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Formula: ☐Ca2(Mg2.5-0.0Fe2+2.5-5.0)Si8O22
(OH)2
Mg/(Mg+Fe2+) < 0.5
Inosilicate (chain silicate)
Crystal System: Monoclinic
Specific gravity: 3.24 to 3.48 measured, 3.34 calculated
Hardness: 5 to 6
Streak: greenish white
Colour: black-green, dark green
Common impurities: Ti,Al,Mn,Na,K,F,P
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).
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