Geological Properties Of Tremolite

 

Geological Properties Of Tremolite. Tremolite is an important rock-forming mineral and occurs most commonly as a white to light green, granular to fibrous component of metamorphosed carbonate rocks, especially skarns and dolomite marbles, where the crystals and aggregates can become quite large. Any light colored amphibole in a marble will normally be tremolite. Large isolated and well-terminated crystals are rare. In this environment, it commonly forms as pseudomorphs of tremolite after diopside, but can also be primary. Tremolite is also common in many metamorphosed mafic and ultramafic rocks such as amphibolites and metabasalts although actinolite and other iron-rich amphiboles usually dominate there. Fibrous (asbestiform) tremolite veins are common in many metamorphic rock types, especially where they are carbonate-bearing. In meta-igneous rocks, cummingtonite and anthophyllite can be difficult to distinguish from tremolite, but these are more restricted to metamorphosed Ca-poor ultramafic rocks like serpentinites.

Tremolite is a silicate mineral and member of the amphibole group. Chemical formula is Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. A calcium magnesium silicate, tremolite forms a solid-solution series with ferroactinolite, where iron substitutes in increasing amounts for magnesium. The color of tremolite varies with increasing iron content from colorless to white in pure tremolite to gray, gray-green, green, dark green and nearly black in other specimens. Traces of manganese may tint tremolite pink or violet. When well-formed, crystals are short to long prisms. More commonly, tremolite forms unterminated bladed crystals, parallel aggregates of bladed crystals, or radiating groups. Tremolite and actinolite both form thin, parallel, flexible fibers up to 10 in (25 cm) in length, which are used commercially as asbestos. Tremolite is known as nephrite jade when it is massive and fine-grained. The mineral is abundant and widespread. It is the product of both thermal and regional metamorphism and is an indicator of metamorphic grade because it converts to diopside at high temperatures (1,065°F/575°C or above).

Tremolite occurs as a result of contact metamorphism of calcium and magnesium rich siliceous sedimentary rocks and in greenschist facies metamorphic rocks derived from ultramafic or magnesium carbonate bearing rocks.

 

Formation and Occurrence

Tremolite forms primarily through metamorphic processes, especially during the contact or regional metamorphism of dolomitic limestones and ultramafic rocks. When magnesium-rich fluids interact with calcium-bearing carbonate rocks under heat and pressure, Tremolite crystallizes as a product of these transformations.

This mineral commonly appears in metamorphosed dolomitic marbles, schists, and serpentinites, where it often coexists with minerals such as calcite, dolomite, talc, diopside, and serpentine. In some geological settings, it may also occur as fine fibrous aggregates forming part of asbestos deposits, though non-fibrous varieties (massive or prismatic Tremolite) are far more common in the crystal and collector’s world.

Notably, the presence of Tremolite indicates specific metamorphic conditions—usually medium to high temperatures (400–700°C) and moderate pressures—making it an important index mineral in metamorphic petrology. Its occurrence helps geologists reconstruct the temperature and chemical environment of ancient metamorphic events.

Geological Significance

Tremolite holds significant geological importance as both a metamorphic indicator mineral and a member of the amphibole group, which provides key insight into the chemical evolution of metamorphic rocks. The magnesium-to-iron ratio in Tremolite and its related species (like actinolite and ferro-actinolite) can reveal detailed information about the original rock composition and metamorphic grade.

Its fibrous varieties were historically mined as part of asbestos deposits, particularly in regions such as Canada, the United States, and Italy. While the fibrous form (tremolite asbestos) is no longer mined due to health concerns, the crystalline, compact forms of Tremolite remain valuable for geological research and crystal collections.

Geologically, Tremolite also plays a role in metasomatic replacement—a process where chemical fluids alter pre-existing rocks, changing their mineral composition without melting them. This makes it a key mineral in studying hydrothermal and metamorphic fluid interactions deep within Earth’s crust.

Varieties and Related Minerals

Tremolite often transitions chemically into other amphiboles, forming solid-solution series:

• Actinolite: The iron-bearing variety of Tremolite, typically green.

• Nephrite Jade: A compact, interlocking fibrous form of Tremolite or Actinolite that has been used for thousands of years as jade.

• Tremolite Marble: Metamorphic rock rich in Tremolite, often white to pale green, used decoratively.

Pure Tremolite is generally colorless to white, while increasing iron content gives rise to green shades that lead into actinolite. In rare cases, chromium impurities can lend Tremolite a soft mint-green tint.

 

Associated minerals:

include calcite, dolomite, grossular, wollastonite, talc, diopside, forsterite, cummingtonite, Riebeckite and winchite.

 

Notable Localities

High-quality Tremolite specimens are found worldwide. Major localities include Switzerland (the original Val Tremola site), Canada (Ontario and Quebec), Italy, China, Myanmar, Madagascar, and the United States (New York, California, and New Jersey). Nephrite jade, the most famous form of compact Tremolite/Actinolite, is particularly abundant in British Columbia and New Zealand, where it holds deep cultural and artistic significance.

 

Crystallography: 

Monoclinic; prismatic. Crystals prismatic in habit; the prism faces make angles of 56° and 124° with each other. The termination of the crystals is almost always formed by the two faces of a low clinodome (Figs. 400 and 401). Tremolite is often bladed and frequently in radiating columnar aggregates. In some cases in silky fibers. Coarse to fine granular. Compact

 

Tremolite Composition: 

Ca2Mg5Si80 22 (0 H )2, is an end member of an isomorphous series. Iron may replace magnesium in part, and when present in amounts greater than 2 per cent, the mineral is called actinolite

 

Tremolite Diagnostic Features: 

Characterized by slender prisms and good prismatic cleavage. Distinguished from pyroxenes by the cleavage angle and from hornblende by lighter color.

 

Countries of Origin

Tanzania, United Republic Of; Afghanistan; United States of America; Switzerland; Canada; Austria; Unknown; Norway; China; Italy; Australia; France; Germany\

 

Occurrence of Tremolite

Tremolite is most frequently found in impure, crystalline, dolomitic limestones where it has formed on the recrystallization of the rock during metamorphism. It is also found in talc schists. Actinolite commonly occurs in the crystalline schists, being often the chief constituent of green-colored schists and greenstones. Frequently the actinolite of such rocks has had its origin in the pyroxene contained in the igneous rock from which the metamorphic type has been derived.

Tremolite Uses Area

The fibrous variety is used to some extent as asbestos material. The fibrous variety of serpentine furnishes more and usually a better grade of asbestos. The compact variety nephrite is used largely for ornamental material by oriental peoples.

 

Tremolite Physical Properties

Crystal habit: Elongated prismatic, or flattened crystals; also as fibrous, granular or columnar aggregates
Color White, brown, colourless, grey, light green, green, light yellow, pink-violet
Streak: White
Luster: Vitreous, Silky
Cleavage: Perfect on {110}
Diaphaneity Transparent, Translucent
Mohs Hardness: 5 – 6
Crystal System: Monoclinic
Tenacity: Brittle
Density 2.99 – 3.03 g/cm3 (Measured) 2.964 g/cm3 (Calculated)
Fracture: Splintery
Parting on: {010} {100}
Tremolite Optical Properties
2V: Measured: 88° to 80°, Calculated: 82° to 84°
R:I values: nα = 1.599 – 1.612 nβ = 1.613 – 1.626 nγ = 1.625 – 1.637
Twinning Simple or multiple: common parallel to {100}, rarely parallel to {001}
Optic Sign Biaxial (-)
Birefringence δ = 0.026
Relief Moderate
Dispersion: r < v weak

 

At Crystals by Rob

At Crystals by Rob, we celebrate Tremolite for its natural elegance and connection to Earth’s transformative cycles. Each piece in our collection highlights the mineral’s crystalline grace and geological heritage—formed from the dance of fire and pressure deep within the planet. Tremolite reminds us that change, though often forged under intensity, can yield strength and clarity. Whether admired for its soft luster or its geological significance, Tremolite embodies the balance between transformation and serenity that lies at the heart of our crystal philosophy.