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Actinolite is a member of the amphibole supergroup, specifically the calcium amphibole subgroup. It forms a solid solution series with tremolite (Mg-rich end-member) and ferro-actinolite (Fe-rich end-member). Its name is derived from the Greek word 'aktis' (ray) due to its common radiating, fibrous habit. It is characterized by its typically green color, prismatic to acicular crystal habit, and two distinct cleavage planes intersecting at approximately 56 and 124 degrees.
How to Identify
- Color
- Typically light to dark green, sometimes grayish-green, blackish-green, or rarely white.
- Luster
- Vitreous to silky (especially in fibrous varieties).
- Texture
- Can be massive, fibrous, columnar, or radiating. Fibrous varieties can be very fine and hair-like.
- Crystal Form
- Prismatic, acicular (needle-like), or bladed crystals. Often found in radiating aggregates.
- Cleavage
- Perfect cleavage in two directions intersecting at approximately 56° and 124°.
- Geological Environment
- Metamorphic rocks such as greenschist, amphibolite, serpentinite, and skarns. Also found in altered mafic igneous rocks.
Key Facts
- Hardness: 5.5-6 on the Mohs scale
- Specific Gravity: 3.0-3.4 (varies with Fe content)
- Crystal System: Monoclinic
- Color: Light to dark green, grayish-green, blackish-green, rarely white
- Luster: Vitreous to silky
- Transparency: Translucent to opaque
- Fracture: Uneven to subconchoidal
- Cleavage: Perfect on {110} (two directions at ~56° and ~124°)
- Composition: Calcium magnesium iron silicate hydroxide
Quick Check
- Color: Green (various shades)
- Luster: Vitreous to silky
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, acicular, bladed, fibrous, columnar, radiating aggregates, massive.
- Cleavage Type: Perfect in two directions, intersecting at approximately 56° and 124°.
- Fracture Type: Uneven to subconchoidal.
- Tenacity: Brittle (non-fibrous); flexible to elastic (fibrous).
- Luster Type: Vitreous to silky.
Formation
Actinolite is a common mineral of the amphibole group, forming primarily during regional and contact metamorphism of mafic and ultramafic rocks, as well as carbonate rocks rich in magnesium. It crystallizes under conditions of low to medium temperature and pressure. It can also form as a hydrothermal alteration product.
Usage
Fibrous varieties of actinolite, particularly those with an asbestiform habit, were historically used as asbestos. However, due to its hazardous nature, this use has been largely discontinued. Non-fibrous actinolite has limited industrial use, but well-formed crystals are sought by mineral collectors. Nephrite jade, a tough ornamental stone, is a microcrystalline variety of actinolite (or tremolite).
Age Distribution
Found in rocks of various ages, typically associated with metamorphic events.
Where to Find
United States
Notable occurrences in California (especially in serpentinites), Vermont, and New York.
Canada
Found in various metamorphic terrains, particularly in Ontario and Quebec.
Switzerland
Classic localities in the Alps, where it was first described.
Austria
Occurrences in metamorphic rocks of the Alps.
Russia
Found in metamorphic complexes, including the Urals.
China
Significant source of nephrite jade, a variety of actinolite.
New Zealand
Another important source of nephrite jade.
Finding Tips
Look for Metamorphic Rocks
Actinolite is a key indicator mineral for greenschist facies metamorphism. Search in areas with mafic or ultramafic rocks that have undergone regional metamorphism.
Identify Associated Minerals
Actinolite often coexists with chlorite, epidote, albite, talc, and serpentine. The presence of these minerals can help narrow down your search.
Observe Crystal Habit
Its characteristic prismatic, acicular, or radiating fibrous habit is a strong diagnostic feature. Look for these forms in outcrops or hand samples.
Check for Cleavage
The two distinct cleavage planes at oblique angles (56° and 124°) are crucial for identification, especially when distinguishing it from pyroxenes (which have cleavage at ~90°).
Safety Precautions for Fibrous Varieties
WARNING: Fibrous actinolite can occur in an asbestiform habit. Asbestiform minerals are known carcinogens. If you encounter fibrous actinolite, DO NOT disturb it, inhale dust, or handle it without appropriate personal protective equipment (respirator, gloves, eye protection). Avoid collecting such samples. If you suspect you have found asbestiform actinolite, consult with a qualified geologist or environmental professional for safe handling and identification.
Similar Rocks
Tremolite
Tremolite
Also known as: Calcium-Magnesium Silicate
Hornblende
Hornblende
Also known as: Common Hornblende
Epidote
Epidote
Also known as: Pistacite
Scientific Classification
- Mineral Class
- Silicates
- Group
- Amphibole Group, Calcium Amphibole Subgroup
- Crystal System
- Monoclinic
- Chemical Formula
- Ca2(Mg,Fe)5Si8O22(OH)2
- Composition
- Calcium magnesium iron silicate hydroxide. Forms a solid solution series with tremolite (Mg-rich) and ferro-actinolite (Fe-rich). The Mg:Fe ratio can vary significantly.
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