Tridymite and Cristobalite - SiO2

Deep mantle, meteorite impact, and other ultrahigh-pressure settings may result in a more densely-packed SiO2-structure, with silicon in octahedral (6-fold) coordiantion with oxygen. This is the structure basis for stishovite, which is isostructural with rutile (TiO2, in which Ti is in octahedral coordination).

silicon coordination in stishovite This is a part of the unit cell for stishovite, showing the coordination of silicon and oxygen. The oxygen is in hexagonal closest packing; in the full unit cell and mineral structure, half of the possible octahedral positions are occupied by silicon.

stishovite structure Stishovite has a tetragonal structure, with strips of edge-sharing octahedra that run parallel to the c-axis.