Aegirine

Aegirine is a member of the clinopyroxene group of inosilicate minerals. 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). Acmite is a fibrous, green-colored variety.

Aegirine
Aegirine (dark) with minor feldspar (light) from Malawi
General
Category	Silicate mineral, pyroxene
Formula (repeating unit)	NaFe3+[ Si2O6]
Strunz classification	9.DA.25
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 9.658, b = 8.795 c = 5.294 [Å], β = 107.42°; Z = 4
Identification
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 occurs as dark green monoclinic prismatic crystals. It has a glassy luster and perfect cleavage. Its Mohs hardness varies from 5 to 6, and its specific gravity is between 3.2 and 3.4.

Syenite with aegirine and acmite from Magnet Cove, Arkansas

This mineral commonly occurs in alkalic igneous rocks, nepheline syenites, carbonatites and pegmatites. It also appears 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.

The acmite variety was first described in 1821, at Kongsberg, Norway,[5] and the aegirine variety in 1835 for an occurrence in Rundemyr, Øvre Eiker, Buskerud, Norway. Aegirine was named after Ægir, the Norse god of the sea.[2] A synonym for the mineral is acmite (from Greek ἀκμή "point, edge") in reference to the typical pointed crystals.[6]

It is sometimes used as a gemstone.[7]