Tridymite and cristobalite are high-temperature polymorphs of SiO that occur in high-silica volcanic rocks. It is associated with troilite, pyroxferroite, cristobalite and alkali feldspar. Cristobalite is a mineral polymorph of silica that is formed at very high-temperatures. It occurs together with allophane, quartz, cristobalite, gibbsite, vermiculite and limonite. The high-temperature forms of tridymite and cristobalite are known from silica-rich volcanic rocks. Tectosilicates such as quartz, and cristobalite are common in soils. Other phases, e.g. the higher-temperature phases tridymite and cristobalite, are not significant for oscillators. Silification is prominent and has formed cristobalite and tridymite. There is more than one form of the cristobalite framework. The structure of solid BeF resembles that of cristobalite.