Tridymite, a high temperature polymorph of SiO2, has many structural modifications and these are described by Heaney (1994). Ideally, the basic structure is comprised of sheets of hexagonal tetrahedral rings with alternate tetrahedra around a ring with apices pointing in opposite directions from adjacent tetrahedra. Adjacent tetrahedral sheets are related by a mirror plane to form channels normal to the sheets. Stacking of the sheets follow ABAB… stacking with A representing the initial sheet and B relating to its mirror image. Tridymite rarely occurs upon heating of SiO2 without the presence of a flux. Tridymite occurs as a devitrification phase of obsidian.
Cf., cristobalite, quartz