mercurial:

[mer-kyoo r-ee-uh l]

–adjective

 

1.  changeable; volatile; fickle; flighty; erratic: a mercurial nature

2.  animated; lively; sprightly; quick-witted

3.  pertaining to, containing, or caused by the metal mercury

4.  of or pertaining to the god Mercury

5.  of or pertaining to the planet Mercury