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