ACMITE, or AEGIRITE, a mineral of the pyroxene (q.v.) group, which may be described as a soda-pyroxene, being 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 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 deg. -5 deg. ). (The hardness is 6-6 1/2, 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 aegirine-diopside.
Acmite 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 Kongsbeig in Rorway, and was named by F. Stromeyer in 1821 from the Gr. akme, 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 pethaps in more general use, especially among petrologists.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)