Abstract
Scandium (Sc), which has a high melting point and a low density, is widely used in lighting, alloys, ceramics, catalysts, and other industrial fields. Under a specific geological condition, Sc could be enriched in certain coal. As one of strategic metals hosted in coals, the geochemistry of Sc is a hot topic in coal geology. In order to clarify the geochemical characteristics of Sc in Chinese coals, based on a comprehensive collection of the data of Sc concentrations in Chinese coals and four cases of the Huainan coals of Anhui, the Nantong coals of Chongqing, the Baoding coals of Sichuan, and the Mengtuo coals of Yunnan, the content, temporal and spatial distribution, modes of occurrence, and enrichment mechanism of Sc in Chinese coals were discussed, and the Sc resources hosted in coal measures was assessed as well. Results indicated that the scandium in Chinese coals was at normal level with an average of 4.02 mg/kg, which was slightly higher than that of world hard coal. The concentrations of scandium in certain coals from Guizhou, Chongqing, Anhui, Guangxi, Inner Mongolia, Shanxi, and Yunnan were elevated, and with the maximum Sc concentration of 139 mg/kg. Regarding a coalbed, Sc was preferably enriched in country rocks, e.g., the roof, floor, and parting rocks, relative to coal benches. Most Sc-rich Chinese coals were bituminous and with a coal-forming period of Permian, while the Late Jurassic - Early Cretaceous coals were of a low Sc content. Overall, due to a high mineral matter content in the high-ash and sapropelic coals, its Sc concentrations increased. As a lithophilic element, Sc in coals was dominantly hosted in minerals, especially silicate minerals, e.g., kaolinite, and some authigenic minerals, e.g., xenotime. Sc was also partially bounded to organic matter, especially the humic matter in low rank coals. The genetic types of Sc enrichment in Chinese coals included: ① Mafic source rock type. As a compatible element, Sc was of a higher concentration in mafic rocks than that of intermediate and felsic ones. Correspondingly, the weathering of mafic source rock could lead to a Sc enrichment in certain coal; ② Paralic facies type. Due to the marine transgression, the increasing pH value of peat bogs with a large amount of humic acid facilitated the migration of Sc and its combination with organic matter; ③ Groundwater leaching type. The leaching of Sc-bearing minerals by groundwater resulted into a local Sc enrichment in coal benches of a coalbed; ④ Hydrothermal solution type. Hydrothermal solutions leached and/or transported Sc into a coal via a mineral precipitation; ⑤ Magmatic and metamorphic type. Igneous intrusion led to a rise of coal rank, resulting in a reduce of organic matter bound Sc. Meanwhile, Sc hosted by minerals in coals might be enriched due to the increased ash yields; ⑥ Volcanic ash type. Mafic tuff layers or tonsteins occurred in coal measures could provide the coalbeds with Sc.
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