Response of Macromolecular Structure to Deformation in Tectonically Deformed Coal

The structural evolution of tectonically deformed coals (TDC) with different deformational mechanisms and different deformational intensities are investigated in depth through X‐ray diffraction (XRD) analysis on 31 samples of different metamorphic grades (R_{o, max}: 0.7%–3.1%) collected from the Huaibei coalfield. The results indicated that there are different evolution characteristics between the ductile and brittle deformational coals with increasing of metamorphism and deformation. On the one hand, with the increase of metamorphism, the atomic plane spacing (d₀₀₂) is decreasing at step velocity, the stacking of the BSU layer (L_c) is increasing at first and then decreasing, but the extension of the BSU layer (L_a) and the ratio of L_a/L_c are decreasing initially and then increasing. On the other hand, for the brittle deformational coal, d₀₀₂ is increasing initially and then decreasing, which causes an inversion of the variation of L_c and L_a under the lower‐middle or higher‐middle metamorphism grade when the deformational intensity was increasing. In contrast, in the ductile deformational coals, d₀₀₂ decreased initially and then increased, and the value of L_c decreased with the increase of deformational intensity. But the value of L_a increased under the lower‐middle metamorphism grade and increased at first and then decreased under the higher‐middle metamorphism grade. We conclude that the degradation and polycondensation of TDC macromolecular structure can be obviously impacted during the ductile deformational process, because the increase and accumulation of unit dislocation perhaps transforms the stress into strain energy. Meanwhile, the brittle deformation can transform the stress into fractional heat energy, and promote the metamorphism and degradation as well. It can be concluded that deformation is more important than metamorphism to the differential evolution of the ductile and brittle deformational coals.