Formula: SiO2 tectosilicate (framework silicate)
A high-pressure paramorph of quartz
Specific gravity: 2.92
Hardness: 7½ - 8
Streak: White
Colour: Colourless
Common impurities: Al,Fe,Na
Environments:
Images
Formula: SiO2 tectosilicate (framework silicate)
A high-pressure paramorph of quartz
Specific gravity: 2.92
Hardness: 7½ - 8
Streak: White
Colour: Colourless
Common impurities: Al,Fe,Na
Environments:
Coesite may be found in
eclogite and
kimberlite.
It is a mineral of the
blueschist and
eclogite facies.
Alteration
Coesite is a high pressure paramorph of
quartz. With increasing pressure, At
800oC alpha quartz alters to coesite at about 30 kbar pressure, then
coesite alters to stishovite at about 90 kbar.
alpha quartz, beta quartz and coesite can
co-exist at a point where the temperature is about 1,360oC and the pressure 34 kbar.
beta quartz and coesite can co-exist in equilibrium with the silica melt at a point
where the temperature is about 2,410oC and the pressure 45 kbar.
With further increase in pressure and temperature, coesite can continue to exist up to about 2,770oC and
110 kbar pressure, at which point coesite, stishovite and the silica melt
are in equilibrium
(QP).
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