Aegirine

inosilicate

primary mineral

Formula:NaFe3+Si2O6 Inosilicate (chain silicate), pyroxene group
Specific gravity: 5.50 to 3.60
Hardness: 6
Streak: Pale yellowish grey
Colour: Black, brown, green. Bright green to yellow-green in thin section.
Solubility: Aegirine is slightly soluble in hydrochloric acid.
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments

Aegirine is a pyroxene. It is a relatively rare primary mineral found chiefly in igneous rocks rich in Na and poor in SiO2 and in syenite pegmatites. It is also found as a secondary mineral in metamorphic rocks.
Aegirine may be found in granite, syenite, nepheline syenite, rhyolite and schist.
In igneous rocks it is associated with orthoclase, feldspathoids, augite and soda-rich amphibole.
In hornfels of contact metamorphic rocks it is found with quartz, spessartine and riebeckite.

Alteration

aegirine and CaO to andradite, quartz and Na2O
2NaFe3+Si2O6 + 3CaO → Ca3Fe3+2 (SiO4)3 + SiO2 + Na2O
This is a high temperature process (DHZ 2A p508).

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O (DHZ 2A p511)

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan (DHZ 2A p512).

serpentine, aegirine and quartz to magnesio-riebeckite and H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 + 2SiO2 → Na2 (Mg3Fe3+2)Si8O22(OH)2 + H2O

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)

jadeite, diopside, magnetite and quartz to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction. (DHZ 2A 512)

Common impurities: Al,Ti,V,Mn,Mg,Ca,K,Zr,Ce

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