Aegirine - NaFe³⁺Si₂O₆

Habit: Dark green to greenish black, reddish brown or black. Prismatic crystals with blunt to steep terminations, striated lengthwise, that can be bent or twisted; also sprays of acicular crystals, fibrous or in radial concretions. Vitreous to slightly resinous luster; translucent to opaque. Yellowish gray streak.

Environment: Common in alkalic igneous rocks, carbonatites and pegmatites. Also found in regionally metamorphosed schists, gneisses and iron formations, blueschist facies rocks and sodium metasomatism in granulites; an authigenic mineral in some shales and marls.

Etymology: From Ægir, the Norse sea god, as the mineral was first described from Norway.