HYPERSTHENE, a rock-forming mineral belonging to the group of orthorhombic pyroxenes. It differs from the other members (enstatite and bronzite) of this group in containing a considerable amount of iron replacing magnesium; the chemical formula is Distinctly developed crystals are rare, the mineral being usually found as foliated masses embedded in those igneous rocks—norite, hypersthene-andesite, etc.—of which it forms an essential constituent. The coarsely grained labradorite-hypersthene-rock (norite) of St. Paul island, Labra dor, has furnished the most typical material. The colour is brownish-black, and the pleochroism strong; the hardness is 6 and specific gravity