Silica (SiO2) occurs naturally in at least nine different varieties (polymorphs), which include tridymite, cristobalite, coesite, and stishovite, in addition to high (β) and low (α) quartz. These forms are characterized by distinctive crystallography, optical characteristics, physical properties, pressure-temperature stability ranges, and occurrences.
The crystal structures of all silica polymorphs except stishovite contain silicon atoms surrounded by four oxygens, thus producing tetrahedral coordination polyhedra. Each oxygen is bonded to two silicons, creating an electrically neutral framework. Stishovite differs from the other silica minerals in having silicon atoms surrounded by six oxygens (octahedral coordination.) Ideal high tridymite is composed of sheets of SiO4 tetrahedra oriented perpendicular to the c crystallographic axis with adjacent tetrahedra in these sheets pointing in opposite directions. High cristobalite, like tridymite, is composed of parallel sheets of SiO4 tetrahedra with neighboring tetrahedra pointing in opposite directions. However, the hexagonal rings are distorted and adjacent sheets are rotated 60° with respect to one another. Coesite also contains silicon atoms tetrahedrally coordinated by oxygen. These polyhedra share corners to form chains composed of four-membered rings. Silicon in stishovite is octahedrally coordinated by oxygen. These coordination polyhedra share edges and corners to form chains of octahedra parallel to the c crystallographic axis.
Chemically, all silica polymorphs are ideally 100% SiO2. However, unlike quartz which commonly contains few impurities, the compositions of tridymite and cristobalite generally deviate significantly from pure silica. This usually occurs because of a coupled substitution in which a trivalent ion such as Al3+ or Fe3+ substitutes for Si4+, with electrical neutrality being maintained by monovalent or divalent cations occupying interstices In the relatively open structures of these two minerals. See also Coesite; Quartz.
