TREMOLITE

Tremolite – Ca2Mg5Si8O22(OH)2 – is an inosilicate of the Amphibole Group. It is an important rock-forming mineral, typical of metamorphic rocks. Due to its fibrous habit, it was occasionally mined as “asbestos”, but its use has been decreasing due to health problems generated by the inhalation of dust from the fibers, which are considered a respirable carcinogen.

Tremolite is part of the Actinolite Group, which is a series of solid solution between tremolite (without Fe), actinolite (with Fe+Mg) and ferroactinolite (with Fe). Tremolite also forms a series with edenite and richterite. Under conditions of high temperatures and pressures, in the upper amphibolite and granulite facies, tremolite forms a series with “hornblendes”. Intermediate compositions between cummingtonite and tremolite are also known. For the detailed classification of amphiboles, see other sources.

Tremolite may contain, as impurities, Ti, Mn, Al, Na, K, F, Cl and H2O. A purple Mn bearing variety of tremolite is “hexagonite”. “Byssolite”, a white variety of amphibole with a capillary habit, is composed in whole or in part of tremolite. Nephrite, one of the materials that makes up jade, is also tremolite in whole or in part.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Usually colorless, white or with pale colors (green, yellow). It can be brown, grey, green, pink-violet.     

Habit: Long to short prismatic, acicular, fibrous, capillary, granular.

Cleavage:  {110} perfect, as other amphiboles.      

Tenacity: Splintery.        

Twinning: Common, // to {100}, single or multiple. 

Fracture: Fibrous.       

Mohs Hardness: 5 – 6

Parting:  On {010} and {100}.        

Streak:  White.        

Lustre: Vitreous, silky.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.99 – 3.03

 

2. Geology and Deposits

Members of the Actinolite Group are restricted to low- and medium-grade metamorphic rocks. Tremolite is very common in limestones that have undergone contact metamorphism or regional metamorphism, such as marbles and skarns, where it can form large crystals. Any light colored amphibole in marbles is usually tremolite. In these cases it can be primary or, more frequently, a pseudomorph over diopside.

Tremolite also occurs in mafic and ultramafic rocks metamorphosed to greenschist facies, such as amphibolites and metabasalts, but in these rocks the dominant amphiboles are actinolite and other Fe-rich amphibole.

Tremolite and actinolite are common alteration products of pyroxene and amphibole, in this case known as uralite (uralitization).

 

3. Mineral Associations

Tremolite occurs with carbonates (calcite, dolomite) and diopside.

Also with quartz, serpentine, garnet, wollastonite, talc, olivine (forsterite), cummingtonite, scapolite, riebeckite, pyrite, epidote, winchite and magnetite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.599 – 1.688    nβ: 1.610 – 1.697       nγ: 1.620 – 1.705

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. Low Fe tremolite is the only colorless monoclinic amphibole.

As Fe contents increase, the color changes to light green and then to dark green, whereupon it exhibits weak pleochroism.

Relief:  Moderate to high.          

Cleavage: {110} perfect: in the basal sections, usually rhomb-shaped, there are two cleavages that intersect at 124º and 56º, typical of amphibole. In the longitudinal sections there is only one cleavage, parallel to the Z axis.           

Habits: Long prismatic. They can be fibrous, tabular or acicular forming fibrous aggregates. Longitudinal sections tend to show rhombic outlines.   

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.026, corresponding to 2nd order interference colors: strong and intense colors in yellow, red, orange, blue.           

Extinction: Longitudinal sections have oblique extinction with an angle of 10 to 15º. Basal sections have symmetric extinction.           

Elongation sign: ES(+) (diagnostic!).            

Twins: Simple and lamellar twins are common.

Zoning: No.             

CONVERGENT LIGHT

Character:  B(-).         

2V angle: 80 – 88o         

Alterations:  Tremolite alters to talc, chlorite and calcite. Can be replaced with hornblende.         

May be confused with:  Tremolite differs from other monoclinic amphibole in being colorless.

Arfvedsonite and other sodic amphiboles show well-defined bluish colors.

Actinolite is very similar, but has pale green colors and pleochroism in PPL.

Cummingtonite in meta-igneous rocks may be very similar, but often has thin lamellar twins and is B(+).

Anthophyllite in serpentinites also may be very similar, but shows parallel extinction in the longitudinal sections and does not present twins.

Wollastonite is similar, but has different habits, almost parallel extinction in the longitudinal sections and a smaller 2V angle.        

 

Tremolite 01: In PPL, diagonally in the image, colorless longitudinal section of tremolite with the typical long prismatic habit and a cleavage paralell to elongation (often difficult to see!).
Tremolite 01: The same section as of the previous image, now in XPL. Intense colors and oblique extinction are characteristic of tremolite.
Basal section of tremolite in XPL. Diagnostic are the rhombic shape, the intense colors and the existence of two cleavages that intersect at 56º and 124º angles, as in all amphiboles. When the basal sections are very small, the cleavage is often difficult to visualize.
Basal section of tremolite in XPL. Diagnostic are the rhombic shape, the intense colors and the existence of two cleavages that intersect at 56º and 124º angles, as in all amphiboles. When the basal sections are very small, the cleavage is often difficult to visualize.
Basal section of tremolite in XPL. Diagnostic are the rhombic shape, the intense colors and the existence of two cleavages that intersect at 56º and 124º angles, as in all amphiboles. When the basal sections are very small, the cleavage is often difficult to visualize.
Tremolite 02: Opaque minerals, calcite and tremolite in PPL. The long prismatic forms are not always well developed, but it is usually possible to find basal sections with the diagnostic cleavages. Care must be taken not to confuse tremolite with wollastonite.
Tremolite 02: The same section as of the previous image, now in XPL. Intense birefringence colors are characteristic for tremolite.
In XPL, basal section of tremolite with simple twin.
In XPL, at the contact between two basal sections of tremolite, a hole (black), formed in the thin section, a very common feature on tremolite thin sections because tremolite shatters easily. As a general rule, holes in the thin section indicates minerals with a splintery tenacity, like wollastonite, tremolite, kyanite and some others.
In XPL, radial aggregate of tremolite in a meta-carbonatite. The fibrous habit is very evident. Some actinolites with similar habit, also present, can be differentiated easily because actinolite show greenish colors with pleochroism in PPL.
In XPL, tremolite in a meta-ultramafic rock with glaucophane, with many magnetites (opaques = black).
Tremolite 03: In PPL, very large crystals of tremolite in a tremolitite, observed with the 2.5 objective and the 6x ocular. Basal sections tend to rhombic and show two cleavages, longitudinal sections show only one cleavage direction.
Tremolite 03: The same section as of the previous image, now in XPL. Tremolite always is very colorful.

5. Reflected Light Microscopy

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

Sample preparation: Due to its excellent cleavages, its long prismatic to acicular habit and its splintery tenacity, polishing tremolite is complicated and generally has a much lower quality than other minerals that accompany it, such as calcite and phlogopite, for example. However, this poor polish, combined with other tremolite characteristics, helps to identify it.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like common silicates and calcite.       

Pleochroism:  No.     

Reflectivity: Low (<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is not observed.        

Internal reflections: Generalized clear, may be almost colorless, greenish or yellowish. Depending on the orientation of the cleavages in relation to the plane of the polished section, they can be multicolored.      

May be confused with: actinolite, which is very similar, usually with a stronger greenish color. It is not possible to differentiate a tremolite with low iron content from an actinolite, as the colors of the internal reflections are very similar, as are the other characteristics.       

General Characteristics: 

Grain shape is usually long prismatic, typical of amphibole.

Relief may be higher than that of some associated minerals (calcite, micas, etc).

Cleavage is usually observed, but in the basal sections it is more difficult because holes are generated.

Polishing pits with triangular shape are very frequent.

Polishing scratches are rare.

Tremolite 01: Tremolite in carbonatite, with calcite on the left and tremolite on the right. The dark grains are phlogopite. In PPL, calcite and tremolite show cleavage and same reflection color, but calcite has strong pleochroism. Phlogopite is much darker.
Tremolite 01: The same section as of the previous image, now in XPL. Tremolite has slightly greenish internal reflections, calcite is colorless to white with strong anisotropy, and phlogopite is brown.
Tremolite 02: In PPL, tremolite with common features. Cleavage, polishing pits and holes (black) are always present. At the top of the image, calcite.
Tremolite 02: The same section as of the previous image, now in XPL. Multicolored reflections ocurr in some crystals as a function of the orientation of the cleavage in relation to the plane of the polished section.