Formula: SiO2 tectosilicate (framework silicate)
A high-pressure polymorph of quartz
Specific gravity: 2.92
Hardness: 7½ - 8
Coesite may be found in
It is a mineral of the blueschist and eclogite facies.
Coesite is a high pressure polymorph 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 temerature, 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.
Common impurities: Al,Fe,Na