Actinolite (ac-tin’-o-lite) is a bright-green or grayish-green monoclinic silicate mineral of the amphibole group: Ca2(Mg,Fe)5 Si8O22(OH)2. It may contain manganese. Actinolite is a variety of asbestos, occurring in long, slender, needlelike crystals and also in fibrous, radiated, or columnar forms in metamorphic rocks (such as schists) and in altered igneous rocks. Actinolite was named by Richard Kirwan in 1794 after the Greek word aktinos for ray, based on the allusion to the mineral’s common radiation fibrous habit.
Table of Content
Mineralogy
Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2, and iron-rich ferro-actinolite, Ca2(Mg2.5-0.0Fe2+2.5-5.0)Si8O22(OH)2. Mg and Fe ions can be freely exchanged in the crystal structure. Like tremolite, asbestiform actinolite is regulated as asbestos.
Occurrence of Actinolite
Contact metamorphic localities and skarn are suitable environment for occurrence of actinolite. Actinolite is produced by low-grade regional or contact metamorphism of magnesium-rich limestones, mafic, or ultramafic rocks. Actinolite mineral occurs in regionally metamorphosed rocks, such as schists. It is a main constituent in greenschists, and in some amphibolites. It is often associated with minerals such as talc, chlorite, glaucophane, epidote, lawsonite, albite, and pumpellyite.
Actinolite also occurs in metasomatic rocks such as scarnites and propylites. It rarely occurs in pegmatites. It also occurs in blueschists as a retrograde mineral. Tremolite and actinolite are also common alteration products of pyroxene and hornblende, in which case they are known as uralite. May occur in any rocks hosting pyroxene and/or hornblende.
Physical Properties of Actinolite
| Chemical Formula | Ca2(Mg,Fe)5Si8O22(OH)2 |
| Composition | Basic calcium, magnesium, iron silicate |
| Color | Light to dark green; grayish green to black. |
| Class | Inosilicate Double chains of linked tetrahedra |
| Crystal system | Monoclinic |
| Habit | Acicular, typically radiating or forming parallel aggregates Columnar masses |
| Hardness | 5 to 6 |
| Diaphaneity | Translucent to transparent |
| Cleavage | Two perfect prismatic {110} |
| Streak | Colorless |
| Specific gravity | 3.0 to 3.3 |
| Fracture | Splintery – Thin, elongated fractures produced by intersecting good cleavages or partings (e.g. hornblende) |
| Luster | Vitreous to dull |
| Luminescence | Non-fluorescent |
Optical Properties
| PPL | Colorless to green Pleochroic “60/120” cleavage (actually 56° and 124°) |
| XPL | Upper 1st to 2nd order interference colors |
| δ | 0.03 |
| Pleochroism (x) | Very pale yellow, colorless, pale brown, or light green |
| Pleochroism (y) | Greenish yellow, pale yellow green, pale brown, or bluish green |
| Pleochroism (z) | Pale green, green, dark green, or pale bluish green |
Chemical Composition
| Elements | Content |
| SiO2 | 55.17 |
| MgO | 16.21 |
| CaO | 12.08 |
| FeO | 11.07 |
| Al2O3 | 2.69 |
| Na2O | 0.82 |
| MnO | 0.18 |
| TiO2 | 0.17 |
| Total | 98.39 |
Estimated Properties
| Electron density | Bulk density (electron density) = 3.02 g/cm3 note: Specific gravity of actinolite = 3.04 g/cm3 |
| Photoelectric | PEActinolite = 4.88 barns/electron U = PEActinolite x ρelectron density = 14.73 barns/cm3 |
| Fermion index | Fermion index = 0.02 Boson index = 0.98 |
| Radioactivity | Actinolite is not radioactive |
How to Identify Actinolite
Actinolite can be identified in the field by its colour variations, such as green, green-black, grey-green, and black. It has {110} perfect cleavage. This mineral has a vitreous luster, with white streak. The fracture on this mineral is splintery, thin, elongated fractures – caused due to the intersecting good cleavages. The density of actinolite is 2.98 to 3.1 g/cm3, with a hardness of 5.5 – approximate to a knife blade.
Actinolite in Hand Sample
Actinolite
Actinolite in quartz
Actinolite replacing spinifex olivine in a meta-komatiite
Scanning electron micrograph of actinolite
Actinolite in Thin Section
Actinolite in actinolite schist, plane polars
Actinolite in actinolite schist, crossed polars