Aegirine
Chemical formula: NaFe<sup>3+</sup>Si<sub>2</sub>O<sub>6</sub>
Aegirine is a basic sodium-iron pyroxene, forming characteristic, long, prismatic crystals of dark green to black color.
Properties
- Mohs hardness
- 6
- Luster
- Vitreous
- Streak
- Pale greenish grey
- Density
- 3.5-3.6
- Cleavage
- Good on {110}
- Fracture
- Uneven to conchoidal
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Aegirine is most easily recognized by its characteristic habit – long, slender, prismatic crystals, often with longitudinal striations. Key features also include its dark green to black color, vitreous luster, and relatively high density. Its occurrence in association with alkaline rock minerals (such as nepheline or eudialyte) is a strong diagnostic indicator. ## Distinguishing from Similar Minerals Aegirine is sometimes confused with other dark prismatic minerals: - **Amphiboles (hornblende, arfvedsonite):** The most reliable way to distinguish them is by the cleavage angle, which for pyroxenes (including aegirine) is approximately 87°, and for amphiboles, it is approximately 56° and 124°. Arfvedsonite often exhibits a bluish tint, which aegirine lacks. - **Tourmaline (schorl):** Schorl typically has a triangular cross-section and does not exhibit cleavage, unlike aegirine. - **Augite:** Distinguishing it from augite (another clinopyroxene) can be very difficult without advanced analysis. However, aegirine usually forms more slender and acicular crystals and occurs in a specific environment of sodium-rich rocks. ## Crystal Forms Aegirine crystals are monoclinic, with a columnar, prismatic, and often acicular habit. They are usually terminated by steep pyramidal faces. They often form radial aggregates resembling suns or fans, as well as fibrous, felted, and granular aggregates.
Geological environment
## Genesis Aegirine is a characteristic mineral of alkaline igneous rocks, poor in silica. It forms during crystallization from magma rich in sodium and iron. It is most commonly found in nepheline syenites, carbonatites, phonolites, and alkaline pegmatites. It can also form as a result of metamorphic processes in the glaucophane schist (blueschist) facies. ## Mineral Associations Minerals typically associated with aegirine include: nepheline, microcline, orthoclase, albite, sodalite, eudialyte, arfvedsonite, riebeckite, astrophyllite, catapleiite, and lorenzenite. Compositions of aegirine with red eudialyte or white nepheline are particularly valued by collectors. ## Localities World-class specimens come from several key localities. The most important include: Mont Saint-Hilaire in Quebec (Canada), famous for its perfect crystals; the Khibiny and Lovozero massifs on the Kola Peninsula (Russia), where it occurs in association with eudialyte; Mount Malosa in Malawi, which provides sharp, lustrous crystals. Historical localities include the Langesundsfjord region in Norway and Magnet Cove in Arkansas (USA).
Rarity
Not very common
For collectors
## Quality Criteria The most highly prized aegirine specimens are characterized by sharply terminated, slender crystals with strong luster and an intense, dark green color. Specimens in the form of radial aggregates ("suns") on a light rock matrix are of great value. Compositions with color-contrasting minerals, such as red eudialyte, pink sodalite, or white analcime or nepheline, are particularly sought after. Crystal size and lack of damage are key factors influencing value. ## Popular Localities Collectors most desire specimens from classic localities. Crystals from Mont Saint-Hilaire (Canada) are considered exemplary in terms of form. Specimens from Mount Malosa (Malawi) are renowned for their sharpness and luster. Those from the Kola Peninsula (Russia), in turn, form spectacular compositions with rare alkaline minerals, making them unique.
Care and storage
## Cleaning Aegirine specimens can be safely cleaned with lukewarm water, mild soap, and a soft brush. Special care should be taken with specimens on a delicate matrix or in fibrous aggregates. Ultrasonic cleaners are generally safe for solid crystals but can damage brittle aggregates. ## What to Avoid The mineral is sensitive to strong acids, which can damage it. Avoid sudden temperature changes. Although stable in light, prolonged exposure to intense sunlight is not recommended for any collector's mineral. ## Storage Aegirine is relatively hard and durable, but brittle, acicular crystals can be susceptible to mechanical damage. It is recommended to store specimens in separate boxes or on stands, away from minerals that could scratch them (e.g., quartz, topaz).