The Structural Forms of Quartz and Other Silicas
Except for water, silica is the most extensively studied MX2 compound. One of the challenges in studying silica is its complex set of structures. Silica has several common polymorphs under different conditions of temperature  and pressure , as seen in Figs. 2 and 3. For instance, cristobalite is the crystalline silica polymorph at atmospheric pressure above 1,470°C. It is built on an fcc lattice with 24 ions per unit cell. This structure is, in fact, the simplest form of silica. In addition to five polymorphs (quartz, coesite, stishovite, cristobalite, tridymite) that have thermodynamic stability fields, a large and increasing number of metastable polymorphs have been synthesized. These include vitreous silica, clathrasils, and zeolites . Except for stishovite, all these structures are based on frameworks of
1,723 (melting point)
High tridymite Hexagonal
Low quartz Hexagonal
Fig. 2 Principal silica polymorphs at atmospheric pressure 
SiO4 tetrahedra. These silica structures have been determined mainly by X-ray and neutron diffraction methods and, more recently, by Si and Al magic angle spinning solid-state NMR studies.
The various framework silica structures arise from the different ways that the (SiO4)4- tetrahedra are linked into 1-, 2-, and 3-dimensional arrangements. Although the basic tetrahedra are present in most silica structures, the connectivity varies widely.
Both ionic and covalent natures of the Si-O bond contribute to the preference for (SiO4)4- tetrahedron formation in both crystalline and glassy silicas. In addition, each O anion is coordinated by two Si cations, corresponding to corner sharing of the oxide tetrahedra, preventing the close-packing of anion layers and resulting in relatively open structures .