In rock-forming minerals, REE occurs as trivalent cations in carbonates, oxides, phosphates, and silicates ( Battsengel et al., 2018). For comparative analysis, the measured REE concentrations can be normalized to some REE reference values, such as the Post-Archean Australian Shale (PAAS) ( De Campos and Enzweiler, 2016). The distribution of REE follows the Oddo-Harkins rule, which states that elements with even atomic numbers are more abundant in the Earth's crust than those adjacent odd-numbered ones ( Nikanorov, 2016). Individually, their abundance in the Earth's crust ranges from 0.14В ppm (Eu) to 70В ppm (Ce and Y) ( Ganguli and Cook, 2018). REEs are usually poorly soluble in natural ecosystems and present low contents in uncontaminated freshwaters ( Gonzalez et al., 2014). da Silva, in Emerging Freshwater Pollutants, 2022 17.2.1 Geochemistry of REE in freshwaters

It is important that all REE exploration projects and mines adopt best practice, thereby encouraging the continued use of REE. The environmental record of many REE mining and processing sites has been poor, and REE mining accordingly gained a bad reputation. After upgrading the ore minerals, cracking (dissolution) of the ore and separating the individual REE from each other are expensive, chemical-intensive processes. Many deposits have intricately intergrown minerals often minerals that have not been extracted commercially. Process mineralogy is a prime concern for REE ores. Owing to its smaller cation size, Sc concentrates in different minerals, such as clinopyroxene and is produced as a by-product of uranium deposits, nickel laterites, and from aegirine at Bayan Obo, China. REE are usually mined by conventional open pit methods, except for the ion adsorption clays in China that are now usually mined by in-situ leaching. Ion adsorption clays, in which the REE are adsorbed to clay particle surfaces and released by leaching, are the most important sources of heavy REE. Other mines produce loparite from nepheline syenite and monazite from mineral sand placer deposits. Most working REE mines are in carbonatite-related deposits, usually where initial mantle derived and magmatic enrichments of light REE have been further upgraded by metamorphic, hydrothermal or weathering processes. The crustal abundance of light REE about the same as copper and even the less abundant heavy REE are much more abundant than precious metal such as gold. They have risen to prominence because production from mining through to manufacturing is dominated by China, making REE the epitome of critical raw materials. Their magnetic and luminescent properties make them essential components of digital and low carbon technologies. The rare earth elements (REE) consist of 17 elements: Sc, Y and the lanthanoids LaвЂ“Lu. Frances Wall, in Encyclopedia of Geology (Second Edition), 2021 Abstract

The meaning of earth as element