Ferro-actinolite
The iron end-member of the actinolite–tremolite amphibole series.
Intro adapted from Wikipedia (CC BY-SA).
Ferro-actinolite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₂(Mg₂.₅₋₀.₀Fe²⁺₂.₅₋₅.₀)Si₈O₂₂(OH)₂.
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Amphibole structural group.
Key properties: Mohs hardness 5–6; specific gravity 3.24–3.48; Vitreous luster; optically biaxial.
IMA symbol Fact, IMA status: approved, named in 1946.
Found in iron-rich greenschist and blueschist facies metamorphic rocks; in metamorphosed iron formations; as part of a vein assemblage cutting Ca-Fe-Si tactites.
It is found associated with Hedenbergite (replacement), andradite, ilvaite (contact metamorphic); cummingtonite, quartz, magnetite, riebeckite, biotite, hematite (iron formations).
The name refers to ferrous iron in its composition and similarity to actinolite.