Augite

Hand Specimen identification: Association, form, two cleavages at 86° to 87°, and color identify pyroxene, but differentiating augite is problematic without X-ray data. White to green or pale green pyroxenes are often diopside; augite tends to be dark green or black. Augite is occasion-ally confused with hornblende. Plate 6.1 is a color photo of massive augite.

Chemical Composition : (Ca,Mg,Fe,Na)(Mg,Fe,Al)(Si,Al)2O6

Hardness : 5 to 6

Specific Gravity3.2 to 3.4

Cleavage/Fracture : two perfect {110}/uneven

Luster/Transparency : two perfect {110}/uneven

Color : black, dark green

Streak : white

Optical Properties : Augite is similar to other clinopyroxenes in thin sec-tion. Grains are prismatic or blocky, depending on orientation. Color may be various shades of light brown, yellow-brown, or green. Augite may exhibit weak pleochroism. Extinction angle, optic sign, and 2V help distinguish the different clinopyroxenes, but telling them apart may be difficult. Biaxial 1 + 2, a = 1.671 to 1.735, b = 1.672 to 1.741, g = 1.703 to 1.761, d = 0.030, 2V = 25° to 60°.

Crystallography : Monoclinic, a = 9.8, b = 9.0, c = 5.25, b = 105°, Z = 4; space group C2/c; point group 2/m.

Habit : Individual crystals are typically poorly formed stubby black or green prisms with octagonal or square cross sections. Simple and polysynthetic twins may be present.

Structure and composition : The structure of augite is similar to that of the other clinopyroxenes (see diopside). Its chemistry is, how-ever, more complex and variable. Ca:Mg:Fe ratios vary. Significant solid solution occurs with jadeite, NaAlSi2O6; aegirine, Na(Al,Fe)Si2O6; and Ca-Tschermak’s pyrox-ene, CaAl2SiO6. Ti, Li, Mn, and a number of other ele-ments may also be present in small amounts.

Occurrence and associations : Augite is the most common pyroxene found in mafic to intermediate igneous rocks, both plutonic and volcanic. Associated minerals include hornblende and plagioclase.

Varieties :

Related Minerals : Augite is equivalent in composition to diopside with many impurities, especially Na. It is structurally and chemically closely related to other pyroxenes, espe-cially pigeonite, and to pyroxenoids. Omphacite is a bright green variety of augite rich in Na and Al.

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Hand Specimen identification: Occurrence as phenocrysts in volcanic rock, form, two cleavages at 86°, and …

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