Objective For vitrinite with weak anisotropy, which proves uniaxial and optically negative, replacing random vitrinite reflectance (R_ran) with the maximum vitrinite reflectance (R_max) allows for the accurate correction of vitrinite reflectance suppression induced by anisotropy. However, as its anisotropy intensifies, vitrinite exhibits increasingly complex optical properties, rendering it challenging to reflect the thermal metamorphic temperature of vitrinite using R_max.

Methods This study investigated the strongly anisotropic vitrinite in coals and natural coke subjected to magma intrusion in the Shitai coal mine, Huaibei City, Anhui Province. Using a reflected-polarized light microscope (R-PLM), an ultra-high-definition scanning electron microscope (SEM), and tests on R_max and the minimum vitrinite reflectance (R_min), this study analyzed the microscopic textures, deep microfabrics, and optical properties of the vitrinite. Accordingly, the microscopic optical mechanisms underlying vitrinite reflectance suppression were explored.

Results The strongly anisotropic vitrinite, formed by the contact metamorphism of magmas, contains flow-texture carbon and mosaic carbon, both composed of lamellar microfabrics. The lamellae in the carbon were uniaxial and optically negative (N_o > N_e), with the long axis N_o of the indicatrix parallel to the lamella surface and its short axis N_e perpendicular to the surface. The vitrinite exhibited the optical properties of pseudo-biaxial crystals in Kilby’s cross plot. The pseudo-biaxial crystals were formed by lamella folding. The underlying reason is that lamella folding changed the orientation of the lamellae’s reflected optical indicatrix (ROI) despite minimally influencing its morphology.

Conclusions Lamella folding represents the direct cause of reflectance suppression in strongly anisotropic vitrinite. The Kilby’s cross plot serves as an effective method used to determine whether strong anisotropy induces vitrinite reflectance suppression. Replacing the average reflectance with R_max corresponding to the maximum bireflectance (ΔR_o) can correct reflectance suppression. This will help accurately determine the metamorphic grade of coals formed by magma intrusion-induced contact metamorphism through inversion.