Augite

Augite is a clinopyroxene. Clinopyroxenes and orthopyroxenes are easily distinguished in thin section by their extinction angles: clinopyroxenes have inclined extinction, and orthopyroxenes have parallel extinction.

Physical Properties
Chemical formula (Ca,Mg,Fe,Na)(Mg,Fe,Al)(Si,Al)2O6
Class Inosilicate
Single chains of linked tetrahedra
Crystal system Monoclinic
Habit Stubby prisms with octagonal or
square cross sections
Color Black, dark green
Hardness 5 to 6
Specific gravity 3.2 to 3.4
Cleavage Two perfect {110}
Fracture Uneven
Luster Vitreous
Transparency Transparent to translucent
Streak White
Optical Properties
PPL Shades of light brown, yellow-brown,
or green
May be weakly pleochroic
XPL 3rd order interference colors
Inclined extinction
δ 0.030
Twinning May show simple or polysynthetic twins
Extinction Inclined
after Perkins, 328

Augite in Hand Sample

Hand sample

Augite phenocrysts from Mt. Vesuvius

Scanning electron micrograph of augite

Another scanning electron micrograph of augite

Augite in Thin Section

Thin Section GigaPans

Augite, plane polars

Augite, crossed polars

Augite, PPL

Augite, XPL

Further Reading

Augite at webmineral.com

Augite at mindat.org