Aegirine

Aegirine is a mineral. It is a member of the clinopyroxene group of inosilicate minerals. Acmite is a Acmite is a fibrous green-colored variety of aegirine, with the name also used as a synonym. It was first described in 1821, in Kongsberg, Norway.

Aegirine
Aegirine (dark) with minor feldspar (light) from Malawi
General
Category	Silicate mineral, pyroxene
Formula	NaFe3+[Si2O6]
IMA symbol	Aeg[1]
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	[2][3][4][5]

Etymology

The name aegirine is derived from Ægir , a Norse mythological figure (god of the sea), as the mineral was first described from Norway.^[2][3]

A synonym for the mineral is acmite (from Greek ἀκμή "point, edge") in reference to the typical pointed crystals.^[6]

Chemistry and description

Syenite with aegirine and acmite from Magnet Cove, Arkansas, US

Aegirine is the sodium endmember of the aegirine–augite series.^{[citation needed]}

It has the chemical formula NaFeSi₂O₆, in which the iron is present as the ion 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. Aluminum also substitutes for the iron(III). Acmite is a fibrous green-colored variety.^{[citation needed]}

Aegirine occurs as dark green monoclinic prismatic crystals.^[3] It has a glassy luster and perfect cleavage, "in two directions at near 90 degree angles".^[7] It is described on Mindat.org as "slightly resinous", with its colour "dark green to greenish black, reddish brown, [or] black" Its Mohs hardness is 6 and its specific gravity is between 3.5 and 3.6.^[3][4]

Associated minerals include augite, nepheline, andradite, baryte, quartz, spessartine, riebeckite, biotite, sodalite, and albite.^[7]

Occurrence

The acmite variety was first described in 1821, at Kongsberg, Norway,^[8] and the aegirine variety in 1835 for an occurrence in Rundemyr, Øvre Eiker, Buskerud, Norway.^[3]

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.^[2]

Major localities include Mont Saint-Hilaire, Quebec, Canada; Kongsberg, Norway; Narsarssuk, Greenland; Kola Peninsula, Russia; Magnet Cove, Arkansas, US; Kenya; Scotland, and Nigeria.^[7]

Aegirine also occurs in the syenite at the Bowral quarries in New South Wales, Australia, as described in a 1906 paper by geologist and later Antarctic explorer Douglas Mawson.^[9][10]

Uses

Aegirine is sometimes used as a gemstone.^[11]

See also

• List of minerals