Actinolite
Actinolite is a calcium-magnesium-iron amphibole with the formula Ca2(Mg,Fe)5Si8O22(OH)2, part of the tremolite-actinolite-ferro-actinolite solid solution series. Its green color comes directly from iron substituting for magnesium within the crystal structure, and it typically grows as elongated, bladed to fibrous crystals. The mineral is best known to rockhounds as a key component of nephrite jade.
Actinolite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Ca2(Mg,Fe)5Si8O22(OH)2 |
| Mohs hardness | 5–6 |
| Crystal system | Monoclinic |
| Luster | Vitreous to silky |
| Streak | White |
| Cleavage | Perfect prismatic, two directions at ~56°/124° |
| Fracture | Uneven to splintery |
| Specific gravity | 3.0–3.2 |
| Transparency | Translucent to opaque |
| Common colors | Green, Dark green, Grayish green, Black |
| Rarity | common |
Geology & background
Actinolite crystallizes chiefly in low- to medium-grade metamorphic settings, forming when mafic and ultramafic rocks such as basalt, gabbro, and peridotite are altered by heat, pressure, and hydrothermal fluids. It is a defining mineral of greenschist-facies metamorphism, appearing in actinolite schist, greenstone, and amphibolite alongside chlorite and epidote. Fine deposits occur in the Alps, the Ural Mountains, and parts of California and Vermont, where it forms elongated crystal masses in metamorphosed volcanic terrain. Rockhounds prize compact, interlocking actinolite fibers under the name nephrite, one of the two minerals sold as true jade, historically carved in China, New Zealand, and among Pacific Northwest peoples. Loose asbestiform actinolite is a recognized health hazard and should be handled with care, unlike the massive or bladed material typical of display specimens. Because tremolite and actinolite grade into one another compositionally, color intensity is often the clearest field clue to which name best applies.
How to identify Actinolite
Look for slender, bladed, or fibrous green to grayish-green crystals with a vitreous luster and a white streak. Hardness sits near 5 to 6, scratching glass but yielding to a steel file. Two cleavage directions meet at roughly 56 and 124 degrees, typical of amphiboles, and cross-fiber masses can feel silky. Actinolite is often confused with tremolite, which is paler and lower in iron, and with pyroxenes, which cleave at nearly right angles instead.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Actinolite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Actinolite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Compact actinolite intergrown with tremolite is the mineral sold worldwide as nephrite jade, distinct from the pyroxene jadeite.
- Its name comes from the Greek aktis, meaning ray, a reference to its radiating fibrous crystal clusters.
- Asbestiform actinolite is one of six minerals regulated as asbestos, even though most collected actinolite specimens are massive and safe to handle.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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