Aegirine

Formula:  NaFeSi₂O₆

Origin of name:  for Aegir, Scandinavian God of the Sea

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  common on {100}, simple or lamellar

Unit cell:  a = 9.66 Å, b = 8.80 Å, c = 5.29 Å, b = 107.4°

Colour:  Dark green to greenish black

Diaphaneity: Translucent to opaque

Luster:  Vitreous to resinous

Habit:  Crystals long prismatic, vertically striated or furrowed; tufts or acicular to capillary.

Hardness:  6

Specific gravity:  3.50 to 3.60

Cleavage:  2; {110} good; {100} parting

Tenacity:  Brittle

Streak:  Pale yellowish gray

Synonyms/varieties:  Acmite, schefferite (Mn, Fe-rich var.)

Comments: Member of the pyroxene group. Common in alkalic igneous rocks, carbonatites and pegmatites. Also in regionally metamorphosed schists, gneisses and iron formations, in blueschist facies rocks and from sodium metasomatism in granulites. Sometimes found as authogenic mineral in some shales and marls.