Aegirine

(mineralogy) NaFe(SiO₃)₂ A brown or green clinopyroxene occurring in alkali-rich igneous rocks. Also known as aegirite.

(click to enlarge)
Aegirine crystals from Magnet Cove, Arkansas (credit: Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard)

Pyroxene mineral, sodium and iron silicate (NaFe⁺³Si₂O₆), commonly found in alkaline igneous rocks, particularly in syenites (composed of an alkali feldspar and a ferromagnesian mineral) and syenite pegmatites. It also occurs in schists. Aegirine is generally dark green to greenish black.

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Environment

Minerals of plutonic rocks so low in silica that they are in part made from a low-silica equivalent of feldspar -- the feldspathoids, minerals such as nepheline and leucite, and called nepheline syenites.

Visually recognizable only when crystallized; the pyroxene cleavage angles and a steep termination then become diagnostic. Usually in prismatic, embedded crystals, several inches (10 cm) long, terminated by steep points. Also in fibrous masses.

Black, brown, or green on thin edges. Luster glassy; hardness 6-6Ɖ; specific gravity 3.4-3.5; fracture uneven; cleavage easy prismatic. Brittle; translucent on thin edges.

Sodium iron silicate (13.4% Na ₂ O, 34.6% Fe ₂ O ₃ , 52.0% SiO ₂ ). Acmite has almost exactly this composition and probably would not be separately recognizable; in any case aegirine is the acceptable identification for the mineral when it is found, since species tailored to exact theoretical compositions are not usual in nature. Only the steep-tipped crystals should be called acmite.

Fuses easily to shiny, black magnetic bead, giving a yellow sodium color to the flame.

These are the common dark minerals of their particular alkaline (low-silica feldspathoid) group of rocks; therefore, the crystal habit and associates are the usual guides to the identity. They are more fusible than most other black silicates they resemble.

Aegirine is common in high-soda, low-silica rocks: the nepheline syenites (very black biotite, albite, and occasional natrolite, nepheline, and red eudialyte) in the fine-grained equivalents. This entire family of high-soda, low-silica rocks, however, is not common. The best U.S. occurrence is at Magnet Cove, Arkansas, where slender crystals several inches (10 cm or more) long are abundant. In Canada, St. Hilaire in Quebec has become famous for its feldspathoid group of hundreds of nepheline syenite associates. In Russia, the Kola Peninsula is equally famed for a like assemblage, commonly with rare-earth, zirconium, and titanium minerals. Acmite needles are set in a dark-hued dike of this material at Beemerville, New Jersey. Mid-size crystals are found in low-silica rocks in the Bear Paw and Highwood mountains in Montana and near Colorado Springs. The same sort of rock is found in Greenland; at Langesundfjord, Norway; and near Poços de Caldas, Brazil.

Aegirine
Aegirine crystals in matrix
General
Category	Mineral
Chemical formula	NaFe3+[ Si2O6]
Identification
Molar mass	231.00
Color	Dark Green, Greenish Black
Crystal habit	acircular
Crystal system	Monoclinic
Cleavage	Distinct/good
Fracture	brittle
Mohs scale hardness	6
Luster	Vitrious
Streak	Yellowish-grey
Diaphaneity	subtransparent to translucent to opaque
Specific gravity	3.52
Refractive index	biaxial
Birefringence	0.037 - 0.061
Dispersion	r > v

Aegirine is an inosilicate member of the clinopyroxene group. Aegirine is the sodium endmenber of the aegirine-augite series. Aegirine has the chemical formula NaFeSi₂O₆ in which the iron is present as Fe³⁺. In the aegirine-augite series the sodium is variably replaced by calcium with iron(II) and magnesium replacing the iron(III) to balance the charge. Aluminium also substitutes for the iron(III). It is also known as acmite, which is a fibrous, green-coloured variety.

Aegirine occurs as dark green monoclinic prismatic crystals. It has a glassy lustre and perfect cleavage. The Mohs hardness varies from 5 to 6 and the specific gravity is 3.2 to 3.4.

Aegirine

It occurs in alkali nepheline syenites and similar igneous rocks. Localities include Mont Saint-Hilaire, Quebec, Canada; Kongsberg, Norway; Narsarssuk, Greenland; Kola Peninsula, Russia; Magnet Cove, Arkansas, USA; Kenya; Scotland and Nigeria.

Aegirine was named after Ægir, the Teutonic god of the sea. A synonym for the mineral is acmite (from Greek ἀκμή "point, edge") in reference to the typical pointed crystals.

References

• Mindat
• Webmineral
• Mineral Galleries
• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7

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