Tridymite

Silica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. Tridymite is found in terrestrial volcanic rocks, lunar rocks and martian meteorites, IVA and lIE iron meteorites, eucrites, mesosiderites, and enstatite chondrites.
Tridymite structure
Image source: http://www.auburn.edu/~hameswe/Tridy_Cristobpage.html.