Yes, MIXTURES is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, X 8, T 1, U 1, R 1, E 1, S 1.

Is it a valid word?

MIXTURES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(chemistry) a substance consisting of two or more substances mixed together (not in fixed proportions and not with chemical bonding)
any foodstuff made by combining different ingredients
a collection containing a variety of sorts of things
an event that combines things in a mixture
the act of mixing together

Also: mixed bag.