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The abundances of the Lanthanide or rare earth elements ( REE ) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled.

In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly.

It arises because Eu < sup > 2 +</ sup > can substitute for Ca < sup > 2 +</ sup > in plagioclase feldspar, unlike any of the other Lanthanides, which tend to only form 3 + cations.