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In page Erbium:

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The concentration of erbium in the Earth crust is about 2.8 mg/kg and in seawater 0.9 ng/L.[1] (Concentration of less abundant elements may vary with location by several orders of magnitude[2] making the relative abundance unreliable). Like other rare earths, this element is never found as a free element in nature but is found in monazite and bastnäsite ores.[3] It has historically been very difficult and expensive to separate rare earths from each other in their ores but ion-exchange chromatography methods[4] developed in the late 20th century have greatly reduced the cost of production of all rare-earth metals and their chemical compounds.[citation needed]