Volatile or lively (9)

I believe the answer is:

mercurial

I believe this is a double definition.

'volatile' is the first definition.
(I've seen this before)

'lively' is the second definition.
(I've seen this before)

(Other definitions for mercurial that I've seen before include "Volatile" , "lively" , "rapidly changing" , "Changing unpredictably" , "Sprightly" .)

I've seen this clue in The Mirror.

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