ACMITE or AEGIRITE, a mineral of the pyroxene (q.v.) group, essentially a sodium and ferric metasilicate, NaFe(SiO3)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 characteis ; 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 6-63-, 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. It is isomorphous with augite; mixed crystals are not uncommon.
The mineral is a characteristic constituent of igneous rocks rich in soda, such as nepheline-syenites, phonolites, etc. 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. IcK,u7), a point, in allusion to the pointed terminations of the crystals. Acmite forms an important constituent of a granitic rock (rockallite) occurring on the islet of Rockall in the North Atlan tic, and is notable for its content of zirconia 2.68%. Aegirite (named from Aegir, the Scandinavian sea-god) was described in 1835 from the elaeolite-syenite of southern Norway. The essential identity of acmite and aegirite has long been established.