Quartz - SiO₂

 Two polymorphs of quarts are shown, with the (SiO₄)^-4 tetrahedra in blue, each linked along corners to four other tetrahedra. Alpha-Quartz, or simply 'quartz' as we know it, has a hexagonal structure with a three-fold axis of symmetry viewed along [0001]. There is also a high-temperature, beta-quartz polymorph, with six-fold symmetry along [0001]. The displacive transformation between these two types requires sufficiently little energy to happen very quickly, and the beta form readily changes to the alpha form upon cooling. The outlines of beta-quartz polymorphs are sometimes preserved in quartz crystals of volcanic rocks, although six-sided crystal habits are also common for the low-temperature polymorph. Note that the higher temperature polymorph has the higher symmetry.