Guide to minerals: Aegirine

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Ben Elick (USA)

Aegirine is a beautiful, dark-coloured pyroxene, which is somewhat rare. It is named after Aegir, a figure in Norse mythology. The mineral has also been called acmite, derived from the Greek word “acme,” meaning point in reference to the mineral’s usually pointed crystals. The name acmite is now obsolete. The mineral was discovered in Norway and was named in 1835.

Fig. 1. specimen of aegirine on a microcline matrix from, Mount Malosa, Domasi Zomba District, Malawi. This specimen is pictued with a cenitimeter scale, and measeures 4.5cm wide, 4.5cm tall and 1.5cm thick. (A Ben Elick specimen; photo by Ben Elick.)

Aegirine crystals are deep green, brown or even black in colour, and typically have lengthwise striations. Crystals can have steep or blunt termination points, and the terminations are often etched and dull. They are commonly twinned.

Aegirine’s clearest identifying characteristics are its distinct dark colour, crystal habit and optical properties, as aegirine can be confused with other pyroxenes. It forms a series with augite, a common pyroxene of mafic igneous rocks and commonly forms with other pyroxenes, feldspathoids, albite, sodalite and barite.

Aegirine is most commonly formed in silica deficient intermediate igneous rocks, often intrusive in nature, such as nepheline syenite and syenite pegmatites. It is also found in phonolites. Although less common, it can also be found in certain types of metamorphic rocks.

Notable aegirine localities include: Kongsberg, Norway; Mont Saint Hilaire, Québec, Canada; Magnet Cove, Garland County, Arkansas, USA; Kola Peninsula, Russia; Libby, Montana, USA; and Mount Malosa, Domasi Zomba District, Malawi.

Aegirine is an exciting and attractive mineral that will make a fine addition to a collector’s mineral collection.

Physical properties
Chemical formula: NaFe3 + Si2O6
Composition: Sodium Iron Silicate
Group: Pyroxene Group
Crystal system: Monoclinic
Crystal class: Prismatic
Fracture: uneven/irregular
Hardness: 6-6.5
Specific gravity: 3.5-3.6
Streak: pale, yellowish-grey
Colour: green, dark green, brown, black
Luster: Vitreous
Cleavage: good

Further reading

Bonewitz, R. L. (2012). Nature Guide – Rocks and Minerals. New York: Dorling Kindersley Inc.

Chesterman, C. W. (1979). Field Guide to Rocks and Minerals – North America. New York: Alfred A. Knopf. (2019). Aegirine. Retrieved from

Mottana, A., Crespi, R., & Liborio, G. (1977, 1978). Simon & Schuster’s – Guide to Rocks and Minerals. New York: Simon & Schuster Inc.

Pellant, C. (2002). Rocks and Minerals. New York: Dorling Kindersley Inc.

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