Page "Mineral" Paragraph 44

Forming 12 % of the Earth's crust, quartz ( SiO < sub > 2 </ sub >) is the most abundant mineral species.

It is characterized by its high chemical and physical resistivity.

Quartz has several polymorphs, including tridymite and cristobalite at high temperatures, high-pressure coesite, and ultra-high pressure stishovite.

The latter mineral can only be formed on Earth by meteorite impacts, and its structure has been composed so much that it had changed from a silicate structure to that of rutile ( TiO < sub > 2 </ sub >).

The silica polymorph that is most stable at the Earth's surface is α-quartz.

Its counterpart, β-quartz, is present only at high temperatures and pressures ( changes to α-quartz below 573 ° C at 1 bar ).

These two polymorphs differ by a " kinking " of bonds ; this change in structure gives β-quartz greater symmetry than α-quartz, and they are thus also called high quartz ( β ) and low quartz ( α ).