aegirine

Pyroxene mineral, sodium and iron silicate (NaFe⁺³Si₂O₆), commonly found in alkaline igneous rocks, particularly in syenites (composed of an alkali feldspar and a ferromagnesian mineral) and syenite pegmatites.

It also occurs in schists. Aegirine is generally dark green to greenish black.

Aegirine crystals from Magnet Cove, Arkansas

By courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard

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▪ mineral

 a pyroxene mineral, sodium and iron silicate (NaFe⁺³Si₂O₆), that is commonly found in alkaline igneous rocks, particularly in syenites and syenite pegmatites. It also occurs in crystalline schists. Aegirine forms a continuous chemical series with aegirine-augite, in which calcium replaces sodium, and magnesium and aluminum replace iron. In this series, the name acmite is given to crystals with the composition NaFeSi₂O₆ as well as to the reddish brown or greenish black pointed crystals approximating that composition. Aegirine generally is restricted to the dark green to greenish black, blunt crystals of the same composition. For detailed physical properties, see pyroxene (table).

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