| Aegirine | |
| Category: | Silicate mineral, pyroxene |
| Imasymbol: | Aeg[1] |
| Molweight: | 231.00 g/mol |
| Strunz: | 9.DA.25 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | C2/c |
| Unit Cell: | a = 9.658, b = 8.795 c = 5.294 [Å], β = 107.42°; Z = 4 |
| Color: | Dark Green, Greenish Black |
| Habit: | Prismatic crystals may be in sprays of acicular crystals, fibrous, in radial concretions |
| Twinning: | Simple and lamellar twinning common on |
| Cleavage: | Good on, (110) ^ (10) ≈87°; parting on |
| Fracture: | Uneven |
| Tenacity: | Brittle |
| Mohs: | 6 |
| Luster: | Vitreous to slightly resinous |
| Refractive: | nα = 1.720 – 1.778 nβ = 1.740 – 1.819 nγ = 1.757 – 1.839 |
| Opticalprop: | Biaxial (−) |
| Birefringence: | δ = 0.037 – 0.061 |
| Dispersion: | moderate to strong r > v |
| Pleochroism: | X = emerald green, deep green; Y = grass-green, deep green, yellow; Z = brownish green, green, yellowish brown, yellow |
| 2V: | Measured: 60° to 90°, Calculated: 68° to 84° |
| Streak: | Yellowish-grey |
| Gravity: | 3.50–3.60 |
| Diaphaneity: | Translucent to opaque |
| 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.This mineral commonly occurs in alkalic igneous rocks, nepheline syenites, carbonatites and pegmatites. It also appears in regionallymetamorphosed 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]