Aegirine-augite is a clinopyroxene mineral, representing a solid-solution series between aegirine (NaFe³⁺Si₂O₆) and augite (Ca(Mg,Fe)Si₂O₆). It is an important rock-forming mineral in alkaline igneous systems and is valued by petrologists for the insights it provides into magmatic chemistry.
🌍 Origins and Naming
- Name: Aegirine-augite reflects its intermediate composition between aegirine and augite.
- Classification: Clinopyroxene subgroup of the pyroxene group.
- Occurrence: Common in alkaline volcanic rocks (phonolites, trachytes) and syenites, as well as in some metamorphic rocks.
🔬 Chemical and Structural Properties
- General Formula: (Na,Ca)(Fe³⁺,Mg,Fe²⁺)Si₂O₆
- Crystal System: Monoclinic
- Color: Dark green to brownish-green, sometimes nearly black
- Habit: Prismatic crystals, often elongated and spear-like
- Hardness: 5.5–6 on Mohs scale
- Specific Gravity: ~3.4–3.6
- Luster: Vitreous
- Optical Properties: Strong pleochroism (green to brown tones), typical of pyroxenes
⚙️ Geological Context
Aegirine-augite typically forms in:
- Alkaline igneous rocks such as nepheline syenites, phonolites, and trachytes
- Pegmatites associated with alkali-rich magmas
- Metamorphic rocks under high-pressure conditions, especially in sodium-rich environments
Associations: Often found with feldspar (orthoclase, albite), nepheline, sodalite, and other alkaline minerals.
📍 Notable Localities
- Langesundsfjorden, Norway – classic locality for aegirine and aegirine-augite
- Mount Malosa, Malawi – famous for spectacular crystals
- Kola Peninsula, Russia – alkaline complexes with aegirine-augite pyroxenes
- Kenya and Tanzania – East African Rift alkaline volcanics
📖 Scientific and Collector Significance
- Petrology: Aegirine-augite is a key indicator of alkaline magmatic processes, helping geologists trace sodium and iron enrichment.
- Collectors: Crystals are less common than pure aegirine but can be attractive, especially when associated with feldspar or quartz.
- Research Value: Provides insights into solid-solution behavior in pyroxenes and the geochemistry of alkaline magmas.
✨ Conclusion
Aegirine-augite is a clinopyroxene mineral bridging aegirine and augite, notable for its dark green crystals and role in alkaline igneous petrology. It is scientifically important as a marker of sodium-rich magmatic systems and aesthetically appealing when found in sharp, lustrous crystals.
In short: Aegirine-augite is a sodium-calcium iron-magnesium clinopyroxene, key to understanding alkaline magmas.
