Augite

Formula:  (Ca, Na)(Mg, Fe, Al, Ti)(Si, Al)₂O₆

Origin of name:  From Greek auge = bright or luster, apparently based on the appearance of its cleavage surface

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  Common on {100}, contact, simple, repeated; on {001}, lamellar

Unit cell: a = 9.70 Å, b = 8.84 Å, c = 5.27 Å, b = 106.97°

Colour:  Pale brown to dark brown or purplish brown, greenish, black

Diaphaneity: Transparent to opaque

Luster:  Vitreous, resinous to dull

Habit:  Crystals short prismatic; massive, granular ,rarely fibrous

Hardness:  5.5 to 6

Specific gravity:  3.23 to 3.52

Cleavage:  3; {110} good; {100} and {010} parting

Tenacity:  Brittle

Streak:  Grayish green

Synonyms/varieties: Fassite  (Fe³⁺, Al-rich var.), ferrohedenbergite, jeffersonite (Mn, Zn-rich var.), violan (Fe=Mg var.), asteroite (Fe-rich var.)

Comments: Member of pyroxene group. Essentially found in mafic igneous rocks, basalt, gabbro; common in ultramafic rocks; in some high-grade metamorphic rocks and metamorphosed iron formations.