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Tremolite and Actinolite are two very similar minerals that form a series with each other and
essentially share the same chemical formula. Tremolite has a greater
presence of magnesium over iron, whereas Actinolite has a
greater presence of iron over magnesium. Tremolite and Actinolite share several recognized varieties.
Mountain Leather, a thickly fibrous and leathery variety, has a
silky luster, a soft felt-like feel, and elastic fibers.
Nephrite, another fibrous variety, is made up of tough, interlocking
fibers, so dense that the fibers are not discernible. Actinolite
and Tremolite both contain a form of asbestos which is made of
movable and elastic fibers. Actinolite asbestos is less common;
most forms are in fact Tremolite. This form of the mineral contains
significant health hazards and should never be brought near the mouth. If its fibers or particles enter the lungs, they can cause asbestosis. Asbestosis is a lung disease caused by inhalation of asbestos particles, which causes several cancers, particularly lung cancer and mesothelioma. Symptoms of asbestosis do not arise until about 20 years after the inhalation. Due to the hazards, washing hands after handling specimens is highly recommended. Many mineral collectors avoid collecting asbestos minerals out of safety concerns. |
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Chemical Formula |
Ca2(Mg,Fe)5Si8O22(OH)2 |
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Composition |
Basic magnesium, calcium, iron silicate |
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Color |
White, light to dark gray, black, light yellow, light to dark green, emerald green, pink to purple. Rarely colorless. |
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Streak |
Colorless |
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Hardness |
5 - 6 |
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Crystal System |
Monoclinic |
Crystal Forms
and Aggregates |
As elongated prismatic crystals, in bladed groups, columnar,
fibrous, reticulated and acicular. Also occurs
radiating, as wheat sheaf formations, as thin hairlike masses,
and as tough interlocking fibers which may appear massive. |
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Transparency |
Transparent to translucent |
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Specific Gravity |
2.9 - 3.2 |
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Luster |
Vitreous, silky |
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Cleavage |
2,2 - prismatic |
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Fracture |
Uneven, splintery |
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Tenacity |
Brittle. Fibrous forms are elastic. |
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Other ID Marks |
Often fluorescent, usually white to yellow. |
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Complex Tests |
Insoluble in acid |
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In Group |
Silicates; Inosilicates; Amphibole Group |
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Striking Features |
Crystal habit and cleavage angle of amphiboles. |
| Environment |
In contact metamorphic rocks in hornfels and skarns, and Serpentine deposits. Commonly in marble of metamorphasized Calcite and Dolomite, and also as a secondary mineral in igneous basalt and diabase.
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| Rock Type |
Igneous, Metamorphic |
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Popularity (1-4)
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2 |
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Prevalence (1-3)
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2 |
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Demand (1-3) |
1 |
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Note: Some of these varieties are also varieties of other amphibole minerals, especially Actinolite.
- Pink to purple variety of Tremolite found primarily in St. Lawrence Co., New York.
- Variety of Actinolite (or sometimes Tremolite) that is made up of tough, hard, interwoven fibers that are extremely dense.
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The finely fibrous variety of Tremolite is used for industrial asbestos. Although fibrous
Serpentine is the main source of asbestos, Tremolite and Actinolite are also asbestos producers. Because it is not affected by
fire and is a poor heat conductor, asbestos is used in fire retardant
devices and for heat protection.
The variety Nephrite is used as the gemstone
Jade. Although most forms of Nephrite Jade are of the Actinolite type, Tremolite may also form Jade, which is generally lighter in color than the more common Actinolite form of Jade.
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Europe contains some very good localities for Tremolite, specifically Campolungo in Ticino, Switzerland; Saint-Marcel, in the Val D'Aosta, Italy; Alvito, Beja District, Portugal; and Storakersvatn, Rana, Norway (where it occurs as a pseudomorph after Diopside). An exceptional emerald green, chromium-rich variety comes from the Merelani Hills in Arusha, Tanzania.
Important Canadian occurrences in Ontario are at Wilberforce, Tory Hill, and Dancey Farm, both in Haliburton Co.; and the Bancroft District, Hastings Co.
In the U.S., the lilac Hexagonite variety is well-known at Balmat, Fowler, Edwards, and Richville, all in St. Lawrence Co., New York. Excellent green Tremolite comes from West Pierrepont, St. Lawrence Co., New York. Very good Tremolite crystals also come from nearby Diana, Lewis Co., New York. Radiating sprays come from Canaan, Litchfield Co., Connecticut; and a fibrous form from Ashland, Middlesex Co., Massachusetts. Tremolite crystal masses have come from Franklin and Sparta, Sussex Co., New Jersey.
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Tourmaline - Lacks cleavage, harder (7 - 7½).
Wollastonite - Softer (4½ - 5), different cleavage angle.
Epidote- Different cleavage angle, crystals are more glassy.
Actinolite - No distinction can be made without x-ray equipment.
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