Aegirine is an inosilicate member of the clinopyroxene group. Aegirine is the sodium endmenber of the aegirine-augite series. Aegirine has the chemical formula NaFeSi2O6 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). It is also known as acmite.

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

It occurs in alkali nepheline syenites and similar igneous rocks. Localities include Mont Saint-Hilaire, Quebec, Canada; Kongsberg, Norway; Narsarssuk, Greenland; Kola Peninsula, Russia; Magnet Cove, Arkansas, USA; Kenya; Scotland and Nigeria.

Aegirine was named after Ægir, the Teutonic god of the sea. A synonym for the mineral is acmite (from Greek) in reference to the typical pointed crystals.