Aegirine is a member of the clinopyroxene group of inosilicate minerals, the sodium endmember of the aegirine-augite series. The chemical formula NaFeSi2O6 in which the iron is present as Fe3+. Acmite is a fibrous, green-colored variety.

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.

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. This stone  named after the Norse god of the sea. A synonym for the mineral is acmite in reference to the typical pointed crystals.

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