Silica Polymorphs

The name quartz comes from the German word “quarz,” of uncertain origin. Quartz and the other main polymorphs of silica are related in the phase diagram [4] shown in Fig. 3. Under ambient conditions, a-quartz is the thermodynamically favored polymorph of silica. At 573°C, a-quartz is transformed into P-quartz, generally similar in structure but with less distortion. This thermal transformation preserves the optical activity of quartz. Heating quartz to 867°C leads to the transformation of P-quartz into P-tridymite, involving the breaking of Si-O bonds to allow the oxygen tetrahedra to rearrange themselves into a simpler, more open hexagonal structure of lower density. The quartz-tridymite transformation involves a high activation energy process that results in loss of the optical activity of quartz. Heating of P-tridymite to 1,470°C gives P-cristobalite that resembles the structure of diamond with silicon atoms in the diamond carbon positions and an oxygen atom midway between each pair of silicon atoms. Further heating of cristobalite results in melting at 1,723°C. A silica melt is easily transformed into vitreous silica by slow cooling, resulting in a loss of long-range order but retaining the short-range order of the silica tetrahedron.

In the last ten years, at least a dozen polymorphs of pure SiO2 have been reported [6]. Stishovite, another form of silica obtained at high temperatures and pressures, has, rather than a tetrahedral-based geometry, a rutile (TiO2) structure in which each Si atom is bonded to six O atoms and each O atom bridges three Si atoms [6]. Stishovite (found in Meteor Crater, Arizona) is more dense and chemically more inert than nor­mal silica but reverts to amorphous silica upon heating.

The distinction among polymorphs other than stishovite arises from the different arrangements of connected tetrahedra. Important examples are quartz and cristobalite. The structures of these polymorphs are relatively complicated. These structures are also relatively open, as corner sharing of oxide tetrahedra prevents the close-packing of anion layers as found in the fcc – and hcp-based oxides [5]. One consequence is that these crystalline structures have low densities, e. g., quartz has a density of 2.65 g cm-3. This low density facilitates structural changes and phase transitions at high pressures. Finally, the high strength of the Si-O interatomic bond corresponds to the relatively high melting temperature of 1,723°C.

When crystalline silica is melted and then cooled, a disordered 3-dimensional network of silica tetrahedra (vitreous silica) is generally formed. Glass manufacturing in the USA is a 10 billion dollar per year industry. It directly benefits from studies of quartz as one of the main raw materials of commercial glasses is almost pure quartz sand, with other raw materials being primarily soda ash (Na2CO3) and calcite (CaCO3) [1].