Hedenbergite

inosilicate

metamorphic

Formula: CaFe2+Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity: 3.5 to 3.6
Hardness: 5 - 6
Streak: Greenish, brownish, grey, also colourless
Colour: Dark green, greenish black, brownish black, black
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Metamorphic environments

Hedenbergite is common in iron-rich metamorphic rocks, such as skarn, in alkaline granite and as xenoliths in kimberlite.

Alteration

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4
(DHZ 2A p651)

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2
(DHZ 2A p274)

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction (DHZ 2A p272).

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction (DHZ 2A p272).

Fe-rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2
(DHZ 2A p126)

diopside-hedenbergite and CO2 to enstatite-ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2
(DHZ 2A p136)

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies (DHZ 2A p139).

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).

hematite, wüstite, quartz and calcite to andradite, hedenbergite and magnetite
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2

titanomagnetite (ilmenite combined with magnetite), quartz, and aegirine-hedenbergite to aenigmatite, hedenbergite, magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) + 12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6) ⇌ 3Na4[Fe2+10Ti2]O4[Si12O36] + 12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 + 5O2
(DHZ 2A p652)

Common impurities: Mn,Zn,Ti,Al,Mg,Na,K

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