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 La–Lu. Frances Wall, in Encyclopedia of Geology (Second Edition), 2021 Abstract
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