## 4 Results and Discussion
### Mineralogical Characteristics  
The coal mineral composition in the study area is dominated by calcite, pyrite, and clay minerals. Optical microscopy reveals diverse pyrite morphologies, including euhedral \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Coal Seam** & \(\mathbf{R_{\text{o,max}}}\) & \(\mathbf{M_{\text{ad}}}\) & \(\mathbf{A_{\text{d}}}\)/\% & \(\mathbf{V_{\text{d}af}}\) & \(\mathbf{FC}\) & \(\mathbf{S_{\text{t,d}}}\) & \(\mathbf{Q_{\text{gr,ad}}}\) \\ \hline \multirow{2}{*}{No. 7} & 0.67–0.85 & 0.52–1.90 & 7.15–20.58 & 35.51–43.38 & 47.61–59.06 & 0.17–2.36 & 26.79–32.40 \\ \cline{2-8}  & 0.69 & 1.28 & 12.49 & 38.48 & 53.59 & 0.70 & 30.13 \\ \hline \multirow{2}{*}{No. 8} & 0.66–0.87 & 0.18–1.76 & 7.19–16.57 & 35.16–40.60 & 51.46–58.43 & 0.10–2.03 & 28.48–31.80 \\ \cline{2-8}  & 0.72 & 1.13 & 11.06 & 37.93 & 54.98 & 0.93 & 30.44 \\ \hline \end{tabular}
\end{table} 
Table 1: Statistical table of coal quality indicators of Shanxi Formation in Northern Jiangsu region. Figure 3: Correlation diagram of coal quality parameters. and subhedral fine-grained crystals, framboidal aggregates, fracture infillings, irregular blocky forms, and disseminated particles. Euhedral and subhedral pyrite grains (1-20 \(\upmu\)m) occur alongside calcite within fractures (Figure 4a). Fracture-filling pyrite, forming strips 0.1-2.0 \(\upmu\)m wide, lines coarse-grained mineral surfaces (Figure 4b), while oxidized filamentous and semi-filamentous pyrite infills fractures (Figure 4c,d), a morphology linked to late-stage coalification processes [42]. Irregular blocky pyrite (2-30 \(\upmu\)m), characterized by smooth surfaces with minor pitting, commonly infills pores and fractures in association with calcite (Figure 4e). Disseminated pyrite (1-5 \(\upmu\)m) is predominantly hosted within calcite (Figure 4f), with localized nodular clusters observed in matrix vitrinite (Figure 4g), reflecting the selective affinity between gel components and pyrite [43; 44]. Calcite occurs as veins or blocky masses (5-60 \(\upmu\)m wide), infilling homogeneous and matrix vitrinite (Figure 4a,e-h). Clay minerals appear as granular or striated inclusions within vitrinite, with some exhibiting a roe-like texture (1-6 \(\upmu\)m) embedded in vitrinite matrices (Figure 4i). Meanwhile, coal quality parameters are closely related to mineralogy; for example, the thermal decomposition of calcite will lead to an increase in the yield of volatile matter in coal quality [45]. Scanning electron microscopy with energy-dispersive spectroscopy reveals stacking and infilling of 3 \(\upmu\)m-diameter pores within massive iron dolomite (Figure 5a). Fractures in the coal seam are occupied by band-like and irregular blocky calcite (Figure 5b,f), while cylindrical calcite fills pores measuring 3-15 \(\upmu\)m in diameter (Figure 5h). Long columnar gypsum is observed within organic matter (Figure 5c). Granular pyrite (2-20 \(\upmu\)m) is  
Figure 4: Microscopic photograph of minerals in coal. (**a**) Kongzhuang Coal Mine (KZ-W), No. 7 coal seam; (**b**) Kongzhuang Coal Mine (KZ-W), No. 7 coal seam; (**c**) Kongzhuang Coal Mine (KZ-W), No. 7 coal seam; (**d**) Xuzhuang Coal Mine (XZ-W), No. 8 coal seam; (**e**) Xuzhuang Coal Mine (XZ-W), No. 7 coal seam; (**f**) Xuzhuang Coal Mine (XZ-W), No. 7 coal seam; (**g**) Xuzhuang Coal Mine (XZ-W), No. 7 coal seam; (**h**) Xuzhuang Coal Mine (XZ-W), No. 7 coal seam; (**i**) Xuzhuang Coal Mine (XZ-W), No. 7 coal seam; ID, Iron dolomite; Ca, Dolomite; Gy, Gypsum; Py, Pyrite; Ka, Kaolinite; Do, Dolomite; Ma, Macrinite; Of, Oxyfusinite; Vi, Vitrinite. embedded within vitrinite, exhibiting octahedral, tetrahedral, and transitional crystal habits (Figure 5d,g). Fine-grained aggregated pyrite (1-10 \(\upmu\)m) is hosted in kaolinite, with irregular blocky pyrite infilling coal fractures (Figure 5e). Agglomerated and disseminated kaolinite occur within vitrinite or pore spaces (Figure 5d,f,g,i), while columnar dolomite occupies 10-20 \(\upmu\)m pores (Figure 5i).