White gypsum and basalt are mixed (9)
I believe the answer is:
alabaster
'white gypsum' is the definition.
(I've seen this before)
'basalt are mixed' is the wordplay.
'mixed' is an anagram indicator.
'basalt'+'are'='basaltare'
'basaltare' with letters rearranged gives 'ALABASTER'.
'and' is the link.
(Other definitions for alabaster that I've seen before include "Type of gypsum" , "Blast area for white mineral" , "Altar base (anag)" , "statue material" , "White substance" .)