Ferro-actinolite

inosilicate

hedenbergite

Formula: ☐Ca2(Mg2.5-0.0Fe2+2.5-5.0)Si8O22 (OH)2
Mg/(Mg+Fe2+) < 0.5
Inosilicate (chain silicate)
Specific gravity: 3.24 to 3.48
Hardness: 5 - 6
Streak: greenish white
Colour: black-green, dark green
Environments

Metamorphic environments

Ferro-actinolite occurs in iron-rich greenschist and blueschist facies metamorphic rocks and in metamorphosed iron formations. In regional metamorphic environments it is associated with hedenbergite. In contact metamorphic environments it is associated with andradite and ilvaite. In iron formations it is associated with cummingtonite, quartz, magnetite, riebeckite, biotite and hematite.

Alteration

ferro-actinolite to hedenbergite, grunerite, quartz and H2O
7Ca2Fe2+5Si8O22(OH)2 → 14CaFe2+Si2O6 + 3Fe2+2Fe2+5 Si8O22(OH)2 + 4SiO2 + 4H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, probably due to the instability of ferro-actinolite with rising temperature. The association of hedenbergite and grunerite has been widely described, and its formation may be due to the above reaction (DHZ 2A p273).

ferro-actinolite, calcite and quartz to hedenbergite, CO2 and H2O
Ca2Fe2+5Si8O22(OH)2 + 3CaCO3 + 2SiO2 ⇌ 5CaFe2+Si2O6 + 3CO2 + H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, according to the above reaction, probably due to the instability of ferro-actinolite with rising temperature (DHZ 2A p273).

Common impurities: Ti,Al,Mn,Na,K,F,P

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