Augite

The most common pyroxene, augite is named after the Greek word augites, which means “brightness”—a reference to its occasional shiny appearance. Most augite has a dull, dark green, brown, or black finish. Augite occurs chiefly as short, thick, prismatic crystals with a square or octagonal cross section and sometimes as large, cleavable masses. It occurs in a solid-solution series in which diopside and hedenbergite are the end-members. Augite is common in silica-poor rocks and various other dark-colored igneous rocks, as well as igneous rocks of intermediate silica content. It also occurs in some metamorphic rocks formed at high temperatures (1,065°F/575°C or above). Augite is a common constituent of lunar basalts and some meteorites. Notable crystal localities are in Germany, the Czech Republic, Italy, Russia, Japan, Mexico, Canada, and USA. Because it is difficult to distinguish between augite, diopside, and hedenbergite in hand specimens, all pyroxenes are often identified as augite.

Name: From the Greek for luster, apparently based on the appearance of its cleavage surface.

Mineral Group: Pyroxene group.

Association: Orthoclase, sanidine, labradorite, olivine, leucite, amphibole, pyroxene

Chemical Properties of Augite

Chemical Classification	Inosilicates
Chemical Composition	(CaxMgyFez)(Mgy1Fez1)Si2O6

Physical Properties of Augite

Color	Brown-green, black, green-black, brown, purplish brown
Streak	Greenish gray, light to dark brown
Luster	Vitreous
Cleavage	Distinct/Good Good on {110}
Diaphaneity	Translucent, Opaque
Mohs Hardness	5½ – 6 on Mohs scale
Crystal System	Monoclinic
Tenacity	Brittle
Parting	on {100} and {010}
Fracture	Irregular/Uneven, Sub-Conchoidal
Density	3.19 – 3.56 g/cm3 (Measured)    3.31 g/cm3 (Calculated)

Optical Properties of Augite

Type	Anisotropic
Crystal Habit	Grains often anhedral; May be granular, massive, columnar or lamellar
Color / Pleochroism	x=pale green or bluish green y=pale greenish, brown, green or bluish green z=pale brownish green, green or yellow-green
Optical Extinction	Z : c = 35°-48°
2V:	Measured: 40° to 52°, Calculated: 48° to 68°
RI values:	nα = 1.680 – 1.735 nβ = 1.684 – 1.741 nγ = 1.706 – 1.774
Twinning	Commonly displays simple and lamellar twinning on {100} and {001}; They may combine to form a herringbone pattern. Exsolution lamellae may be present.
Optic Sign	Biaxial (+)
Birefringence	δ = 0.026 – 0.039
Relief	High
Dispersion:	r > v weak to distinct

Occurrence

Essential in mafic igneous rocks, basalt, gabbro; common in ultramafic rocks; in some high-grade metamorphic rocks and metamorphosed iron formations.

Uses Area

Augite is a mineral of interest to geologists, and collectors. While it has little to no industrial value, the presence and development of augite may help tell scientists and geologists about Earth’s history in certain regions.

Distribution

Widespread; only a few classic localities, much studied or providing

• From Arendal, Norway.
• In Italy, from Vesuvius, Campania; around Frascati, Alban Hills, Lazio; on Mt. Monzoni, Val di Fassa, Trentino-Alto Adige; at Traversella, Piedmont; and on Mt. Etna, Sicily. Around the Laacher See, Eifel district, On the Azores and Cape Verde Islands.
• In Canada, from Renfrew and Haliburton Cos., Ontario; at Otter Lake, Pontiac Co., Quebec; and many other localities.
• In the USA, from Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey; and at Diana, Lewis Co., and Fine, St. Lawrence Co., New York.
• From Tomik, Gilgit district, Pakistan.
• At Kangan, Andhra Pradesh, India.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
• Mindat.org. (2019). Halite: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
• Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].