(from Aegir, the Scandinavian god of the sea), also aegirine, a rock-forming mineral, a silicate of the group of monoclinic pyroxenes, with the chemical composition NaFe[Si₂O₆]. It contains admixtures of Mn, Zn, Ti, Nb, Zr, and V. As a result of the isomorphism Na⁺ ⇆ Ca²⁺ accompanied by the replacement of Fe³⁺ by (Mg, Fe)²⁺ or Si⁴⁺ by Al³⁺, transitions are observed to aegirite-diopside, aegirite-hedenbergite, and aegirite-augite.

Aegirite occurs in the form of elongated tabular, columnar, or acicular crystals, radiated and fibrous (”mermaid hair”) concretions, and sometimes spherulites. The color ranges from green to greenish black; sometimes the mineral is almost colorless. Aegirite has a vitreous luster, a hardness of 5.5–6 on Mohs’ scale, and a density of 3,400–3,600 kg/m³.

Aegirite is a typical mineral in alkali rocks and associated pegmatites; it is also characteristic of zones of alkaline metasomatism that develop after various intrusive and sedimentary-metamorphic rocks (gneisses, schists, and ferruginous quartzites). In agpaitic nepheline syenites it is found together with nepheline, orthoclase, eudialite, and apatite. In alkali granites it occurs with albite, riebeckite-arfvedsonite, and zircon. In zones of alkaline metasomatism it is found together with albite, hematite, and pyrochlore. Aegirite alters to chlorite, hematite, and limonite.