Occurrence State of Rare Earth Elements in the Paleo-Weathering Crust of the Tieling Formation, Northern Hebei Province by X-Ray Diffraction and Electron Probe Microanalysis

In recent years, the discovered paleo-weathering crustal rare earth ores have the advantages of stability and easy mining. However, due to the low degree of research on their element occurrence state, the study of the enrichment mechanism of the deposits is restricted. In this paper, the occurrence state of rare earth elements in the paleo-weathering crust of the Tieling Formation was systematically studied by using the techniques of stepwise chemical extraction, X-ray diffraction, heavy minerals identification, and electron probe microanalysis. Stepwise chemical extraction shows that rare earth elements in the paleo-weathering crust mainly exist in mineral phases, accounting for about 99.38% of the total content. The rare earth elements in aqueous solution, ionic phase, and colloidal deposition phase account for 0.01%, 0.22% and 0.39%, respectively; X-ray diffraction analysis shows that a small amount of rare earth elements in the ionic adsorption state in the paleo-weathering crust samples may exist on the surface of clay minerals such as illite; electron probe microanalysis results show that the anatase, leucoxene and barite in the paleo-weathering crust samples contain about 0.1% of light rare earth elements such as Ce, Nd and Sm. In summary, it is preliminarily concluded that the main rare earth elements in the paleo-weathering crust exist in mineral phases, some of them exist in anatase, leucoxene and barite as similar images, and the rest may exist on the surface of anatase and a large number of clay minerals in nano-scale fine particles. A very small amount of ionic rare earth elements may be adsorbed on the surface of clay minerals such as illite and illite-smectite mixed layer. The content of rare earth elements in the water-soluble phase and colloidal sedimentary phase is very low. The study on the occurrence state of rare earth elements in the ancient weathering crust helps to optimize development and utilization technology and provides a theory for the selection and smelting utilization of rare earth resources.