Mineral box (4)
I believe the answer is:
spar
I believe this is a double definition.
'mineral' is the first definition.
(I know that spar is a type of mineral)
'box' is the second definition.
(spar can mean to box)
(Other definitions for spar that I've seen before include "Box; various minerals" , "Stout pole, nautically" , "Act as boxer" , "microcrystalline mineral?" , "Mineral; strong pole" .)