samples with a relatively high Eu or EuN/EuN* content are relatively lower than those in samples with a relatively low Eu or EuN/EuN* content, such as the samples HB-23 (Eu = 1.47 μg/g, EuN/EuN* =1.18, Ba = 288 μg/g) and HB-1 (Eu = 0.28 μg/g, EuN/EuN* = 0.78, Ba = 317 μg/g). Therefore, the high Eu or EuN/EuN* contents in this study were generally not interfered with by the Ba concentration.  Table 6) are lower than those of common Chinese coals (136 μg/g) [12] and the upper continental crust (168.37 μg/g) [91], but slightly higher than those of average world hard coals (68.6 μg/g) [69].
The REY contents of the partings and floor from the Hebi No. 6 mine are approximately two to four times higher than those of the coal bench samples (Table 6). Specifically, the REY concentrations of the partings range from 209.99 to 285.36 μg/g, with an average of 242.41 μg/g, which are slightly higher than those of average world clays (226.42 μg/g) [90] and those of the upper continental crust (168.37 μg/g) [91]. The contents of REY in the floor are the highest among all the non-coal samples, with values of up to 365.9 μg/g, which are much higher than those of average world clays (226.42 μg/g) [90] and the upper continental crust (168.37 μg/g) [91].  Table 6) are lower than those of common Chinese coals (136 µg/g) [12] and the upper continental crust (168.37 µg/g) [91], but slightly higher than those of average world hard coals (68.6 µg/g) [69].
The REY contents of the partings and floor from the Hebi No. 6 mine are approximately two to four times higher than those of the coal bench samples (Table 6). Specifically, the REY concentrations of the partings range from 209.99 to 285.36 µg/g, with an average of 242.41 µg/g, which are slightly higher than those of average world clays (226.42 µg/g) [90] and those of the upper continental crust (168.37 µg/g) [91]. The contents of REY in the floor are the highest among all the non-coal samples, with values of up to 365.9 µg/g, which are much higher than those of average world clays (226.42 µg/g) [90] and the upper continental crust (168.37 µg/g) [91].    Figure 6A), except for the sample HB-23, which shows an HREY enrichment and a positive Eu anomaly. Like the coal samples, the REY distribution pattern of the floor is also characterized by LREY enrichment with negative Eu-and Ce-anomalies ( Figure 6B). While compared with the distribution curves of the REY in coal samples, the curves of the REY in the the partings are relatively flat ( Figure 6B), indicating no obvious fractionation of LREY and HREY.

Other Trace Elements
In addition to Li, compared to the average values for world hard coals reported by Ketris and Yudovich [69], the No. 2 1 coal is also enriched with Sr (CC = 5.07) and slightly enriched with Pb (CC = 2.47) and Th (CC = 2.19), while Mn, Zn, Rb, Mo, Cd, Bi, and Tl in the No. 2 1 coal are depleted and the remaining elements have normal concentrations ( Figure 7A). The concentration of Sr in the No. 2 1 coal samples varies from 110 to 1183 µg/g and has an average value of 557.4 µg/g.
In addition to Li, compared to the average values for world hard coals reported by Ketris and Yudovich [69], the No. 21 coal is also enriched with Sr (CC = 5.07) and slightly enriched with Pb (CC = 2.47) and Th (CC = 2.19), while Mn, Zn, Rb, Mo, Cd, Bi, and Tl in the No. 21 coal are depleted and the remaining elements have normal concentrations ( Figure 7A). The concentration of Sr in the No. 21 coal samples varies from 110 to 1183 μg/g and has an average value of 557.4 μg/g.
Compared to the average values of trace elements for world clays [90], the parting samples from the Hebi No. 6 mine are slightly enriched in Ga (CC = 2.31), Nb (CC = 2.36), and Pb (CC = 2.05), while Mn, Zn, Rb, Cd, Cs, and Tl are depleted and the remaining trace elements have concentrations close to the average values for world clays [90] ( Figure 7B). The concentrations of Ga, Nb, and Pb in the parting samples range from 34.8 to 39.9 μg/g, 24.9 to 27.9 μg/g, and 26.3 to 31.7 μg/g, respectively. Figure 7C shows the ratio of the average values of trace elements in the floor sample and the average values of trace elements for world clays [90]. Gallium (CC = 2.

Minerals
Minerals in coal are often regarded as a nuisance, being responsible for most of the problems arising during coal utilization, but minerals are also seen as a potentially valuable source of critical metals. With a few exceptions, minerals are the major hosts of the majority of elements present in coal [96].
According to the results of X-ray diffraction analysis (Figure 8), clay minerals such as illite and kaolinite are the main mineral in the No. 2 1 coal of Hebi No. 6 mine, followed by quartz and calcite, with a small amount of ankerite and K-feldspar. The mineral phases in the partings and floor are dominantly kaolinite and, to a lesser extent, illite, quartz, and calcite, with smaller proportions of K-feldspar, plagioclase, and ankerite.
Minerals in coal are often regarded as a nuisance, being responsible for most of the problems arising during coal utilization, but minerals are also seen as a potentially valuable source of critical metals. With a few exceptions, minerals are the major hosts of the majority of elements present in coal [96].
According to the results of X-ray diffraction analysis (Figure 8), clay minerals such as illite and kaolinite are the main mineral in the No. 21 coal of Hebi No. 6 mine, followed by quartz and calcite, with a small amount of ankerite and K-feldspar. The mineral phases in the partings and floor are dominantly kaolinite and, to a lesser extent, illite, quartz, and calcite, with smaller proportions of Kfeldspar, plagioclase, and ankerite.
Under the SEM-EDS, the morphology and principal elements of the main minerals in the samples are shown in Figures 9 and 10. The morphology of illite, with some occurrences as Ti-bearing illite, mainly appears as irregular flaky and fine slats ( Figure 9A-D), and the kaolinite exists in form of thin sheets in the No. 21 coal (Figure 9E,F). Although usually regarded as being relatively immobile, Ti may be mobilized at low pH levels [97], allowing it to be leached from detrital components and reprecipitated in other parts of the coalbed [98], leading to the Ti-bearing illite found in the present paper.
Calcite mainly occurs as fracture infillings ( Figure 3B) and parallel-developed columnar ( Figure  10A), indicating an epigenetic and calcium-rich solution origin.
Pyrite in Hebi No. 6 coal occurs mainly as discrete crystals ( Figure 10B,C) and fracture fillings ( Figure 10D). The former probably suggests a syngenetic origin while the later an epigenetic origin [99].  Under the SEM-EDS, the morphology and principal elements of the main minerals in the samples are shown in Figures 9 and 10. The morphology of illite, with some occurrences as Ti-bearing illite, mainly appears as irregular flaky and fine slats ( Figure 9A-D), and the kaolinite exists in form of thin sheets in the No. 2 1 coal ( Figure 9E,F). Although usually regarded as being relatively immobile, Ti may be mobilized at low pH levels [97], allowing it to be leached from detrital components and re-precipitated in other parts of the coalbed [98], leading to the Ti-bearing illite found in the present paper.     Calcite mainly occurs as fracture infillings ( Figure 3B) and parallel-developed columnar ( Figure 10A), indicating an epigenetic and calcium-rich solution origin.
Pyrite in Hebi No. 6 coal occurs mainly as discrete crystals ( Figure 10B,C) and fracture fillings ( Figure 10D). The former probably suggests a syngenetic origin while the later an epigenetic origin [99].

Lithium
Based on the correlation coefficients (r) between the elemental concentrations and the ash yield, Li in the No. 2 1 coal has a very high positive correlation coefficient with the ash yield (r = 0.87, Figure 11A, Table 7, Group 1), indicating a highly inorganic affinity. In addition, the relatively high correlation coefficients of Li-Al 2 O 3 (r = 0.95, Figure 11B, Table 7), Li-SiO 2 (r = 0.91, Figure 11C, Table 7), Li-K 2 O (r = 0.50, Figure 11D, Table 7), and Li-TiO 2 (r = 0.79, Figure 11E, Table 7) indicate that Li is mainly present in the aluminosilicate minerals, most likely the clay minerals. The relatively high correlation coefficient of Li-TiO 2 (r = 0.79, Figure 11E, Table 7) and the weekly positive correlation coefficient of Li-K 2 O (r = 0.50, Figure 11D, Table 7) indicate that, to a lesser extent, Ti-bearing illite is probably also a carrier of Li. Furthermore, the conclusion above can be confirmed by the Ti-bearing illite, which has also been found in the No. 2 1 coals ( Figure 9C

Lithium
Based on the correlation coefficients (r) between the elemental concentrations and the ash yield, Li in the No. 21 coal has a very high positive correlation coefficient with the ash yield (r = 0.87, Figure  11A, Table 7, Group 1), indicating a highly inorganic affinity. In addition, the relatively high correlation coefficients of Li-Al2O3 (r = 0.95, Figure 11B, Table 7), Li-SiO2 (r = 0.91, Figure 11C, Table  7), Li-K2O (r = 0.50, Figure 11D, Table 7), and Li-TiO2 (r = 0.79, Figure 11E, Table 7) indicate that Li is mainly present in the aluminosilicate minerals, most likely the clay minerals. The relatively high correlation coefficient of Li-TiO2 (r = 0.79, Figure 11E, Table 7) and the weekly positive correlation coefficient of Li-K2O (r = 0.50, Figure 11D, Table 7) indicate that, to a lesser extent, Ti-bearing illite is probably also a carrier of Li. Furthermore, the conclusion above can be confirmed by the Ti-bearing illite, which has also been found in the No. 21 coals (Figure 9C,D) detected by SEM-EDS, and the intimate association of Li and TiO2-, Al2O3-, and SiO2 indicated by cluster analysis (Figure 12, Group  B), indicating that at least some Ti-bearing aluminosilicates are carriers of Li in No. 21 coal from the Hebi No. 6 mine.
Based on sequential chemical extraction results, Finkelman et al. [36] suggested that in most coals about 90% of Li is associated with clays and micas and the remainder is either associated with organics or acid-insoluble phases, such as tourmaline. The high Li in the Haerwusu [11] and Buertaohai-Tianjiashipan [52] coals in the Junger Coalfield is deduced to be associated with aluminosilicate minerals. Wang et al. [47] also considered that Li in the coal of the Qinshui Basin is probably bound to clay minerals such as kaolinite and illite. However, cookeite, a Li-rich member of the chlorite group, was considered to be the primary Li carrier in the coals from the Jincheng Coalfield, southeastern Qinshui Basin [62]. Ti-bearing aluminosilicates were not uncommon in coals [35,100,101], but those occurring as the main carrier of Li have rarely been reported. As mentioned above, this study and a number of studies have shown relatively strong correlations between Li and aluminosilicates/clay minerals [96,102]. This is because clay minerals, being usually negatively charged in nature, have high surface to volume ratios, which enable trace elements such as Li, usually positively charged, to be adsorbed on their surface. Additionally, some clay minerals have an interlayer space where cation exchange may take place [96]. Titanium may be leached from detrital components at low pH levels [97] and re-precipitated in these clay minerals, leading to a close relationship between Li and Ti-bearing clays/aluminosilicates.

Other Trace Elements
The affinity of some elevated elements in the No. 21 coal (Sr, Pb, Th, except Li) and some enriched trace elements in the partings and floor, such as Ga, Nb, Pb, and REY, are discussed. Three groups of elements can be differentiated according to their correlation coefficients with the ash yield ( Table 7). The first group has a very high positive correlation coefficient with ash yield (>0.7; Ga 0.87, Nb 0.91, Th 0.76, SiO2 0.98, TiO2 0.81, Al2O3 0.97, K2O 0.80; Table 7; Figure 13A-C) and includes elements with a high inorganic affinity. Most of these elements normally relate with aluminosilicate minerals (rAl-Si  Table 7. Element affinities between the concentrations of elevated elements and the ash yield or selected elements in coal samples. Based on sequential chemical extraction results, Finkelman et al. [36] suggested that in most coals about 90% of Li is associated with clays and micas and the remainder is either associated with organics or acid-insoluble phases, such as tourmaline. The high Li in the Haerwusu [11] and Buertaohai-Tianjiashipan [52] coals in the Junger Coalfield is deduced to be associated with aluminosilicate minerals. Wang et al. [47] also considered that Li in the coal of the Qinshui Basin is probably bound to clay minerals such as kaolinite and illite. However, cookeite, a Li-rich member of the chlorite group, was considered to be the primary Li carrier in the coals from the Jincheng Coalfield, southeastern Qinshui Basin [62]. Ti-bearing aluminosilicates were not uncommon in coals [35,100,101], but those occurring as the main carrier of Li have rarely been reported. As mentioned above, this study and a number of studies have shown relatively strong correlations between Li and aluminosilicates/clay minerals [96,102]. This is because clay minerals, being usually negatively charged in nature, have high surface to volume ratios, which enable trace elements such as Li, usually positively charged, to be adsorbed on their surface. Additionally, some clay minerals have an interlayer space where cation exchange may take place [96]. Titanium may be leached from detrital components at low pH levels [97] and re-precipitated in these clay minerals, leading to a close relationship between Li and Ti-bearing clays/aluminosilicates.

Other Trace Elements
The affinity of some elevated elements in the No. 2 1 coal (Sr, Pb, Th, except Li) and some enriched trace elements in the partings and floor, such as Ga, Nb, Pb, and REY, are discussed. Three groups of elements can be differentiated according to their correlation coefficients with the ash yield ( Table 7). The first group has a very high positive correlation coefficient with ash yield (>0.7; Ga 0.87, Nb 0.91, Th 0.76, SiO 2 0.98, TiO 2 0.81, Al 2 O 3 0.97, K 2 O 0.80; Table 7; Figure 13A-C) and includes elements with a high inorganic affinity. Most of these elements normally relate with aluminosilicate minerals (r Al-Si > 0.7, Figure 13D-I). The second group with an r ash of 0.4 to 0.69 includes Fe 2 O 3 and Pb. Although the correlation coefficient of Pb-ash (r = 0.78, Figure 13J, Table 7) is high, there are only two points falling in the high-ash areas (ash > 30%) of the Pb-ash plot, and the other points in the low-ash areas (with ash < 30%) have a lower correlation coefficient (r = 0.49, Figure 13J), indicating that Pb has an inorganic-organic mixed affinity rather than an inorganically dominated affinity. The elements of Group 3 have correlation coefficients with an ash yield of less than 0.40, indicating no statistical significance. This group includes CaO, REY, Sr, and Na 2 O (Table 7). However, in some cases, accessory Sr-bearing barite and Sr-sulphate minerals can occur in the cleats/fractures of coal due to marine influence and/or being associated with syngenetic pyrite clusters in low ash yield coals, leading to a very weak correlation between Sr and ash yields; thus, this negative correlation does not always indicate organically bound Sr and Ba in coal [40,103,104]. However, sulphate minerals in the studied coals are low, which can be evidenced by the low contents of total sulfur (0.2-0.4%, 0.3% on average; Table 2) and sulphate sulphur (bdl-0.04%, 0.02% on average; Table 2) [99]. Therefore, it is unlikely that Sr exists in any significant amount in Sr-bearing barite or other sulphate minerals in the studied coals.

Source of Lithium
The Al 2 O 3 /TiO 2 ratio has been widely used in the source analysis of sedimentary rocks [34,50,52] and sediments associated with coal deposits [105]. aluminosilicate minerals, further indicating a similar provenance-the moyite of the Yinshan Upland. This conclusion is consistent with the results inferred from the Al2O3/TiO2 ratio, as mentioned above.
In northern China, during the Late Paleozoic the Xingmeng Trough subducted beneath the North China Platform, resulting in the uplift and orogeny of the Yinshan Upland [111]. Thus, the Yinshan Upland, which is located to the north of the North China Plate, served as the dominant sediment-source region for the Permo-Pennsylvanian coals on the North China Plate itself [112]. The source of two super-large Li-Ga deposits in the Pingyu mining area and the Jungar Coalfield is closely related to the Yinshan Upland [4,45]. During the Middle Permian, the source materials for the study area were mainly from the Yinshan Upland located in the north of the study area [113].
Therefore, according to the comprehensive analysis, the enriched Li in the No. 21 coal of the Hebi No. 6 mine is mainly derived from the moyite of Yinshan Upland.

Depositional Conditions during Peat Accumulation
The Sr/Ba ratio is one of the mostly widely used indicators, not only for sedimentary rocks [114][115][116] but also for coals [99,[117][118][119]. The ratio of Sr/Ba in marine-influenced coals is mostly >1; in contrast, if the Sr/Ba < 1, the coals are generally influenced by fresh water. In this paper, the ratios of Sr/Ba in the floor, coal sample HB-23, and partings, coal sample HB-18, from bottom to top are lower than 1 ( Figure 15A), indicating that the lower part of the profile was mainly affected by fresh water. From the top of sample HB-17, the ratios of Sr/Ba in almost all the No. 21 coal samples (except the samples HB-8) are above 1 ( Figure 15A), indicating that these coals were influenced by marine water The sediment-source region of the No. 2 1 coal can be further inferred by their rare earth element and trace-element assemblages [108,109]. Dai et al. [11] attributed the high Li concentration in the Pennsylvanian coal from the Junger Coalfield, Inner Mongolia, to moyite (a variety of biotite granite in which quartz exceeds orthoclase [110]) of the Yinshan Upland to the north of the coalfield [53]. The REY plots for the No. 2 1 coal are similar to those for Li-enriched coals from the Haerwusu mine, Junger Coalfield [11], which are mainly characterized by LREY and have negative Eu anomalies. In addition, those two mines above also have similar enriched trace element associations (e.g., Li, Ga, Pb, and Th) and similar modes of occurrence of Li, which are believed to be associated with aluminosilicate minerals, further indicating a similar provenance-the moyite of the Yinshan Upland. This conclusion is consistent with the results inferred from the Al 2 O 3 /TiO 2 ratio, as mentioned above.
In northern China, during the Late Paleozoic the Xingmeng Trough subducted beneath the North China Platform, resulting in the uplift and orogeny of the Yinshan Upland [111]. Thus, the Yinshan Upland, which is located to the north of the North China Plate, served as the dominant sediment-source region for the Permo-Pennsylvanian coals on the North China Plate itself [112]. The source of two super-large Li-Ga deposits in the Pingyu mining area and the Jungar Coalfield is closely related to the Yinshan Upland [4,45]. During the Middle Permian, the source materials for the study area were mainly from the Yinshan Upland located in the north of the study area [113].
Therefore, according to the comprehensive analysis, the enriched Li in the No. 2 1 coal of the Hebi No. 6 mine is mainly derived from the moyite of Yinshan Upland.

Depositional Conditions during Peat Accumulation
The Sr/Ba ratio is one of the mostly widely used indicators, not only for sedimentary rocks [114][115][116] but also for coals [99,[117][118][119]. The ratio of Sr/Ba in marine-influenced coals is mostly >1; in contrast, if the Sr/Ba < 1, the coals are generally influenced by fresh water. In this paper, the ratios of Sr/Ba in the floor, coal sample HB-23, and partings, coal sample HB-18, from bottom to top are lower than 1 ( Figure 15A), indicating that the lower part of the profile was mainly affected by fresh water. From the top of sample HB-17, the ratios of Sr/Ba in almost all the No. 2 1 coal samples (except the samples HB-8) are above 1 ( Figure 15A), indicating that these coals were influenced by marine water during coal deposition. The conclusion above is consistent with the sedimentary setting that the No. 2 1 coal formed both in upper delta plain and lower delta plain environment ( Figure 16) [71]. Furthermore, the variation trend of the Li content and Sr/Ba ratio is opposite, which can be seen from Figure 15A, indicating that Li is more enriched in coals influenced by fresh water ( Figure 15B).     [71]). LST, low-stand systems tracts; TST, transgressive systems tracts; HST, highstand system tracts. Figure 16. A sequence stratigraphic model of the No. 2 1 coal member from the Anhe Coalfield (modified from [71]). LST, low-stand systems tracts; TST, transgressive systems tracts; HST, high-stand system tracts.
During oxidizing conditions, low pH conditions (e.g., pH < 3) may occur in some parts of the peat. Aluminum is generally insoluble in the conditions of natural pH values but is soluble under low pH values and would be easily leached from many existing Al-bearing materials (including volcanic ash) under such solubility. Then, the Al-bearing leachates would transfer to other parts of the peat bed with relatively higher but still acidic pH values and subsequently be precipitated initially to bauxite-group minerals (e.g., gibbsite [98,119]), which are unstable if silica was present during their crystallization [97]. Interaction between the precipitated alumina and silica in solutions would therefore result in the formation of authigenic kaolinite rather than other clay minerals such as illite, because the K, Na, and Ca would be expected to have been leached out in the acidic system. However, a marine-influenced environment would favor the formation of illite [120]. Although illite in some cases can be of terrigenous origin [98,109,119,121], in others it can be of syngenetic authigenic origin if the ions required for their formation (e.g., K+ for illite in a marine-influenced environment) were available in a weakly alkaline peat swamp. As shown in Figure 17, the contents of kaolinite and illite in the samples from the Hebi No. 6 mine show the opposite "X" type trend from bottom to top; that is, in the upper part of the samples (from sample HB-13 to the top), the content of illite is significantly higher than that of kaolinite. Meanwhile, in the lower part (from the bottom of sample HB-19) the content of kaolinite increases sharply while the content of illite decreases sharply, showing that the content of kaolinite is significantly higher than that of illite. Thus, the variation trend of the kaolinite and illite content indicates that the lower part was mainly affected by fresh water while the upper coal seam was affected by seawater transgression. This conclusion is consistent with the Sr/Ba value and sedimentary setting of No. 2 1 coal.

Conclusions
The petrology, mineralogy, and geochemistry of the No. 21 coal from the Hebi No. 6 mine, Anhe Coalfield, China, were reported in this paper, with an emphasis on the distribution, modes of occurrence, and origin of Li. The main conclusions drawn from this work are summarized as follows.  In addition, syngenetic and authigenic kaolinite in coal usually reflects the acidic conditions of the peat swamp [50]. The cell-filling kaolinite ( Figure 3C,F) found in coal samples HB-8 and HB-18, which have higher Li concentrations than those of the adjacent coal samples ( Figure 15A), indicates that the acidic conditions of the peat swamp may be favorable to the enrichment of Li ( Figure 15B). The acidic conditions of the peat swamp can be further supported by the Ti-bearing clays found in sample HB-8 ( Figure 9C,D). In the Junger-Hebaopian mining districts, Li in coals has also been found to be more enriched in acidic peat depositional environments [122].
The Sr/Cu and Rb/Sr ratios of fine-grained sediments are generally sensitive to climatic variations during depositional processes [123]). Coals with a Sr/Cu ratio of~1.3-5.0 were formed in humid conditions, while those with a Sr/Cu ratio >5.0 were formed in arid conditions [124]. In addition, the Rb/Sr ratio declines with increasing aridity and decreasing temperature [125,126]. Note that Sr/Ba values of >1 and <1 also indicate arid and humid climatic conditions, respectively [127][128][129], and this indication is probably based on the dominant occurrence of fresh water, where it occurs, with an Sr/Ba < 1 value corresponding to a humid climate [50]. The conclusion about peat environment deduced from these three factors mentioned above is consistent; that is to say, the floor, coal sample HB-23, four partings, and coal sample HB-18 (from bottom to top) were formed in humid conditions, while the samples from HB-17 to top were basically formed in arid conditions, which probably represent one wetting-upward and drying-upward coal cycle of No. 2 1 coal in the Anhe Coalfield, as mentioned by Li et al. [71]. (Figure 15A), Li enrichment mainly occurs in coals or non-coal samples formed in humid conditions ( Figure 15B).

Conclusions
The petrology, mineralogy, and geochemistry of the No. 2 1 coal from the Hebi No. 6 mine, Anhe Coalfield, China, were reported in this paper, with an emphasis on the distribution, modes of occurrence, and origin of Li. The main conclusions drawn from this work are summarized as follows.