Monoclinic crystal of aegirine with orthoclase, from Mount Malosa, Zomba District, Malawi (size: 85 mm x 83 mm; 235 g)
Category Silicate mineral, Pyroxene
(repeating unit)
NaFe3+[ Si2O6]
Strunz classification 9.DA.25
Crystal system Monoclinic
Crystal class Prismatic (2/m)
H-M symbol: (2/m)
Space group C2/c
Unit cell a = 9.658, b = 8.795
c = 5.294 [Å], β = 107.42°; Z = 4
Formula mass 231.00 g/mol
Color Dark Green, Greenish Black
Crystal habit Prismatic crystals may be in sprays of acicular crystals, fibrous, in radial concretions
Twinning Simple and lamellar twinning common on {100}
Cleavage Good on {110}, (110) ^ (110) ≈87°; parting on {100}
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 6
Luster Vitreous to slightly resinous
Streak Yellowish-grey
Diaphaneity Translucent to opaque
Specific gravity 3.50 - 3.60
Optical properties Biaxial (-)
Refractive index nα = 1.720 - 1.778 nβ = 1.740 - 1.819 nγ = 1.757 - 1.839
Birefringence δ = 0.037 - 0.061
Pleochroism X = emerald green, deep green; Y = grass-green, deep green, yellow; Z = brownish green, green, yellowish brown, yellow
2V angle Measured: 60° to 90°, Calculated: 68° to 84°
Dispersion moderate to strong r > v
References [1][2][3][4]

Aegirine is a member of the clinopyroxene group of inosilicates. Aegirine is the sodium endmember 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-colored variety.

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


Commonly occurs in alkalic igneous rocks, nepheline syenites, carbonatites and pegmatites. Also in regionally metamorphosed schists, gneisses, and iron formations; in blueschist facies rocks, and from sodium metasomatism in granulites. It may occur as an authigenic mineral in shales and marls. It occurs in association with potassic feldspar, nepheline, riebeckite, arfvedsonite, aenigmatite, astrophyllite, catapleiite, eudialyte, serandite and apophyllite.[1]

Localities include Mont Saint-Hilaire, Quebec, Canada; Kongsberg, Norway; Narsarssuk, Greenland; Kola Peninsula, Russia; Magnet Cove, Arkansas, US; Kenya; Scotland and Nigeria.

It was first described in 1835 for an occurrence in Rundemyr, Øvre Eiker, Buskerud, Norway. Aegirine was named after Ægir, the Teutonic god of the sea.[2] A synonym for the mineral is acmite (from Greek ἀκμή "point, edge") in reference to the typical pointed crystals.[5]

It is sometimes used as a gemstone.[6]


  1. 1 2 Handbook of Mineralogy
  2. 1 2 Mindat
  3. Webmineral
  4. Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
  5.  "Acmite". Encyclopædia Britannica. 1 (11th ed.). 1911. p. 149.
  6. Dictionary of Gems and Gemology By Mohsen Manutchehr-Danai p.5
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