Actinolite is a green amphibole silicate mineral, part of the inosilicate group, notable for its fibrous habit and occurrence in metamorphic rocks. It is both scientifically important and visually striking, often forming radiating sprays or bladed crystals.
🔬 Chemical and Structural Properties
- Formula: Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
- Crystal System: Monoclinic, prismatic habit
- Color: Pale to dark green, yellowish-green, bluish, or black
- Hardness: 5–6 on Mohs scale
- Luster: Vitreous to silky; dull in fibrous forms
- Streak: White
- Specific Gravity: ~3.0–3.2
- Optical Properties: Biaxial (−), moderate pleochroism from yellow to dark green
🌍 Geological Occurrence
Actinolite is common in metamorphic rocks, especially:
- Greenschist facies rocks (where it gives the name “actinolite schist”)
- Contact metamorphosed limestones
- Altered igneous rocks
It is an intermediate member of the tremolite–ferro-actinolite series, with magnesium-rich tremolite on one end and iron-rich ferro-actinolite on the other.
📍 Notable Localities
- Norway – classic type localities
- Pakistan (Astore Valley, Gilgit-Baltistan) – fine crystals
- Austria (Tyrol) – greenschist occurrences
- USA (Massachusetts, California) – metamorphic deposits
- Namibia – attractive specimens
⚠️ Safety Considerations
Fibrous actinolite can occur as a form of asbestos, which is hazardous if inhaled. Non-fibrous crystals are safe to handle, but fibrous material requires caution.
✨ Collector and Scientific Significance
- Petrology: Indicator mineral for metamorphic grade (greenschist facies).
- Collectors: Attractive green sprays and radiating crystals are prized.
- Historical Note: Named in 1794 by Richard Kirwan from the Greek aktinos (“ray”), referencing its fibrous habit.
📖 Conclusion
Actinolite is a versatile amphibole mineral, bridging geology and mineral collecting. Its green coloration, fibrous habit, and role in metamorphic petrology make it scientifically valuable, while its striking sprays and crystals appeal to collectors. However, fibrous actinolite highlights the dual nature of minerals—beautiful yet potentially hazardous.
In short: Actinolite is a green amphibole mineral found in metamorphic rocks, valued for its beauty and geological significance, but fibrous forms can be hazardous.
