Geochemistry of scandium in pelagic sediments from the Pacific Ocean: Implication to the resource assessment of deep-sea REE-rich mud

With the rapid development and wide use of fuel cells and scandium (Sc)-aluminum (Al) alloy, the demand for Sc may increase significantly in the future. Due to the lack of independent deposits, Sc is mainly derived from comprehensive utilization at present, which may cause a potential resource risk for the green economy in the future. It has been reported that deep-sea rare earth elements (REE)-rich mud is also enriched in Sc, but the geochemical properties of Sc in pelagic sediments are still poorly understood. In this study, bulk sediments geochemical data of China Ocean Mineral Resources R&D Association cruises at the Western Pacific and the Clarion-Clipperton Fracture Zone combined with in-situ chemical composition of fish teeth (ichthyoliths or bioapatite) and ferromanganese micronodules are used to reveal the geochemical characteristics of Sc in pelagic sediments. The frequency distribution of Sc contents in sediments from the Western Pacific and the CCFZ showed regional variability, with median values of ∼19 ppm and ∼ 24 ppm, respectively. Scandium is commonly enriched in REE-rich sediment layers unassociated with hydrothermal activity, with a median content of ∼38.4 ppm. The Sc contents of the pelagic clay are generally higher than that of calcareous ooze and siliceous ooze. Compared to REE-rich sediments, Sc is concentrated in bioapatites (up to ∼250 ppm) but deficient in micronodules (∼2–20 ppm). Based on the in-situ geochemical results and the correlations between Sc and other major elements, clay minerals and bioapatites are likely to be the main Sc host phases in deep-sea sediments. Considering the high commercial price of Sc and its highest value proportion (>20 %) among REEs, Sc should not be ignored in the resource assessment of REE-rich sediments.