Omphacite

inosilicate

metamorphic

Formula: (Ca,Na)(Mg,Fe,Al)Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity: 3.29 to 3.39
Hardness: 5 - 6
Streak: Greenish white
Colour: Dark green
Environments:

Metamorphic environments

Omphacite is an essential constituent of eclogite as a result of high pressure and high temperature metamorphism.
It is also found in kimberlites.
It is a characteristic mineral of the eclogite facies.

Alteration

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above (DHZ 2A p453).

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction (DHZ 2A p453).

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2
This reaction occurs at high temperature and pressure (DHZ 2A p449).

diopside and albite to omphacite and quartz
CaMgSi2O6 + xNaAlSi3O8 ⇌ CaMgSi2O6.xNaAlSi2O6 + SiO2
(DHZ 2A p453)

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
(DHZ 2A p449)

Common impurities:Ti,Cr,Mn,K,H2O

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