Actinolite

For the community in Canada, see Actinolite, Ontario.
Actinolite

Actinolite crystal from Portugal
General
Category Inosilicates
Formula
(repeating unit)		 Ca2(Mg4.5-2.5Fe2+0.5-2.5)
Si8O22(OH)2
Strunz classification 9.DE.10
Crystal system Monoclinic
Crystal class Prismatic (2/m)
H-M symbol: (2/m)
Space group C2/m
Identification
Color pale to dark green, yellowish green and black. White or grey when in asbestos
Crystal habit bladed, fibrous, radial
Twinning simple or lamellar
Cleavage perfect along {110}.
Fracture uneven
Mohs scale hardness 5–6
Luster vitreous to dull
Streak white
Diaphaneity translucent to transparent
Specific gravity 3.00 (+0.10, -0.05)
Polish luster vitreous
Optical properties Biaxial (-)
Refractive index nα = 1.613–1.628
nβ = 1.627–1.644
nγ = 1.638–1.655
Birefringence 0.0250-0.0270
Pleochroism moderate, yellow to dark green (in stones that are transparent)
2V angle 78–82° (calc), 84–73° (meas)
Dispersion r < v
Ultraviolet fluorescence inert
Absorption spectra faint line at 503 nm
References [1][2][3][4][5]

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

Etymology

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

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. 1 2 Actinolite. Webmineral.org
  3. ↑ Actinolite. Handbook of mineralogy
  4. 1 2 3 Gem Reference Guide. City: Gemological Institute of America (GIA), 1988. ISBN 0-87311-019-6
  5. ↑ IMA Master List
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