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Formula: (Ca,Mg,Fe)2Si2O6
Inosilicate (chain silicate), clinopyroxene
Varieties
Fassaite is a variety of augite with a low iron content.
Properties
Crystal System: Monoclinic
Specific gravity: 3.19 to 3.56 measured 3.31 calculated
Hardness: 5½ to 6
Streak: Greenish gray, light to dark brown
Colour: Black, brown, green
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Common impurities: Ti,Cr,Na,Mn,K
Environments:
Plutonic igneous environments (typical)
Pegmatites
Carbonatites
Metamorphic environments
Meteorites
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.
Localities
Amity, Town of Warwick, Orange county, New York, USA, is an area of
granite intrusions into
marble and associated
gneiss. The marble is
mostly composed of white crystalline calcite that often has small flakes
or spheres of graphite and
phlogopite. Augite forms very large, well-formed crystals to
30 cm at contact zones between the
granite and marble.
The crystals are dark green to black, prismatic, opaque, and commonly have brown
goethite staining on their surfaces
(R&M 96.5.434-435).
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
enstatite-ferrosilite,
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).
enstatite-ferrosilite,
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).
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