Formula: ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22 (OH)2
Mg/(Mg + Fe2+) is 0.5 to 0.9
Inosilicate (chain silicate) amphibole
Specific gravity: 3.03 to 3.24
Hardness: 5 to 6
Streak: White
Colour: Green, green-black, grey-green, or black. Colourless, pale green to deep green in thin section.
Solubility: Insoluble in water and hydrochloric acid
Common impurities: Mn, Al, Na, K, Ti

Metamorphic environments
Hydrothermal environments

Actinolite is an amphibole mineral that is produced by low-grade regional or contact metamorphism of magnesium carbonate. Hydrothermal actinolite occurs in veins and as replacement of mafic minerals. In active geothermal systems hosted by intermediate to mafic volcanic rocks, actinolite is present at temperatures above 280oC (AofA).
It may be found in schist.
Associated minerals are calcite, quartz, epidote, glaucophane, pumpellyite and lawsonite.
Actinolite is a mineral of the albite-epidote-hornfels, prehnite-pumpellyite, greenschist, amphibolite and glaucophane-bearing blueschist facies.

At Coed-y-Brenin deposit, Ganllwyd, Gwynedd, Wales, UK, actinolite occurs within narrow veins, less than 5 mm in width, cutting the amphibole porphyry. Specimens consist of 2–5 cm long, plumose sprays of fibrous actinolite, in which 1–2 mm pyrite crystals are embedded. The actinolite is surrounded by clinochlore and a little quartz is also present (JRS 21.117).

In the Bronx, New York City, USA, actinolite occurs associated with quartz and calcite. (R&M 84.3.221.

At the Belvidere Mountain Complex in Vermont, USA, actinolite occurs in coarse-grained amphibolite, as pods along fault-zone contacts with serpentinite and as fine-grained masses with chlorite in fine-grained amphibolite (R&M 90.6.525-526).


Actinolite may be altered to chlorite (AofA).

chlorite (clinichlore), actinolite and albite to glaucophane, iron-poor epidote, SiO2 and H2O
9Mg5Al(AlSi3O10)(OH)8 + 6☐Ca2Mg5Si8O22(OH)2 + 50Na(AlSi3O8) → 25☐Na2(Mg3Al2)Si8O22(OH)2 + 6Ca2Al3[Si2O7][SiO4]O(OH) + 7SiO2 + 14H2O
This is a metamorphic reaction (DHZ 3 p156).

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