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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). It is also known as acmite, which is a fibrous, green-colored variety.

Aegirine
Aegirine-Feldspar-Group-233473.jpg
Aegerine (dark) on feldspar (light) from Malawi
General
CategorySilicate mineral, pyroxene
Formula
(repeating unit)
NaFe3+[ Si2O6]
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[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.

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 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.[5]

It is sometimes used as a gemstone.[6]

ReferencesEdit

  1. ^ a b http://rruff.geo.arizona.edu/doclib/hom/aegirine.pdf Handbook of Mineralogy
  2. ^ a b http://www.mindat.org/min-31.html Mindat
  3. ^ http://webmineral.com/data/Aegirine.shtml 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