Aegirine is a member of the clinopyroxene group of inosilicate minerals. Aegirine is the sodium endmember of the aegirine-augite series. Aegirine has the chemical formula Na 6 in which the iron is present as Fe3+. In the aegirine-augite series the sodium is variably replaced by calcium with iron(II) and magnesium replacing the iron(III) to balance the charge. Aluminium also substitutes for the iron(III). Acmite is a fibrous, green-colored variety.
Aegirine occurs as dark green monoclinic prismatic crystals. It has a glassy luster and perfect cleavage. Its Mohs hardness varies from 5 to 6, and its specific gravity is between 3.2 and 3.4.
This mineral commonly occurs in alkalic igneous rocks, nepheline syenites and pegmatites schists , and iron formations; in blueschist facies rocks, and from sodium metasomatism. It may occur as an authigenic mineral in shales. It occurs in association with potassic feldspar , riebeckite, astrophyllite , eudialyte
Localities include Mont Saint-Hilaire , Canada; Kongsberg; Kola Peninsula , Russia; Magnet Cove ; Scotland.
The acmite variety was first described in 1821, at Kongsberg, Norway, and the aegirine variety in 1835 for an occurrence in Rundemyr, Øvre Eiker, Buskerud, Norway. Aegirine was named after Ægir , the Norse A synonym for the mineral is acmite (from Greek ἀκμή "point, edge") in reference to the typical pointed crystals.
It is sometimes used as a gemstone.