ACHITE, or AEGIRITE  , a mineral of the pyroxene (q.v.) group, Which may be described as a soda-pyroxene, being essentially asodium and ferric metasilicate, NaFe(SiOs)2 . In its crystallographic characters it is close to ordinary pyroxene (augite and diopside), being monoclinic and having nearly the same angle between the prismatic cleavages . There are, however, important differences in the optical characters: the birefringence of acmite is negative, the pleochroism is strong and the extinction angle on the plane of symmetry measured to the vertical axis is small (3°-5°) . The hardness is 6-0, and the specific gravity 3'55 . Crystals are elongated in the direction of the vertical axis, and are blackish green (aegirite) or dark brown (acmite) in colour . Being isomorphous with augite, crystals intermediate in composition between augite or diopside and aegirite are not uncommon, and these are known as aegirine-augite or aegirinediopside . Acmite is a characteristic constituent of igneous rocks rich in soda, such as nepheline-syenites, phonolites, &c . It was first discovered as slender crystals, sometimes a foot in length, in the pegmatite veins of the granite of Rundemyr, near Kongsberg in Norway, and was named by F . Stromeyer in 1821 from the Gr. aKo, a point, in allusion to the pointed terminations of the crystals . Aegirite (named from Aegir, the Scandinavian sea-god) was described in 1835 from the elaeolite-syenite of southern Norway . Although exhibiting certain varietal differences, the essential-identity of acmite and aegirite has long been established, but the latter and more recent name is perhaps in more general use, especially among petrologists .