Actinolite
Actinolite crystal from Portugal
General
Category	Inosilicates
Formula (repeating unit)	Ca2(Mg4.5-2.5Fe2+0.5-2.5) Si8O22(OH)2
IMA symbol	Act[1]
Strunz classification	9.DE.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	pale to dark green, yellowish green, blue 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	[2][3][4][5][6]

Actinolite is an amphibole silicate mineral with the chemical formula Ca₂(Mg_4.5–2.5Fe²⁺_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.^[3]

Mineralogy

Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, Ca₂(Mg_5.0-4.5Fe²⁺_0.0-0.5)Si₈O₂₂(OH)₂, and iron-rich ferro-actinolite, ☐Ca₂(Mg_2.5-0.0Fe²⁺_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).^[5]

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.^[5]