The Collaborative International Dictionary of English v.0.48:

lanthanon \lanthanon\ n.
   Same as lanthanide.

   Syn: rare earth, rare-earth element, lanthanide.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lanthanon
    n 1: any element of the lanthanide series (atomic numbers 57
         through 71) [syn: rare earth, rare-earth element,
         lanthanoid, lanthanide, lanthanon]