Actinolite

Actinolite is an intermediate member Actinolite-Tremolite Series. 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. The name actinolite has been retained for petrological reasons in the nomenclature (Leake et al., 1997; Hawthorne et al. 2012), since it is entrenched in the literature. It is an important indicator mineral of the greenschist facies, and many petrologists use the actinolite-pumpellyite facies to describe a defined set of pressure and temperature conditions. It has therefore been decided to keep the mineral actinolite.



Actinolite rarely forms good crystals. It is most frequently found as radiating fibrous to asbestiform masses. It may also occur as bladed crystals to 15 cm; columnar. Crystals may be kinked or bent.

Varieties of actinolite include:

Strahlstein: Crystals forming rays or rosettes

Smaragdite: Bright green variety colored by a small chrome content.


There are several different amphibole varieties that often are actinolite, these are:

Nephrite jade:

A compact massive amphibole with interlocked and intertwined fibers, giving a very tough rock. This material can often be cut into rather attractive cabochons that are often used for jewelry purposes. The amphibole is often in the tremolite/actinolite series.

Byssolite:

Intergrown hair-like masses formed in low-temperature environments, typically miarolic cavities or alpine clefts. Some of these specimens may not be actinolite but perhaps tremolite or some other mineral. Arfvedsonite also forms byssolite, but that would be in alkaline, igneous rocks. Often you will see quartz crystals with green hair like inclusions that is labeled actinolite in quartz. It is much easier to put an actinolite label on such specimens rather than confirm the identification by analysis. In the polar Urals, Russia, there was apparently a walk-in sized pocket lined with quartz crystals and fabulous axinite specimens that was stuffed full in places with fibrous green byssolite. Some of the byssolite had perfect small, two to three cm, gemmy, lustrous, floater ferroaxinite crystals in them. The miners would grab a handful of the byssolite and shake the axinites out. It would be great to have a handful of the stuff with the axinites still in it, but I doubt if any of it was saved.

Uralite:

Fibrous amphibole pseudomorphs after pyroxene crystals. These pseudomorphs are often actinolite.

Mountain leather: Densely matted inter-growths of microscopic, long fibrous crystals. These mats are usually whitish in color and you can bend them around a little like leather. A few other minerals like palygorskite and tremolite also form similar leather-like masses.

Actinolite occurs in the following paragenetic environments:

1. Contact metamorphic localities and skarn

This is a common type of occurrence for actinolite. Magnetite bearing skarns are the main paragenetic environment for ferro-actinolite. Also, the amphibole variety uralite is common in skarns. There are several skarn localities represented in this article.

2. Regional metamorphic localities

Actinolite is a main constituent in greenschists, and in some amphibolites. These types of localities do normally not produce nice crystals. Actinolite is also often found in altered ultramafic rocks, often together with talc. Here, a small Cr content can give actinolite with a very nice green color: the variety smaragdite.

3. Alpine type veins

The variety byssolite is typical for alpine veins, and numerous examples are known from the Alps. The Pakistani and Namibian localities producing amphiboles with excellently developed crystals appear to be from alpine clefts or veins in the host rock.

4. Miarolitic cavities in granites and other igneous rocks
Also in this paragenesis, byssolite is common.