ACTINOLITE

Actinolite – Ca2(Mg,Fe)5Si8O22(OH)2 – is an inosilicate of the Amphibole Supergroup. It is a very common mineral, typical of metamorphic rocks. It is mined when it occurs with fibrous habit and is marketed as “asbestos”, but this usage is less and less due to health concerns. The variety “nephrite” is one of the two minerals that make up jade and is extremely expensive.

Actinolite is part of the Actinolite Group, which is a series of solid solutions between tremolite (without Fe), actinolite (with Fe+Mg) and ferroactinolite (with Fe). For the detailed classification of amphibole, see other bibliography.

There is a variety of actinolite with an intense green color due to a content of Cr, called “smaragdite”, which occurs in eclogites with retrograde metamorphism as pseudomorphosis on omphacite. “Nephrite” is a green rock composed basically of micro- to cryptocrystalline fibrous actinolite, one of the two minerals (with jadeite) being marketed as jade.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Green, green-black, gray-green.     

Habit: Tabular crystals, may be fibrous radiating, asbestiform. Massive, granular.

Cleavage: {110} good.       

Tenacity: Brittle.        

Twinning: By {100} or {001} common, simple or lamellar.       

Fracture:  Splintery.      

Mohs Hardness: 5 – 6

Parting: By {100}.         

Streak: White.         

Lustre:  Vitreous to pearly.         

Diaphaneity: Transparent.           

Density (g/cm³): 3.03 – 3.24

 

2. Geology and Deposits

Actinolite is a mineral restricted to low-grade metamorphic rocks. Thus, it is common in sediments rich in Ca-Mg-Fe that were regionally metamorphosed to schists, the so-called greenschists. It is found in metamorphosed mafic and ultramafic rocks such as meta-basalts, meta-gabbros and “greenstones”. May be present in contact metamorphic rocks such as adinoles.

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.

 

3. Mineral Associations

It occurs associated with other typical minerals of low-grade metamorphic paragenesis: quartz, albite, Chlorite Group minerals, lawsonite, glaucophane, epidote, pumpellyite and talc.

It is also associated with calcite, titanite, magnetite, axinite (Fe), prehnite, plagioclase (oligoclase) and dyscrasite.               

When it occurs as “nephrite”, it is associated with diopside, garnet (grossular), rutile, graphite, apatite, vesuvianite, prehnite, datolite, talc, serpentine, titanite, chromite, magnetite and pyrite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.613 – 1,646     nβ: 1.624 – 1,656        nγ: 1.636 – 1,6

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, pale green to darker green with shades of blue. Typical pleochroism:

X = colorless, pale yellowish green;

Y = pale yellow-green, pale blue-green,

Z = pale green, green, blue-green.

Darker colors and stronger pleochroism are associated with higher Fe content.    

Relief: Moderate. Increases with increasing Fe content.           

Cleavage: {110} perfect.

Longitudinal sections show only one cleavage, parallel to the crystallographic (vertical) z-axis.

Basal sections show two cleavages that intersect at 124º and 56º angles, forming rhombs (as in all amphiboles).           

Habits: Long prismatic crystals, may be fibrous. Subhedral to anhedral forms, but bladed forms can occur; basal sections may form rhombs. As a typical mineral of low-grade metamorphic rocks, crystals are often very small.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.017 – 0.027: 1st order colors to colors of low 2nd order: grey, yellow, orange, red. They may be masked by the mineral’s own color. Color intensity decrease with increasing Fe content.           

Extinction: Oblique, with an angle of 10 – 17º.           

Elongation sign: ES(+).            

Twins:  They are relatively common, both simple and lamellar.        

Zoning: Often zoned.             

CONVERGENT LIGHT

Character: B(-)          

2V angle: 65 – 86º          

Alterations: actinolite alters to talc, chlorite and calcite.          

May be confused with: some other minerals, but the pleochroism and the cleavage are very diagnostic.

Tremolite is very similar, but it is colorless and does not show pleochroism.

Green hornblende is very similar, but have a stronger colors and stronger pleochroism. Actinolite never shows brown colors like hornblende.

Arfvedsonite have much stronger (darker) colors in PPL and always with blue tones.

Orthorhombic amphiboles (antophyllite, holmquistite, ferrogedrite, etc.) show parallel extinction.

Cummingtonite is very similar but tends to have a positive optical signal and has a higher refractive index. In addition, cummingtonite often has very fine lamellar twins.

Wollastonite is similar but has nearly parallel extinction, a smaller 2V angle and occur in other paragenesis.        

 

Actinolite 01: In PPL, actinolite crystals with its first pleochroism color. Long prismatic habit is typical for actinolite.
Actinolite 01: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color. Cleavage is well-defined.
Actinolite 01: The same section as of the two previous images, now in XPL. First order and lower second order interference colors are diagnostic.
Actinolite 02: A group of actinolite crystals in PPL, showing the first pleochroism color.
Actinolite 02: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color.
Actinolite 02: The same section as of the two previous images, now in XPL. The prismatic habit is very diagnostic.
In PPL, basal sections of actinolite with their typical rhombic shapes and showing two cleavage directions that intersect at 56 and 124º.
In PPL, basal sections of actinolite with their typical rhombic shapes and showing two cleavage directions that intersect at 56 and 124º.
Simple twin in actinolite. XPL.
Actinolite 03: In PPL, at left, a large longitudinal section of actinolite with bluish-green colors. In the center of the image, a light green chlorite. On the right, a basal section of actinolite, in darker green.
Actinolite 03: The same section as of the previous image, now rotated 90 degrees to show the other pleochroism color.
Actinolite 03: The same section as of the two previous images, now in XPL. Actinolite with first order interference colors and chlorite with a violet-purple anomalous interference color.
Actinolite 04: In meta-ultramafic rock, aggregate of chlorite and actinolite, both with an almost fibrous habit and green with weak pleochroism.
Actinolite 04: The same section as of the previous image, now in XPL. , Actinolite show its low interference colors (yellowish) and chlorite has a strong anomalous blue interference color.
Actinolite 04: From the same thin section of the two previous images, a detail of the almost fibrous actinolites and chlorites, both in light green. PPL.
Actinolite 04: The same section as of the previous image, now in XPL. Actinolite in yellow/reddish, chlorite in strong "Prussian" or "Berlin" blue.
Actinolite 05: In PPl, early clinopyroxene phenocrystal, probably augite, in acidic volcanic rock. The crystal is intensely uralitized, forming green hornblende and/or actinolite, with a fibrous appearance and pleochroism in green. Macroscopically it resembles a green tourmaline.
Actinolite 05: The same section as of the previous image, now in XPL. The amphiboles show strong colors and almost paralell extinction. Such altered clinopyroxenes are a common feature in these acidic volcanic rocks, classified as "rhyolites" ou "rhyodacites".

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of actinolite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with actinolite.

Sample preparation: The polishing of actinolite is not difficult and follows the polishing of other silicates from the actinolite paragenesis.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, a bit ligther than quartz.       

Pleochroism: No.      

Reflectivity: Very low (~4%)        

Bireflectance:  No.      

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.       

Internal reflections:  Light green, widespread, very luminous to very dark. Multicolored reflections are relatively common, depending on the orientation of the cleavages to the plane of the polished section.      

May be confused with: other amphiboles with a prismatic habit and dark green color. The very dark reflections make it difficult to recognize actinolite.       

General Characteristics: 

Grain shape: the long prismatic form is very typical of amphibole and helps to recognize actinolite.

Cleavage is usually visible.

Polishing pits are common in many of the sections.

Extinction is symmetrical in relation to the cleavages and the shape in the rhombic basal sections.

Actinolite 01: In PPL, long prismatic actinolite crystals, especially well developed. When with smaller sizes, its recognition is not so easy.
Actinolite 01: The same section as of the previous image, now in XPL. Light greenish internal reflections are diagnostic for actinolite.
Actinolite 02: In PPL, magnetite (pink gray) and actinolite (dark gray). In the center of the image are two actinolite crystals side by side: the one on the right shows cleavage and the one on the left, because of its orientation, shows no cleavage.
Actinolite 02: The same section as of the previous image, now in XPL+2 degrees. Actinolite show generalized internal reflections in very dark green. Multicolored reflections are possible.
In XPL, rhombic basal sections of actinolite with its characteristic dark green internal reflections. In some of the sections, it is possible to see the typical amphibole cleavage