Seventeen elements-the 15 lanthanides plus scandium and yttrium-comprise the category of rare earth metals, as defined by the International Union of Pure and Applied Chemistry (lUPAC) (Figure 1). In earlier definitions, scandium and/or yttrium weresometimes excluded. The amount of rare earth metals within the earth is between those of base metals and precious metals. Their similar properties and occurrence together with other elements (e.g., niobium, uranium, and thorium) make their mining, separation, and purification both complex and expensive.
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