mineral

Actinolite

Ca2(Mg,Fe)5Si8O22(OH)2

Hardness: 5.5 - 6 on the Mohs scale. Color: Dark green to brownish-green or grayish-green. Luster: Vitreous to silky/pearly. Crystal system: Monoclinic, often forming elongated prismatic, bladed, or fibrous aggregates. Cleavage: Perfect in two directions intersecting at approximately 56° and 124°.

Hardness: 5
Color: Dark green to brownish-green or grayish-green
Luster: Vitreous to silky/pearly

Identified July 24, 2026More mineral →

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Physical properties

Formation & geological history

Forms through low- to medium-grade regional metamorphism of mafic and ultramafic igneous rocks or impure dolomitic limestones, often associated with greenschist facies metamorphism.

Uses & applications

Used primarily as a collector mineral specimen and occasionally as a ornamental carving stone (when in its massive, fine-grained variety known as nephrite jade). Historically, fibrous forms were used as asbestos.

Geological facts

Actinolite forms an isomorphous series with tremolite and ferro-actinolite. The name 'actinolite' derives from the Greek word 'aktis' (beam or ray), referring to its fibrous or radiated crystal habit.

Field identification & locations

Identified in the field by its characteristic dark green color, bladed to fibrous habit, and amphibole cleavage. Found in metamorphic terranes worldwide, including locations in Vermont, California, Austria, Italy, and Canada.