Formula: ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22 (OH)2
Mg/(Mg + Fe2+) is 0.5 to 0.9
Byssolite is a fibrous form of actinolite.
Inosilicate (chain silicate), amphibole
Crystal System: Monoclinic
Specific gravity: 3.03 to 3.24 measured, 3.07 calculated
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.


The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. Actinolite is a primary silicate that forms aggregates up to 6 cm, intergrown with diopside and grossular (AJM 22.1.33).

At Kwun Yum Shan, Yuen Long District, New Territories, Hong Kong, China, the deposit is a hydrothermal deposit which lies along a fault zone withi altered acid volcanic rocks, consisting mainly of chlorite, biotite, sericite and actinolite with scattered quartz (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).

At the Lin Ma Hang mine, North District, New Territories, Hong Kong, China, the lead-zinc deposit is a hydrothermal deposit which lies along a fault zone within altered acid volcanic rocks, consisting mainly of chlorite, biotite, sericite and actinolite, with scattered quartz. (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned iron mine, with both underground and open cast workings. The iron ores contain magnetite as the ore mineral and occur predominantly as masses of all sizes enclosed in a large skarn body formed by contact metasomatism of dolomitic limestone at the margins of a granite intrusion. In parts of the underground workings magnetite is also found in marble in contact with the granite. The skarn rocks consist mainly of tremolite, actinolite, diopside and garnet.
Actinolite is the dominant constituent of the skarn rocks and the chief gangue mineral of the iron ores. It is intimately associated with tremolite, and other associated minerals include diopside, biotite, garnet, chondrodite, fluorite, magnetite and pyrite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

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).

At Goose Creek, Leesburg, Loudoun county, Virginia, USA, there are several mines. Byssolite was quite commonly found in the Luck Fairfax Plant, both inside prehnite pipes and surrounding the exterior of the pipes; it is also commonly found there as matted fibres and as inclusions in prehnite, quartz and hydroxyapophyllite-(K).
At the Mitchells quarry large masses of byssolite have been found containing actinolite crystals to about 0.6 cm across and 3.3 cm in length and clusters of the same. Also suspended within the byssolite as “floaters” were titanite crystals to 1 cm, tiny calcite crystals, and tiny clusters of epidote crystals.
Actinolite crystals to 1.7 cm in length, in and out of matrix, have been collected at the Luck Bealeton Plant (R&M 98.2.123).


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).

Back to Minerals