Actinolite - Encyclopedia

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Monoclinic
    Chemistry : Ca2(Mg,Fe)5Si8O22(OH)2
    Rarity : Very common

Actinolite is a common species of the group of calcic amphiboles, which designates the very common intermediate terms of the continuous series formed by tremolite (magnesium pole) and ferro-actinolite (ferriferous pole). Its name comes from the Greek aktis (ray) and lithos (stone) because its crystals are often grouped in radiated aggregates. It is a common product of low-grade regional metamorphism (greenschist facies), and contact metamorphism of basic and ultrabasic rocks as well as dolomites. They are frequently associated with chlorite, epidote and talc, as well as many other metamorphic minerals. It appears in the form of very elongated crystals that can reach 25 cm, longitudinally striated and often curved, fibroradiated and fibrous aggregates, or fibrous masses. Its color is most often bottle green with different shades ranging from light greenish gray to dark green. Jade is the trade name for nephrite, an extremely compact fibrous cryptocrystalline variety of actinolite (or tremolite). Byssolite is an asbestiform variety of actinolite, common in alpine deposits. Nephrite is used as an ornamental stone.

Actinolite in the World

The actiote is known in many localities. The most famous deposits are those of Mount Greiner (Zillertal, Austria), where actinolite forms centimetric aggregates, Zermatt in Switzerland, Val Malenco and Traversella, in the Italian Alps, and those of California. But the largest crystals are probably those of Pedra Preta (Brumado, Bahia, Brazil) where the curved individuals reach 25 cm.

Actinolite in France

In France, dark green crystals exceeding 10 cm, embedded in talc, are known on the island of Groix (Morbihan).


The twins are simple and parallel on {100}; lamellar and parallel to {001}.

Fakes and treatments

No fakes known for this mineral species.

Hardness : 5 to 6
Density : 3.03 to 3.24
Fracture : Irregular
Trace : White

TP : Translucent to transparent
IR : 1.613 to 1.666
Birefringence : 0.023
Optical character : Biaxial -
Pleochroism : Weak
Fluorescence : None

Solubility : Hydrofluoric acid

Magnetism : Paramagnetic
Radioactivity : None