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Aegirine crystals from Magnet Cove, Arkansas—Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard
Pyroxene mineral, sodium and iron silicate (NaFe+3SiO), 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.
This entry comes from Encyclopædia Britannica Concise. For the full entry on aegirine, visit Britannica.com.