## 4 Results
### Mineralogy  
Different depositional environments and geological processes affect the mineral composition and characteristics of coal. The XRD results (Figure 4) show that the main minerals in the No. 15 coal are kaolinite, NH\({}_{4}\)-illite, cookeite, quartz, calcite, pyrite, anhydrite, anatase, and rutile. Microscopy further confirms the presence of pyrite, calcite, and clay minerals (Figure 5). The distribution of pyrite in the coal seams varies. It is concentrated in the upper C1 and C2 layers and lower C8 and C9 layers. The pyrite morphologies in C1 and C2 are mainly framboidal (Figure 5A), veined (Figure 5B,F), and disseminated (Figure 5C), whereas in C8 and C9, the pyrite morphology is mainly massive (Figure 5H,I). In C8, pyrite filled the cell cavities (Figure 5G). Framboidal and massive pyrites typically form during peat accumulation or early diagenesis under the influence of seawater [40]. Conversely, disseminated and veined pyrites are generally associated with later epigenetic stages [39]. Interestingly, XRD analysis (Figure 4) did not detect pyrite in the C5 and C6 layers. The overall prevalence of pyrite in the coal indicated that the depositional environment was significantly influenced by reducing environment. The XRD analysis (Table 3) shows that the clay minerals in the coal include kaolinite, illite, NH\({}_{4}\)-illite, and cookeite (Figure 4E), among which kaolinite and NH\({}_{4}\)-illite occurred more frequently (Figure 4). The SEM results show that the kaolinite is vermicular (Figure 6C), and clay minerals are visible under the microscope, mainly filling the fissures and cell cavities. The calcite in the coal occurred in C2, C3, and C8; the calcite was primarily veined under the microscope and was of epigenetic origin according to the relationship between calcite and pyrite. Organic maceral fragments were also observed on the surface of the calcite veins (Figure 5E), and two group cleavages of the calcite were observed under the scanning electron microscope (Figure 6A). Figure 5: Pyrite and calcite in coal samples, (**A–F**) were taken with an oil immersion objective (Reflected light, 10 \(\times\) 50), and (**G–I**) were taken with dry objective (Reflected light, 10 \(\times\) 10). (**A**) Framboidal pyrite in C1; (**B**) Veined pyrite in C1; (**C**) Disseminated pyrite on the surface of calcite in C2; (**D**) Calcite veins and pyrite particle in C8; (**E**) Calcite with vitrinite on the surface in C4; (**F**) Veined pyrite in C2; (**G**) Filled pyrite in C8; (**H,I**) Massive pyrite in C9. tion peak, Si-O stretching vibration peak, OH bending vibration peak, Si(Al)-O stretching vibration peak, and Si(Al)-O bending vibration peak. The three absorption bands 3692, 3651, and 3620 cm\({}^{-1}\) belong to OH stretching vibration peaks, which may be caused by OH stretching vibration inside kaolinite and NH\({}_{4}\)-illite [45; 46; 47; 48; 49]. The absorption peak at 1580 cm\({}^{-1}\) is attributed to the skeleton vibration of the benzene ring. The presence of NH\({}_{4}\)-illite is further confirmed by the bending vibration peaks of NH\({}_{4}^{+}\) at 1432 cm\({}^{-1}\) and the weaker stretching vibration peaks of NH\({}_{4}^{+}\) at 3308 and 3040 cm\({}^{-1}\)[45; 46; 47; 48; 49; 50; 51]. The absorption peaks of 1095, 1033, and 1008 cm\({}^{-1}\) belong to Si-O stretching vibration, which may be caused by Si-O bond vibration in the tetrahedron of kaolinite, while the vibration peaks of 936 and 915 cm\({}^{-1}\) belong to in-plane and out-of-plane bending vibration of OH in octahedron [45; 49]. The absorption peaks of 790, 750, and 690 cm\({}^{-1}\) are caused by Si(Al)-O stretching vibration [49]. The absorption peaks at 537, 471, and 430 cm\({}^{-1}\) are caused by Si(Al)-O bending vibration [49]. In summary, kaolinite, NH\({}_{4}\)-illite, and other aluminosilicate minerals were present in the samples. The absorption peaks in 3692-3620 cm\({}^{-1}\) in C2 and C7 samples showed weak peaks, suggesting less kaolinite content [45].