actinolite

(ăk-tĭn'ə-līt')
n.
A greenish variety of amphibole.

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Ca
Monoclinic -- prismatic

Environment

An isomorphous pair resulting from the metamorphism of impure limestones or dolomites. Also in green schists and gneisses (actinolite) possibly derived from pyroxenes. The ultra-compact, fine-needled jade variety called nephrite seems to develop large nodular masses in hot-spot nodes of more intense alteration in serpentine during regional metamorphism.

Crystal description

Single crystals of amphiboles tend toward elongation. In a marble, single tremolite or actinolite crystals may be a bit stubby but can also be long, large, and well formed. Deeper-hued actinolite crystals often form coarse splintery bundles of dark green, almost black crystals. Fine, very tough and tightly bonded, interlocked masses of needles make nephrite jade, which ranges from white through green to black. Soft, loose, and light bundles of white fibers make amphibole asbestos, bysssolite, and "mountain leather."

Physical properties

White, light green, violet (hexagonite), and dark green. Luster glassy; hardness 5-6; specific gravity 3.0-3.3; fracture subconchoidal to uneven; cleavage perfect prismatic. Transparent to translucent; sometimes fluorescent yellow-orange (tremolite).

Composition

Alkaline calcium magnesium (and iron) silicate. When free of iron it is light in color, or white, and is called tremolite. Iron replacing 2% or more of the magnesium makes it green and changes the name to actinolite. Other than color, there is no simple nor logical way of identifying intermediate examples (CaO, MgO, and FeO total about 42%; and SiO ₂ , about 57%).

Tests

Thin splinters of tremolite and actinolite fuse to a black or white glass, the more iron-rich varieties fusing more easily. Insoluble in acid.

Distinguishing characteristics

This series of minerals is most likely to be confused with some of the related series, the pyroxenes. The well-crystallized varieties can be recognized by their prismatic habit and the characteristic cleavage angles. Wollastonite is commonly fluorescent (lost after heating without fusing), and is decomposed by hydrochloric acid. Scapolite melts more easily and is (or on heating becomes) fluorescent in longwave ultraviolet light. Tourmaline has no cleavage. Epidote melts more readily to black magnetic slag.

Occurrence

Tremolite and actinolite are minerals that have formed through metamorphism of impure limestones and serpentine under conditions of moderately high temperature and pressure, in the presence of some water. They will therefore be found in marbleized limestones, and in gneisses and schists with serpentines. Locally they may form considerable formations of rock, when they tend to be in compact masses of rather slender crystals. The best crystals are found piercing coarsely crystalline calcite, from which they are readily freed by careful chiseling or by acid solution of the host calcite.

Good greenish or white tremolite crystals are found in the calcite of Canaan, Connecticut. Coarse green or gray crystals are abundant at many localities in St. Lawrence Co., New York, and in Ontario. An attractive lilac variety known as hexagonite occurs in masses, sometimes shot with more robust and gemmy 1-cm-broad blades, at Fowler, St. Lawrence Co., New York. Gemmy chrome-tremolite, an emerald green variety, has been found in Ontario, Brazil, Finland, and Tanzania. A gray-brown variety in splendid crystals is known as fluo-richterite, found in a coarse marble near Wilberforce, Ontario.

Actinolite also forms solid masses of elongated slender intergrowing crystals and is quite common in this form. Chester, Vermont, is a typical locality, but there are hundreds of others. Actinolite sometimes fills quartz crystals with finely fibrous green "byssolite" hairs, notably at French Creek, Pennsylvania; it appears to influence a steeper crystal shape when included in quartz in Greece and in Plumas County, California. In the form of loose bundles, byssolite mulches the roots of the famed epidote blades of Untersulzbachtal in Austria.

Remarks

Nephrite jade is also actinolite or tremolite, but earns its title by being very compact and massive. It is one of two mineral species called jade (the other, jadeite, is a pyroxene). Amianthus is the ancient name for a white amphibole asbestos. It melts more easily and its dust has been said to be more dangerous to human health than serpentine asbestos. A related sodium iron silicate known as crocidolite (crow-SID o lite) forms the economically worked blue asbestos veins of South Africa where, after alteration to quartz, it is known as "tiger eye." Mountain leather is a natural mat of soft and flexible light-colored interlocking amphibole asbestos fibers.

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For a list of words related to actinolite, see:

• Mineral compounds

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