The Collaborative International Dictionary of English v.0.48:

Lavoesium \La*v[oe]"si*um\, n. [NL., fr. Lavoisier, the
   celebrated French chemist.] (Chem.)
   A supposed new metallic element, which was said to have been
   discovered in pyrites, and some other minerals, and to be of
   a silver-white color, and malleable. It is not currently
   (1998) a recognized element.
   [1913 Webster +PJC] Lavolt