Silica Group

Mminerals formed exclusively from silica. There are ten known silica polymorphs, two of which are synthetic. Five of the naturall polymorphs are related by reconstructive transformations and can exist metastably: stishovite, coesite, quartz, tridymite, and cristobalite. Conditions to form coesite and stishovite are attained only during meteoroid impacts where there are extremely high shock pressures.

Three of these have polymorphs related by displacive transformations between low- and high-temperature forms (α and β): quartz (shown above), tridymite, and cristobalite. The low temperature forms have lower degrees of crystal symmetry than the high temperature forms.