Aegirine
Aegirine (dark) with minor feldspar (light) from Malawi
General
CategorySilicate mineral, pyroxene
Formula
(repeating unit)		NaFe3+[Si2O6]
IMA symbolAeg[1]
Strunz classification9.DA.25
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/c
Unit cella = 9.658, b = 8.795
c = 5.294 [Å], β = 107.42°; Z = 4
Identification
Formula mass231.00 g/mol
ColorDark Green, Greenish Black
Crystal habitPrismatic crystals may be in sprays of acicular crystals, fibrous, in radial concretions
TwinningSimple and lamellar twinning common on {100}
CleavageGood on {110}, (110) ^ (110) ≈87°; parting on {100}
FractureUneven
TenacityBrittle
Mohs scale hardness6
LusterVitreous to slightly resinous
StreakYellowish-grey
DiaphaneityTranslucent to opaque
Specific gravity3.50–3.60
Optical propertiesBiaxial (−)
Refractive indexnα = 1.720 – 1.778 nβ = 1.740 – 1.819 nγ = 1.757 – 1.839
Birefringenceδ = 0.037 – 0.061
PleochroismX = emerald green, deep green; Y = grass-green, deep green, yellow; Z = brownish green, green, yellowish brown, yellow
2V angleMeasured: 60° to 90°, Calculated: 68° to 84°
Dispersionmoderate to strong r > v
References[2][3][4][5]

Aegirine is a member of the clinopyroxene group of inosilicate minerals. It is the sodium endmember of the aegirine–augite series. 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.

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.[2]

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,[6] 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.[3] A synonym for the mineral is acmite (from Greek ἀκμή "point, edge") in reference to the typical pointed crystals.[7]

It is sometimes used as a gemstone.[8]

See also

References

  1. ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. 1 2 Handbook of Mineralogy
  3. 1 2 Mindat
  4. ↑ Webmineral
  5. ↑ Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
  6. ↑ Dana, James Dwight (1855) [1837]. Manual of Mineralogy (7th ed.). Philadelphia, PA: Durrie & Peck.
  7. ↑ "Acmite" . Encyclopædia Britannica. Vol. 1 (11th ed.). 1911. p. 149.
  8. ↑ Dictionary of Gems and Gemology By Mohsen Manutchehr-Danai p.5
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