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Aegirine

Aegirine
Aegirine crystals in matrix
General
Category	Mineral
Chemical formula	Na Fe³⁺[ Si₂O₆]
Identification
Molecular Weight	231.00
Color	Dark Green, Greenish Black
Crystal habit	acircular
Crystal system	Monoclinic
Cleavage	Distinct/good
Fracture	brittle
Mohs Scale hardness	6
Luster	Vitrious
Refractive index	biaxial
Birefringence	0.037 - 0.061
Dispersion	r > v
Streak	Yellowish-grey
Specific gravity	3.52
Diaphaneity	subtransparent to translucent to opaque

Aegirine is an inosilicate member of the clinopyroxene group. Aegirine is the sodium endmenber of the aegirine-augite series. Aegirine has the chemical formula Na Fe Si₂O₆ 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.

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Aegirine occurs as dark green monoclinic prismatic crystals. It has a glassy lustre and perfect cleavage. The Mohs hardness varies from 5 to 6 and the specific gravity is 3.2 to 3.4.

It occurs in alkali nepheline syenites and similar igneous rocks. Localities include Mont Saint-Hilaire, Quebec, Canada; Kongsberg, Norway; Narsarssuk, Greenland; Kola Peninsula, Russia; Magnet Cove, Arkansas, USA; Kenya; Scotland and Nigeria.

Aegirine was named after Ægir, the Teutonic god of the sea. A synonym for the mineral is acmite (from Greek) in reference to the typical pointed crystals.