Lan·tha·num n.  Chem. A rare element of the rare earth group of the metals, of atomic number 57, allied to aluminum.  It occurs in certain rare minerals, as cerite, gadolinite, orthite, etc., and was so named from the difficulty of separating it from cerium, didymium, and other rare earth elements with which it is usually associated. Atomic weight 138.9. Symbol La. [Formerly written also lanthanium.]

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