Pyroxene

inosilicate

aegirine

Pyroxenes are single chain inosilicates with the general formula M2M1(Si2O6).
There are two groups of pyroxenes, orthopyroxenes such as enstatite Mg2(Si2O6) and clinopyroxenes such as aegirine NaFe3+Si2O6, augite (Ca,Mg,Fe)2Si2O6 and diopside CaMg(Si2O6).
In the discontinuous branch of the Bowen reaction series pyroxene is intermediate between olivine (higher temperature) and amphibole (lower temperature).
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Carbonatites
Metamorphic environments

Pyroxenes are characteristic of the pyroxene-hornfels facies, and are also minerals of the granulite facies.
Pyroxenes are primary minerals.
They are essential constituents of kimberlite, ultramafic rocks and basalt.
Pyroxenes are common but not essential constituents of rhyolite, andesite and eclogite.
They also may be found in granite, syenite, diorite, gabbro, trachyte and hornfels.

Alteration

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.449)

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)

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction (DHZ 2A.123)

Mg-rich siderite and quartz to olivine, orthopyroxene and CO2
3(Fe,Mg)(CO3)→ (Fe,Mg)2SiO4 + 2SiO2 → (Fe,Mg)2SiO4 + 3CO2 (DHZ 1A.266)

Back to Minerals