Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of quartz. It has only been found in Martian and lunar meteorites where it is presumably formed from either tridymite or cristobalite – other polymorphs of quartz – as a result of heating during the atmospheric re-entry and impact to the Earth, at an estimated minimum pressure of 35 GPa.