Discovery of the largest natural carbon onions on Earth

• Published in: Science China. Earth sciences 2022-09, Vol.65 (9), p.1736-1750
• Main Authors: Chen, Yilin, Qin, Yong, Li, Jiuqing, Li, Zhuangfu, Yang, Tianyu, Lian, Ergang
• Format: Article
• Language: English
• Subjects: Carbon; Chemical synthesis; Chemical vapor deposition; Coal; Earth; Earth and Environmental Science; Earth Sciences; Electron microscopy; Fractures; Geological samples; High resolution electron microscopy; Intrusion; Metamorphism; Onions; Permian; Reflectance; Research Paper; Strata; Transmission electron microscopy; Vapors; Vegetables
• ISSN: 1674-7313; 1869-1897
• DOI: 10.1007/s11430-021-9951-x

The synthesis of carbon onions in the laboratory by various methods is common; however, naturally occurring carbon onions have only been found in a few geological samples on Earth. This study used high-resolution transmission electron microscopy to identify natural carbon onions in seven intrusion-affected coal samples collected from Permian coal-bearing strata in the Yongan Coalfield, Fujian Province, South China. This study identified the largest natural carbon onions ever recorded on Earth; their outer diameter was ∼55 nm. Granite porphyry intrusions and quartz hydrothermal veins are abundant in the Permian coal-bearing strata in this coalfield. All samples collected were tectonically deformed coals with highly developed structural fractures, friction mirror planes, and maximum vitrinite reflectance values of 4.0–9.5%. Natural carbon onions observed in the coal samples had single or multiple cores, with 24–46 graphitic shells characterized by outer diameters of 24–55 nm. The maximum vitrinite reflectance, outer diameter, and graphitic shell number of carbon onions in the intrusion-affected coal were positively correlated, indicating that the carbon onions were secondary products formed during coal metamorphism owing to magmatic intrusion. Our results suggest that carbon onions in intrusion-affected coal are synthesized by chemical vapor deposition. We speculate that natural carbon onions exist mainly in fractures or cavities, similar to vapor-deposited pyrolytic carbon, which is the leading cause of the uneven distribution of carbon onions in intrusion-affected coals in the study area.