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Formula: (Ca,Na)(Mg,Fe,Al)Si2O6
Inosilicate (chain silicate), single chains,
clinopyroxene subgroup,
pyroxene group
Jadeite, kosmochlor and
omphacite are all members of the
clinopyroxene subgroup, found in metamorphic
environments, and components of Fei Cui (翡翠), the name preferred by modern gemmologists for "jade", which is a
rock comprising intergrowths of the three minerals. The preferred terminology is "jadeite jade", "kosmochlor jade" and
"omphacite jade" indicating the dominant mineral. NB "jade" is not synonymous with "jadeite".
Nephrite is a rock composed mainly of
amphiboles; it is not fei cui and preferably should not be referred to as
jade.
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:
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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