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.
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