Actinolite: Information, Properties, Occurrence

Actinolite is a member of the amphibole group, a family of silicate minerals known for their long, needle-like or prismatic crystals. Actinolite is a relatively common mineral in some metamorphic rocks. 

It belongs to a series with the minerals tremolite and ferro-actinolite. A series occurs when ions can freely substitute between each other. In this case, when iron is predominant the mineral is ferro-actinolite and when magnesium is predominant the mineral is tremolite. Actinolite is the intermediate member.

A variety of actinolite is composed of microscopically fibrous crystals (asbestos) and is called byssolite. Other minerals also form asbestos such as serpentine and the minerals of the series mentioned above. Serpentine asbestos is more widely used and of a better grade in general.

Actinolite crystals from Switzerland. Photo: Henk Smeets/Tomeik Minerals.

Although asbestos has been shown to cause cancer in humans when inhaled in high enough concentrations, it still has many valuable applications. Asbestos is used for fire retardant materials and brake shoes and pads. Its prior use as insulation has been all but eliminated due to health concerns.

Actinolite is commonly found in metamorphic rocks, such as contact aureoles surrounding cooled intrusive igneous rocks. It also occurs as a product of metamorphism of magnesium-rich limestones.

Some forms of actinolite are used as gemstones. One is nephrite, one of the two types of jade (the other being jadeite, a variety of pyroxene). Another gem variety is the chatoyant form known as cat's-eye actinolite. This stone is translucent to opaque, and green to yellowish green color. This variety has had the misnomer jade cat's-eye.

The name actinolite is derived from the Greek word aktis (ἀκτίς), meaning "beam" or "ray", because of the mineral's fibrous nature. 
 

Actinolite in quartz crystal Photo: KimhoaMineral/Hoa Doan Collector

Simple actinolite occasionally forms interesting crystal habits and specimens. Crystals can be similar to hornblende, but are always translucent or even transparent. Typically they are prismatic, flattened and elongated. Crystal specimens of actinolite can make very interesting mineral specimens.

Actinolite Occurrence

Found in various geological environments, including:

Metamorphic rocks: Actinolite is a common constituent in greenschists and some amphibolites, formed during the metamorphism of magnesium-rich rocks.

Altered igneous rocks: In altered ultramafic rocks, like serpentinite, actinolite can form with a beautiful green color known as smaragdite.

Contact aureoles: The zone around cooled intrusive igneous rocks can host actinolite due to the heat-induced alteration of pre-existing minerals.

Hydrothermal veins: Mineral-rich fluids can deposit actinolite crystals in veins and cavities.

Properties of Actinolite

• Formula: Ca₂Si₈O₂₂(OH)₂.
• Class: Silicates
• Subclass: Inosilicates
• Group: Amphibole
• Color is usually green.white or gray.
• Luster is vitreous.
• Transparency: examples are translucent to transparent.
• Crystal System is monoclinic; 2/m
• Crystal Habits include the flattened prismatic and elongated crystal with a dome-like termination that is actually a two of the four faces of a prism. Also as a fibrous mass (asbestos) and as a fibrous and very compact mass (nephrite jade).
• Cleavage: is perfect in two directions at close to 60- and 120-degree angles.
• Fracture is splintery to uneven.
• Hardness is 5.5 - 6.
• Specific Gravity is approximately 2.9 - 3.3 (very slightly above average for translucent minerals).
• Streak is white.
• Associated Minerals are quartz, lawsaonite, epidote and glaucophane.
• Other Characteristics: the compact nephrite variety is extremely tough and is actually stronger than steel.
• Notable Occurrences include the Lake Baikal Region, Russia; China; New Zealand; British Columbia, Canada and Taiwan.
• Best Field Indicators are toughness (nephrite jade), fibrosity (asbestos), typical green color, crystal habit and hardness.