Omphacite

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Formula: (Ca,Na)(Mg,Fe,Al)Si₂O₆
Inosilicate (chain silicate), clinopyroxene subgroup, pyroxene group
Crystal System: Monoclinic
Specific gravity: 3.29 to 3.39
Hardness: 5 - 6
Streak: Greenish white
Colour: Dark green
Common impurities:Ti,Cr,Mn,K,H₂O
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)

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