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The Mines and Minerals of Lavrion - Actinolite

Last Updated: 29th Nov 2018

By Branko Rieck

ACTINOLITE, ☐{Ca2}{Mg4.5-2.5Fe0.5-2.5}(Si8O22)(OH)2, monoclinic

General Appearance
The name actinolite is derived from Greek ακτίνα ("aktina") for "ray" and λίθος ("lithos") for "stone" in allusion to the fibrous nature of the specimens. This describes it pretty well, although also granular masses have been reported. It is green, green-black, grey-green, or almost black. Actinolite is an (unnecessary) intermediate member of the tremolite-actinolite series that should actually not be mineral at all, but it is being retained because of its entrenchment in the literature. Tremolite is defined as having an Mg/(Mg+Fe2+) ratio ≥ 0.9; actinolite has a ratio of 0.5–0.9 and ferro-actinolite has a ratio of less than 0.5.

Paragenesis
Actinolite is a common mineral in metamorphic rocks that have their origin in magnesium-rich sediments. The contact aureole of the Plaka intrusion, but also the tectonic metamorphism throughout the Lavrion Mining District is riddled with occurrences of actinolite. Also, solutions enriched in magnesium may be responsible for the formation of actinolite. It is often accompanied by the other members of the tremolite-actinolite series, most notably ferro-actinolite, with which it is readily confused. Common associates include calcite, epidote, clinozoisite, chlorite, chamosite-clinochlore solid solution series, quartz, and hematite, as well as pyrite, chalcopyrite, and pyrrhotite formed during the retrograde metamorphic stage.

Localities
The Lavrion Mining District offers no locality for actinolite that stands out. There are numerous places where samples can be picked up. With a high degree of certainty, specimens can be found at the Plaka Granodiorite Mine and in the area between the Adami 2 Mine and Plaka Sulfate Locality in the vicinity of the granodioritic intrusion in the Plaka area.


Fig. 1: Actinolite "suns" on altered granodiorite from the Plaka Granodiorite Mine. FOV 12 mm.


Fig. 2: SEM micrograph (BSE mode) of a polished section of a grey-green, fine-grained, pyrrhotite- and galena-bearing (in narrow calcite veins) skarn sample found near the parking area close to the Plaka Sulfate Locality. This sample was rich in epidote, chamosite, ferro-actinolite and contained violarite and siegenite as rare accessories.
Prismatic crystals of zoned actinolite-ferro-actinolite (dark grey) are embedded in sphalerite (grey). At the bottom are prisms of actinolite. Calcite is dark, and galena is white.


Fig. 3: SEM micrograph (BSE mode) of a polished section of a grey-green, fine-grained, pyrrhotite- and galena-bearing (in narrow calcite veins) skarn sample found near the parking area close to the Plaka Sulfate Locality. This sample was rich in epidote, chamosite, ferro-actinolite and contained violarite and siegenite as rare accessories.
Prismatic crystals of actinolite (dark grey) are intergrown with calcite (bit darker, at the top and the left side) and embedded in galena (white). Minor sphalerite (grey) in bottom right area.


Acknowledgements
Thanks go to Dr. Uwe Kolitsch for allowing me to use his well-established sample data and photos, as well as for constructive comments and diligent proofreading to improve this article.





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