Augite

inosilicate

hypersthene

Formula: (Ca,Mg,Fe)2Si2O6 Inosilicate (chain silicate), pyroxene group
Specific gravity: 3.19 to 3.56
Hardness: 5½ to 6
Streak: Greenish gray, light to dark brown
Colour: Black, brown, green
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments (typical)
Pegmatites
Carbonatites
Metamorphic environments

Augite is a primary and secondary mineral and it is the most common pyroxene. It is an important rock-forming mineral, found chiefly in dark coloured igneous rocks. The melting point of pyroxenes is about 1,000oC.

Augite is a common but not essential constituent of gabbro, anorthosite, peridotite, kimberlite, rhyolite, trachyte, andesite and basalt.
It also may be found in granite.

Alteration

ankerite, dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2 (DHZ 2A p384).

augite and CO2 to hypersthene, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe)SiO3 + CaCO3 + SiO2 (DHZ 2A p384).

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).

augite and andalusite to enstatite- ferrosilite and anorthite
Ca(Fe,Mg)Si2O6 +Al2SiO5 → (Mg,Fe2+)SiO3 + Ca(Al2Si2O8) (DHZ 2A p126).

Fe-rich dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2 (DHZ 2A p 384).

hypersthene, augite and Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4 (DHZ 2A p258).

meionite (scapolite series) and augite to garnet, calcite and quartz
Ca4Al6O24(CO3) + 3Ca(Mg,Fe2+)Si2O6 ⇌ 3Ca2(Mg,Fe2+)Al2(SiO4)3 + CaCO3 + 3SiO2 (DHZ 4 p334).

Common impurities: Ti,Cr,Na,Mn,K

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