name		system	temperature
-quartz (low quartz)	2.65	trigonal
-quartz (high quartz)	2.53	hexagonal	C
tridymite	2.27	hexagonal	C
cristobalite	2.33	cubic	C
coesite	3.01
stishovite	4.29	octahedral

The transition between and -quartz is displacive, which means that no bonds are broken. It therefore transforms spontaneously to come to equilibrium with its environment. The transformation between -quartz and tridymite is displacive, so bonds are actually broken and reformed. Both tridymite and cristobalite are metastable, however, and will revert to and -quartz at surface conditions. Coesite is a stable polymorph found in the interior of the earth and at meteor crater. When a small amount of coesite is dissolved in HF, a small residue remains. This residue contains SiO₂ in the rutile structure, i.e., Si in octahedral coordination. Only 3 out of the 3500 surface minerals contain octahedral silicon, two out of three of which are only found in meteorites or impact craters. The octahedral polymorph of silica is stishovite. A phase diagram of the phases of quartz can be found in Klein and Hurlbut (1985).