Guide to minerals: Aegirine

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.

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.

Mindat.org. (2019). Aegirine. Retrieved from Mindat.org: https://www.mindat.org/min-31.html

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.