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From The Collaborative International Dictionary of English v.0.48:
Gadolinium \Gad`o*lin"i*um\, n. [NL. See Gadolinite.] (Chem.) A rare earth metallic element of the Lanthanide series, with a characteristic spectrum, found associated with yttrium and other rare earth elements. Symbol, Gd; it has an atomic number of 64, an atomic weight of 157.25 (C=12.011), and a valence of +3. [1913 Webster +PJC]