Topic summary
SiO2
Extracted from the Wikipedia article Silicon dioxide.
Geology
SiO2 is most commonly encountered in nature as quartz, which comprises more than 10% by mass of the Earth's crust. Quartz is the only polymorph of silica stable at the Earth's surface. Metastable occurrences of the high-pressure forms coesite and stishovite have been found around impact structures and associated with eclogites formed during ultra-high-pressure metamorphism. The high-temperature forms of tridymite and cristobalite are known from silica-rich volcanic rocks. In many parts of the world, silica is the major constituent of sand.