Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is abundant as it comprises several minerals and as a synthetic products. All forms are white or colorless, although impure samples can be colored.
Silicon dioxide is a common fundamental constituent of glass.
Structure
In the majority of silicon dioxides, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom (see 3-D Unit Cell). Thus, SiO2 forms 3-dimensional network solids in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms. In contrast, CO2 is a linear molecule. The starkly different structures of the dioxides of carbon and silicon are a manifestation of the double bond rule.
Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous). In the form of crystalline, this substance can be found naturally occurring as quartz, tridymite, cristobalite, stishovite, and coesite. On the other hand, amorphous silica can be found in nature as opal, infusorial earth and diatomaceous earth. Quartz glass is the form of intermediate state between this structure.
All of this distinct crystalline forms always have the same local structure around Si and O. In α-quartz the Si–O bond length is 161 pm, whereas in α-tridymite it is in the range 154–171 pm. The Si–O–Si angle also varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz, the Si–O–Si angle is 144°.
Polymorphism
Alpha quartz is the most stable form of solid SiO2 at room temperature. The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. The transformation from α-quartz to beta-quartz takes place abruptly at 573 °C. Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature limit. The high-pressure minerals, seifertite, stishovite, and coesite, though, have higher densities and indices of refraction than quartz. Stishovite has a rutile-like structure where silicon is 6-coordinate. The density of stishovite is 4.287 g/cm3, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648 g/cm3. The difference in density can be ascribed to the increase in coordination as the six shortest Si–O bond lengths in stishovite (four Si–O bond lengths of 176 pm and two others of 181 pm) are greater than the Si–O bond length (161 pm) in α-quartz. The change in the coordination increases the ionicity of the Si–O bond. More importantly, any deviations from these standard parameters constitute microstructural differences or variations, which represent an approach to an amorphous, vitreous, or glassy solid.
Faujasite silica, another polymorph, is obtained by the dealumination of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment. The resulting product contains over 99% silica, and has high crystallinity and specific surface area (over 800 m2/g). Faujasite-silica has very high thermal and acid stability. For example, it maintains a high degree of long-range molecular order or crystallinity even after boiling in concentrated hydrochloric acid.
Molten SiO2
Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid water: negative temperature expansion, density maximum at temperatures ~5000 °C, and a heat capacity minimum. Its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C.
Molecular SiO2
The molecular SiO2 has a linear structure like . It has been produced by combining silicon monoxide (SiO) with oxygen in an argon matrix. The dimeric silicon dioxide, (SiO2)2 has been obtained by reacting O2 with matrix isolated dimeric silicon monoxide, (Si2O2). In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si–O–Si angle of 94° and bond length of 164.6 pm and the terminal Si–O bond length is 150.2 pm. The Si–O bond length is 148.3 pm, which compares with the length of 161 pm in α-quartz. The bond energy is estimated at 621.7 kJ/mol.