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Mineral formula: Ca2(Mg4.5-2.5Fe0.5-2.5)Si8O22OH2

Mineral Group: actinolite group, amphibole supergroup

Crystal system: Monoclinic

Crystal habit: Actinolite rarely forms good crystals. It is most frequently found as radiating fibrous to asbestiform masses. It may also occur as bladed crystals to 15 cm; columnar, Crystals are may be kinked or bent. 

Cleavage: Distinct / good on {110}

Fracture: splintery

Color: green, green-black, gray-green, or black

Luster: vitreous, silky

Diaphaneity: transparent to translucent

Moh’s scale hardness: 5 – 6

Streak: White

Specific gravity: 3.03 – 3.24

Named after:  the Greek ακτίνα ("aktina") for "ray" and λίθος ("lithos") in 1794 by Richard Kirwan for "stone" in allusion to the fibrous nature of the original specimens. 

Geological occurrence: Produced by low-grade regional or contact metamorphism of magnesium carbonate, mafic, or ultramafic rocks; also in glaucophane-bearing blueschists.