Source : Webster's Revised Unabridged Dictionary (1913)

Heavy spar \Heav"y spar`\ (Min.)
   Native barium sulphate or barite, -- so called because of its
   high specific gravity as compared with other non-metallic
   minerals.

Source : WordNet®

heavy spar
     n : a white or colorless mineral (BaSO4); the main source of
         barium [syn: {barite}, {barytes}, {barium sulphate}]