| Aegirine | |
|---|---|
.jpg) 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) (sameH-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] |
Aegirine is a member of the clinopyroxene group ofinosilicate 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 greenmonoclinic prismatic crystals. It has a glassy luster and perfect cleavage. ItsMohs hardness varies from 5 to 6 and itsspecific gravity is between 3.2 and 3.4.
This mineral commonly occurs in alkalic igneous rocks,nepheline syenites,carbonatites andpegmatites. It also appears in regionallymetamorphosedschists,gneisses, and iron formations; inblueschist facies rocks, and from sodiummetasomatism ingranulites. It may occur as an authigenic mineral inshales andmarls. It occurs in association with potassicfeldspar,nepheline,riebeckite,arfvedsonite,aenigmatite,astrophyllite,catapleiite,eudialyte,serandite andapophyllite.[2]
Localities includeMont Saint-Hilaire,Quebec, Canada;Kongsberg, Norway; Narsarssuk,Greenland;Kola Peninsula, Russia;Magnet Cove,Arkansas, US;Kenya;Scotland andNigeria.
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 afterÆgir, theNorse god of the sea.[3] A synonym for the mineral isacmite (fromGreek ἀκμή "point, edge") in reference to the typical pointed crystals.[7]
It is sometimes used as a gemstone.[8]
