HYPERSTHENE  , a

rock-forming mineral belonging to the group of orthorhombic pyroxenes . It differs from the other members (enstatite [q.v.] and bronzite) of this group in containing a considerable amount of iron replacing magnesium: the chemical formula is (Mg,Fe)SiO3 . Distinctly developed crystals are rare, the mineral being usually found as foliated masses embedded in those igneous rocks—norite, hypersthene-andesite, &c.—of which it forms an essential constituent . The coarsely grained labradorite-hypersthene-rock (norite) of the island of St Paul off the coast of Labrador has furnished the most typical material; and for this reason the mineral has been known as " Labrador hornblende " or paulite . The colour is brownish-black, and the plbochrism strong; the hardness is 6, and the specific gravity 3.4-3.5 . On certain surfaces it displays a brilliant copper-red metallic sheen or Schiller; which has the same origin as the bronzy sheen of bronzite (q.v.), but is even more pronounced . Like bronzite, it is sometimes cut and polished for ornamental purposes . (L . J .