Seventeen elements that are not especially rare, but are rarely concentrated enough to mine economically.
A group of 17 metallic elements including the 15 lanthanides plus scandium and yttrium. Despite the name, rare earths are relatively abundant but difficult to extract economically. Used in magnets, electronics, catalysts, and defense applications.
The bottleneck is separation and refining, not geology. Separating adjacent rare earths is chemically difficult and capacity is heavily concentrated in China, so a Western deposit without a processing route solves only half the problem.
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