Actinolite is an important mineral in the identification of some hydrothermal alterations related to mineral deposits such as porphyry copper and skarns, in addition, it is a guide mineral that serves to determine the type and degree of metamorphism in metamorphic rocks that contain the mineral.
Actinolite in thin sheet
The optical properties of actinolite indicate that it occurs as crystals showing a variety of shapes including tabular, columnar, fibrous, sheet and rhombic habits, with the latter exhibiting amphibole cleavage.
Alteration textures range from weak, sometimes selective, partial alteration of pre-existing minerals, to widespread replacement.
There is also a partial or total filling of veins and cavities.
The actinolite alteration is pleochroic, from colorless to very pale yellow to pale green.
Characteristics and physical properties of actinolite mineral
| actinolite | Characteristics and physical properties |
|---|---|
| Class | silicates |
| subclass | Inosilicates |
| Cluster | amphiboles |
| Origin and geological environment | Low to medium grade metamorphism and potassic, sodium-calcic and propylitic alteration. |
| associated rocks | Metamorphic rocks (green schists, blue schists), skarns, copper porphyries (diorite, monzonite, granodiorite and volcanic rocks) |
| Associated minerals or paragenesis | Biotite (phlogopite), K-feldspar (orthoclase), magnetite, quartz, anhydrite, albite-sodium plagioclase, rutile, apatite, sericite, chlorite, epidote |
| Formula and chemical composition | Ca2(Mg,Fe2+)5Si8O22(OH)2 |
| Color | Green, green-black, gray-green or black |
| Brightness | Vitreous |
| Stripe | White |
| cleavage | Perfect, in two directions forming angles of 56° and 124° |
| Fracture | splintery, brittle |
| Tenacity | Fragile |
| Mohs hardness | 5 to 6 |
| Density or specific weight | 2.9 to 3.4 gr/cm3 |
| crystalline system | monoclinic |
| Applications | Guide mineral of metamorphism and propylitic, potassic and sodium-calcic hydrothermal alterations. |
In general, it belongs to the silicates, specifically to the inosilicates.
It is characterized by the fact that it usually appears as elongated acicular crystals in dark greenish tones.
It has a vitreous luster, a hardness of 5 to 6, and a streak of white.
Geological setting, origin and formation
It can be generated in hydrothermal environments related to copper porphyries and skarns, and in low to medium grade metamorphic environments.
Metamorphic actinolite
The actinolite It is a mineral that can form due to thermal or contact metamorphism and regional low to medium grade metamorphism.
It occurs most frequently in calcareous sedimentary rocks that have been affected by contact and regional metamorphism.
They can also be formed by half-grade metamorphism of mafic and ultramafic rocks that transform into blueschists.
Therefore, it is common that they are formed by the alteration of pyroxenes and hornblendes.
Hydrothermal actinolite
The actinolite it can form as a hydrothermal alteration mineral in the core of some porphyry copper deposits, where it can be associated with potassic alteration and mineralization.
It can also form in sodium-calcic alteration, which is generated by the interaction of external fluids with magmatic-hydrothermal systems, and in calcic skarn alteration, particularly endoskarn.
In some mineralized systems, it occupies an inner zone (higher temperature) of the propylitic alteration, followed outward by epidote-chlorite dominated assemblages.
In active geothermal systems hosted by intermediate to mafic volcanic rocks, the actinolite it is present at temperatures greater than 280-300 °C and is characteristic of alteration proximal to intrusive bodies.
How to identify actinolite?
Metamorphic actinolite
Specimens range from green to dark green to black, well formed crystals occur as short to long prisms.
The actinolite it generally occurs as layered crystals, parallel aggregates of layered crystals, or radial clusters.
It is sometimes found as needle-like or fibrous crystals up to 10 inches (25 cm) long.
These elongated crystals are parallel to the cleavage or schistosity of the metamorphic rock.
Alteration or secondary actinolite
The actinolite Hydrothermal occurs in veinlets and as a selective or generalized replacement of mafic minerals (phenocrysts in volcanic and intrusive rocks), in addition, it can alter to chlorite.
It generally occurs as dark green columnar fibrous crystals, particularly in veinlets or cavities.
Its green color and the irregular mottled appearance in the original mineral suggest the replacement of mafic minerals by actinolite fine-grained secondary
Actinolite Uses
It is a guide mineral for metamorphism and propylitic, potassic and sodium-calcic hydrothermal alterations.