Characteristics and transformations of mineral phases in burnt rocks

Objective Burnt rocks are special rocks formed by the surrounding rocks that are baked due to the spontaneous combustion of coal seams. They are extensively distributed in the Yan’an Formation on the northeastern margin of the Ordos Basin. However, the characteristics and formation mechanisms of burning-associated mineral phases in these rocks remain unclear.

Methods This study conducted analytical tests and high-temperature heating simulations of protolith to analyze the mineral phases of underground burnt rocks in the Zhangjiamao Coal Mine, Shaanxi Province.

Results and Conclusions The results indicate that the underground burnt rocks exhibit enrichment in mafic components, abnormal migration of trace elements like Zn, Rb, Sr, and Zr compared to protoliths, outward migration of rare earth element (REE) Eu, and a La/Yb-∑REE distribution pattern closer to that of granites. Microscopic observations reveal that the burnt rocks feature chloritization, magnetization, and hematitization, as well as palimpsests and mineral melting, with the presence of high-temperature metamorphic crystalline minerals such as cristobalite, tridymite, mullite, and cordierite. Primary metamorphism types of the burnt rocks include recrystallization and melting, while the metasomatic alteration (or assimilation and contamination) observed in typical thermally metamorphosed rocks and hydrous or hydroxyl-bearing minerals are absent. This characteristic may be due to the fact that the burning process occurred in a near-surface low-pressure and open system, where no H₂O fluids acted on the material exchange between minerals within the metamorphism system. Based on the burning conditions and mineral assemblages, it can be inferred that the burning process is a special metamorphism type different from other thermal metamorphisms. In combination with previous results, this study proposes that the burnt rocks are characterized by special contact thermal metamorphism, which can be divided into four stages based on metamorphic temperatures: low-temperature dehydration(chloritization and the destruction of hydrous minerals), moderate-temperature oxidation (magnetization and hematitization), high-temperature melting (mullite, cordierite, cristobalite, and tridymite), and complete melting (pyroxene and pseudo-vitreous feldspar). This study systematically summarizes the mineral phases and their transformations in burnt rocks, providing a summary of the metamorphic facies series of burnt rocks based on the analysis of the metamorphic rock facies series and their metamorphic conditions. It can be concluded that temperature and a near-surface low-pressure and open system are the causes of the formation of different burnt rocks.