Actinolite: The amphibole mineral Actinolite information and pictures

About

Actinolite and Tremolite are two very similar minerals that form a series with each other and essentially share the same chemical formula. Actinolite has a greater presence of iron over magnesium, whereas Tremolite has a greater presence of magnesium over iron.

The green color of Actinolite is produced by the iron in its chemical structure. The amount of iron varies among specimens, causing different contrasts of green. More iron will give a specimen a darker color.

Actinolite sometimes occurs as thin, splintery inclusions inside Quartz crystals. If the inclusions are parallel and dense, it will cause chatoyancy in the Quartz.

Actinolite and Tremolite 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. Translucent Nephrite with a uniform or interesting color distribution that is extremely tough fits under the category of Jade.

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 is further discussed in the Tremolite mineral detail page.

Crystal Forms & 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.

Complex Tests

Insoluble in acid

Striking Features

Green color, crystal habit, and cleavage angle of amphiboles.

Environment

In contact and regional metamorphic rocks, Serpentine deposits, hydrothermal replacement deposits, and as a secondary mineral in igneous basalt and diabase.

Varieties

✓ common · ✗ uncommon

Amianthus— Old name applied to any fibrous asbestos mineral of the amphibole group, specifically Tremolite and Actinolite.
Asbestos— Asbestos describes any mineral that is extremely fibrous and flexible. The term is very commonly used to describe the fibrous form of Chrysotile Serpentine, and may also be used in regards to the fibrous variety of Tremolite or Actinolite.
Byssolite— Actinolite or Tremolite composed of tiny, interwoven, fibrous crystals that forms a tough, hairlike mass.
Ferro-actinolite— Form of Actinolite where the magnesium is wholly replaced by iron. Ferro-actinolite is scientifically regarded as a distinct mineral species with a chemical formula of Ca₂Fe₅Si₈O₂₂(OH)₂
Mountain Leather— Name applied to fibrous minerals of the amphibole group (such as Tremolite or Actinolite) composed of a matlike felt of fibers that is leathery in books texture and feel.
Nephrite— Variety of Actinolite (or sometimes Tremolite) that is made up of tough, hard, interwoven fibers that are extremely dense.
Asbestos Amphibole— Describes any finely fibrous mineral of the amphibole group, especially Tremolite and Actinolite.
Uralite— Pseudomorph of Actinolite after any mineral of the pyroxene group, especially Augite.

Uses

Nephrite is one of the two minerals that form the important gemstone Jade. Jade is cut into cabochons, beads, and earrings. Rings and bracelets are sometimes carved out of an entire mass of Jade. Ornamental sculptures and figures are also carved from this gem. Nephrite is only one form of Jade; the other form is the more valuable Jadeite.

The finely fibrous variety of Actinolite is occasionally used for industrial asbestos. Although fibrous Serpentine is the main source of asbestos, Actinolite and Tremolite 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.

Noteworthy Localities

Europe contains some excellent localities for Actinolite. The variety Byssolite, in hairy formation associated with Epidote comes from Knappenwand, Untersulzbach valley, Austria; and fine Byssolite and Actinolite included in Quartz from the Miage Glacier in the Val D'Aosta, Italy. Also in Italy is the Val Malenco, Sondrio Province; and the Passo di Vizze on the Italian/Austrian border. Good Actinolite comes from Norway at Selasvatn, Vegardshei; and at Altermark, Rana. A classic Nephrite locality is Jordanow Slaskie, Lower Silesia, Poland.

Large and lustrous Actinolite crystals from come from Otjiwarongo, Otjozondjupa Region, Namibia; and thin slender crystals, often included in Quartz from Alchuri in the Shigar Valley, Skardu, Pakistan.

In the U.S., noteworthy occurrences are the Carlton Quarry, Chester, Windsor Co., Vermont; the French Creek Mine, Chester Co., Pennsylvania; the Keystone Quarry, Cornog, Chester Co., Pennsylvania; the Calumet Mine, Chaffee Co., Colorado; and the Sears Creek in the Wenatchee Ridge, Chelan Co., Washington. In Canada, important Actinolite deposits are in Tory Hill and Wilberforce, Haliburton Co., Ontario; and in the Bancroft District, Hastings Co., Ontario.

Some of the most extensive Nephrite deposits are in the Granite Mountains of Fremont Co., Wyoming. Alaska also contains several large Nephrite deposits, namely the Jade Mountain Mine in the Kiana District; and in the Shungnak River Mine and the Dahl Creek. Other important Nephrite occurences are Bagby, Mariposa Co., California; and Jade Cove and Willow Creek, Monterey Co, California.

Common Mineral Associations

Albite, Barite, Chlorite, Talc, Epidote, Muscovite, Serpentine

Distinguishing Similar Minerals

Tourmaline - Lacks cleavage, harder (7 - 7½).
Wollastonite - Softer (4½ - 5), different cleavage angle, usually fluorescent.
Epidote - Different cleavage angle, crystals are more glassy
Tremolite - No distinction can be made without x-ray equipment, although Tremolite is usually lighter in color.