## 5 Discussion
### Occurrence Modes of Li in Coal  
Although the content of lithium in coal in this paper is low (average 66.59 \(\upmu\)g/g), and does not reach the minimum mining grade (80 \(\upmu\)g/g) and industrial grade (120 \(\upmu\)g/g) of Chinese coals determined by [PERSON] et al. [4], the study of the occurrence modes of lithium in coal has important theoretical significance for exploring the enrichment and metallogenic mechanism of Li in coal. Many studies have demonstrated that lithium is an active element. During surface weathering, elements can easily migrate from the parent rock into clay minerals and be adsorbed, and lithium in coal occurs typically in clay minerals. For example, Li\({}^{+}\) in hectorite and cookeite can enter into Si-O tetrahedron or Al-O octahedron by replacement, whereas kaolinite cannot only replace Li\({}^{+}\) but also adsorb part of Li via ion adsorption [12; 17; 18; 19; 106]. Lithium in coal is generally associated with aluminosilicate minerals (e.g., clay minerals or mica) as well as with organic matter or acid-insoluble substances [2; 16; 107; 108; 109]. To study the occurrence modes of Li in Gaoping No. 15 coal, we used SPSS 27.0 software  
Figure 14: (**A**) Relationship between Li and the (CaO+MgO+Fe\({}_{2}\)O\({}_{3}\))/(SiO\({}_{2}\)+Al\({}_{2}\)O\({}_{3}\)) (C-value) ratio; (**B**) Relationship between Li and Sr/Ba ratio. to calculate the pearson correlation coefficient between the content of lithium and ash, major element, maccral, clay mineral content, and SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) molar ratio. Correlation analysis between Li content and ash yield and clay minerals shows that Li is well correlated with them (r\({}_{\text{Li-Ad}}\) = 0.58, r\({}_{\text{Li-Clay minerals}}\) = 0.66), indicating that Li exhibits a good inorganic affinity and mainly occurs in clay minerals. This result (Figure 15) shows that Li is highly correlated with Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and SiO\({}_{2}\) (r\({}_{\text{Li-SiO2}}\) = 0.72, r\({}_{\text{Li-Al2O3}}\) = 0.78, and r\({}_{\text{Li-TiO2}}\) = 0.56), suggesting that Li has an aluminosilicate affinity, especially in Ti-bearing minerals, because Ti may be leached out of detrital components under low-pH conditions [9; 64] and resettle in minerals, resulting in a positive correlation between Ti and Li content. The occurrence of rutile and anatase was found in the XRD and SEM analyses (Figure 4C,D, and Figure 6D) and the positive correlation between TiO\({}_{2}\) and Li content(Figure 15D), indicating that Li may occur in titanium-bearing minerals such as rutile and anatase. The simultaneous occurrence of rutile and anatase suggests that the geochemical composition is significantly influenced by the depositional environment. The content of Li in coal has a good correlation with K\({}_{2}\)O and P\({}_{2}\)O\({}_{5}\), indicating that Li also occurs in illite and phosphate minerals. When Li shows a strong negative correlation with the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) molar ratio, it may be associated with one or more Al-bearing mineral phases, such as kaolinite and chlorite group minerals [110; 111; 112]. The absorption peaks of 15P and 15F samples at 3692-3620 cm\({}^{-1}\) and 1095-915 cm\({}^{-1}\), respectively were obvious (Figure 7), indicating that there was more kaolinite in the samples. Table 5 shows that the Li content of 15P and 15F samples is higher than that of other samples, indicating that Li occurs in kaolinite. The moderate negative correlation between Li and the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) molar ratio (r = \(-\)0.55, Table 8) confirms that Li occurs in different Al-bearing mineral phases. Some studies have shown that the main Li-bearing mineral in the coal of the Qinshui Coalfield is cookeite [9; 12; 113]. The XRD patterns for the C7 sample also confirmed the discovery of cookeite (Figure 4E), suggesting that cookeite is a Li-carrier mineral in the No. 15 coal.