What is "aegirine"

n. (context mineral English) A mineral with monoclinic crystals with the chemical formula sodiumiron³⁺(silicon₂oxygen₆) belonging to the pyroxene group.

Aegirine is a member of the clino pyroxene group of inosilicates. Aegirine is the sodium endmember of the aegirine- augite series. Aegirine has the chemical formula Na Fe Si O in which the iron is present as Fe. 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). It is also known as acmite, which is a fibrous, green-colored variety.

Aegirine occurs as dark green monoclinic prismatic crystals. It has a glassy luster and perfect cleavage. The Mohs hardness varies from 5 to 6 and the specific gravity is 3.2 to 3.4.

Commonly occurs in alkalic igneous rocks, nepheline syenites, carbonatites and pegmatites. Also in regionally metamorphosed schists, gneisses, and iron formations; in blueschist facies rocks, and from sodium metasomatism in granulites. It may occur as an authigenic mineral in shales and marls. It occurs in association with potassic feldspar, nepheline, riebeckite, arfvedsonite, aenigmatite, astrophyllite, catapleiite, eudialyte, serandite and apophyllite.

Localities include Mont Saint-Hilaire, Quebec, Canada; Kongsberg, Norway; Narsarssuk, Greenland; Kola Peninsula, Russia; Magnet Cove, Arkansas, US; Kenya; Scotland and Nigeria.

It was first described in 1835 for an occurrence in Rundemyr, Øvre Eiker, Buskerud, Norway. Aegirine was named after Ægir, the Teutonic god of the sea. 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.