## 4 Discussion  
The fracture surfaces of coal samples were analyzed using SEM and EDS, with the results presented in Figures 11 and 12. In certain regions, mineral particles were observed forming a three-phase structure consisting of mineral particles, a coal matrix, and clay-type cementing material. These phases were compacted and bonded together, with original pores and fractures randomly distributed between them. Through SEM-EDS mapping analysis, which is shown in Figure 11c-h and Table 4, the mineral particles in the coal may be iron oxide, which could have undergone calcium cementation due to the influence of calcium-rich groundwater or hydrothermal fluids. Coal is a complex geologic material composed mainly of organic matter and mineral matter. Numerous studies have demonstrated that iron-bearing particles are extensively distributed within coal matrices [33; 34; 35; 36]. [PERSON] has proposed that the presence of ductile clay minerals enhances matrix compressibility, whereas inflexible brittle minerals exhibit resistance to matrix compression [37]. Recent research has further demonstrated that iron minerals exhibit a pronounced suppressive effect on the extension of cracks in coal [38]. Figure 10: CT reconstruction images after the fourth loading–unloading cycle. (**a–j**) The top views of ten regions along the Z-axis; (**k**) The side view of the sample; (**l**) The top view of the sample. Figure 11: SEM–EDS analysis of mineral particles. (**a**) SEM image; (**b**) Elements map; (**c**) Carbon element map; (**d**) Oxygen element map; (**e**) Calcium element map; (**f**) Magnesium element map; (**g**) Silicon element map; (**h**) Iron element map. Figure 12: Analysis of flocculent cementing material. (**a**) SEM image; (**b**) Elements map. The macroscopic mechanical properties of the coal samples were closely linked to their microscopic components and structures. Among the three phases, the hard mineral particles acted as aggregates, while the clay fillings served as the cementing material. In uniaxial cyclic loading-unloading experiments, the failure strengths of three coal samples were 15.98 MPa, 14.42 MPa, and 13.65 MPa, respectively. The initial number of CT-scale mineral particles was 30,185, 23,593, and 18,414, respectively. A higher number of mineral particles correlated with greater macroscopic mechanical strength. The influence of mineral particles on crack propagation was dependent on both their quantity and volume. In their initial state, the coal sample contained 30,185 mineral particles, with volumes ranging from 1.26 \(\times\) 10\({}^{-4}\) to 2.98 mm\({}^{3}\), predominantly below the 10\({}^{-2}\) mm\({}^{3}\) level. After the fourth loading-unloading cycle, the number of mineral particles increased to 30,721, a net increase of 536, due to cracks penetrating minerals and dividing them into smaller fragments. As shown in Table 5 and Figure 13, particle volume distribution revealed that the number of particles in the 1-10 mm\({}^{3}\) range remained unchanged, while the number of particles in the 0.1-1 mm\({}^{3}\) range decreased. Conversely, particles in the 10\({}^{-2}\)-10\({}^{-1}\) mm\({}^{3}\) range and smaller increased significantly. This trend indicates that larger particles were fragmented into smaller ones during crack propagation, with smaller particles more susceptible to transgranular fracturing. From a microscopic perspective, iron-rich mineral particles influenced crack propagation through mechanisms such as deflection toughening and blunting toughening, which increased crack propagation resistance and hindered further expansion. 1. Deflection Toughening: Iron ore particles and the surrounding matrix exhibit differing stiffness and elastic moduli. Under cyclic loading and unloading, asynchronous deformation at their interfaces leads to the formation of numerous micron-level secondary cracks. These cracks, when intersecting the propagation paths of larger cracks, cause the larger to deflect and follow tortuous paths.