Actinolite

Actinolite is an amphibole silicate mineral with the chemical formula Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2.

This article is about the minieral. For the community in Canada, see Actinolite, Ontario.
"Uralite" redirects here. For the rail stop, see Uralite Halt railway station.
Actinolite
Actinolite crystal from Portugal
General
CategoryInosilicates
Formula
(repeating unit)		Ca2(Mg4.5-2.5Fe2+0.5-2.5)
Si8O22(OH)2
Strunz classification9.DE.10
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Identification
Colorpale to dark green, yellowish green and black. White or grey when in asbestos
Crystal habitbladed, fibrous, radial
Twinningsimple or lamellar
Cleavageperfect along {110}.
Fractureuneven
Mohs scale hardness5–6
Lustervitreous to dull
Streakwhite
Diaphaneitytranslucent to transparent
Specific gravity3.00 (+0.10, -0.05)
Polish lustervitreous
Optical propertiesBiaxial (-)
Refractive indexnα = 1.613–1.628
nβ = 1.627–1.644
nγ = 1.638–1.655
Birefringence0.0250-0.0270
Pleochroismmoderate, yellow to dark green (in stones that are transparent)
2V angle78–82° (calc), 84–73° (meas)
Dispersionr < v
Ultraviolet fluorescenceinert
Absorption spectrafaint line at 503 nm
References[1][2][3][4][5]

Etymology

The name actinolite is derived from the Greek word aktis (ἀκτίς), meaning "beam" or "ray", because of the mineral's fibrous nature.[2]

Mineralogy

Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2, and iron-rich ferro-actinolite, ☐Ca2(Mg2.5-0.0Fe2+2.5-5.0)Si8O22(OH)2. Mg and Fe ions can be freely exchanged in the crystal structure. Like tremolite, asbestiform actinolite is regulated as asbestos.

Occurrence

Actinolite is commonly found in metamorphic rocks, such as contact aureoles surrounding cooled intrusive igneous rocks. It also occurs as a product of metamorphism of magnesium-rich limestones.

The old mineral name uralite is at times applied to an alteration product of primary pyroxene by a mixture composed largely of actinolite. The metamorphosed gabbro or diabase rock bodies, referred to as epidiorite, contain a considerable amount of this uralitic alteration.

Fibrous actinolite is one of the six recognised types of asbestos, the fibres being so small that they can enter the lungs and damage the alveoli. Actinolite asbestos was once mined along Jones Creek at Gundagai, Australia.

Gemology

Some forms of actinolite are used as gemstones. One is nephrite, one of the two types of jade (the other being jadeite, a variety of pyroxene).[4]

Another gem variety is the chatoyant form known as cat's-eye actinolite. This stone is translucent to opaque, and green to yellowish green color. This variety has had the misnomer jade cat's-eye. Transparent actinolite is rare and is faceted for gem collectors. Major sources for these forms of actinolite are Taiwan and Canada. Other sources are Madagascar, Tanzania, and the United States.[4]

See also

References
  1. Actinolite. Mindat.org
  2. Actinolite. Webmineral.org
  3. Actinolite. Handbook of mineralogy
  4. Gem Reference Guide. City: Gemological Institute of America (GIA), 1988. ISBN 0-87311-019-6
  5. IMA Master List
  • Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
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