Pyroxene

pyroxene

olivine

enstatite

aegirine

Images

Pyroxenes are single chain inosilicates with the general formula M2M1(Si₂O₆).
There are two groups of pyroxenes, orthopyroxenes and clinopyroxenes.

Orthopyroxenes include

enstatite Mg₂(Si₂O₆)
ferrosilite Fe²⁺₂Si₂O₆

Clinopyroxenes include

aegirine NaFe³⁺Si₂O₆
augite (Ca,Mg,Fe)₂Si₂O₆
clinoenstatite Mg₂Si₂O₆
diopside CaMg(Si₂O₆)
hedenbergite CaFe²⁺Si₂O₆
jadeite NaAlSi₂O₆
johannsenite CaMnSi₂O₆
omphacite (Ca,Na)(Mg,Fe,Al)Si₂O₆
pigeonite (Mg,Fe,Ca)₂Si₂O₆
spodumene LiAlSi₂O₆

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 enstatite-ferrosilite
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