Crystal Structure
In the majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen atoms surrounding a central Si atom. The most common example is seen in the quartz crystalline form of silica SiO2. In each of the most thermodynamically stable crystalline forms of silica, on average, all 4 of the vertices (or oxygen atoms) of the SiO4 tetrahedra are shared with others, yielding the net chemical formula: SiO2.
For example, in the unit cell of α-quartz, the central tetrahedron shares all 4 of its corner O atoms, the 2 face-centered tetrahedra share 2 of their corner O atoms, and the 4 edge-centered tetrahedra share just one of their O atoms with other SiO4 tetrahedra. This leaves a net average of 12 out of 24 total vertices for that portion of the 7 SiO4 tetrahedra which are considered to be a part of the unit cell for silica (see 3-D Unit Cell).
SiO2 has a number of distinct crystalline forms (polymorphs) in addition to amorphous forms. With the exception of stishovite and fibrous silica, all of the crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements. Silicon-oxygen bond lengths vary between the different crystal forms, for example in α-quartz the 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°.
Fibrous silica has a structure similar to that of SiS2 with chains of edge-sharing SiO4 tetrahedra. Stishovite, the higher pressure form, in contrast 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. But more important is the observation that any deviations from these standard parameters constitute microstructural differences or variations which represent an approach to an amorphous, vitreous or glassy solid.
Note that the only stable form under normal conditions is α-quartz and this is the form in which crystalline silicon dioxide is usually encountered. In nature impurities in crystalline α-quartz can give rise to colors (see list).
Note also that both high temperature minerals, cristobalite and tridymite, have both a lower density and index of refraction than quartz. Since the composition is identical, the reason for the discrepancies must be in the increased spacing in the high temperature minerals. As is common with many substances, the higher the temperature the farther apart the atoms due to the increased vibration energy.
The high pressure minerals, seifertite, stishovite, and coesite, on the other hand, have a higher density and index of refraction when compared to quartz. This is probably due to the intense compression of the atoms that must occur during their formation, resulting in a more condensed structure.
Faujasite silica is another form of crystalline silica. It is obtained by dealumination of a low-sodium, ultra-stable Y zeolite with a combined acid and thermal treatment. The resulting product contains over 99% silica, has high crystallinity and high 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 silica exhibits several peculiar physical characteristics that are similar to the ones 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. When molecular silicon monoxide, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge, molecular SiO2 is produced which has a linear structure. Dimeric silicon dioxide, (SiO2)2 has been prepared 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.
| Form | Crystal symmetry Pearson symbol, group No. |
ρ g/cm3 |
Notes | Structure |
|---|---|---|---|---|
| α-quartz | rhombohedral (trigonal) hP9, P3121 No.152 |
2.648 | Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K | |
| β-quartz | hexagonal hP18, P6222, No.180 |
2.533 | closely related to α-quartz (with an Si-O-Si angle of 155°) and optically active; β-quartz converts to β-tridymite at 1140 K | |
| α-tridymite | orthorhombic oS24, C2221, No.20 |
2.265 | metastable form under normal pressure | |
| β-tridymite | hexagonal hP12, P63/mmc, No. 194 |
closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K | ||
| α-cristobalite | tetragonal tP12, P41212, No. 92 |
2.334 | metastable form under normal pressure | |
| β-cristobalite | cubic cF104, Fd3m, No.227 |
closely related to α-cristobalite; melts at 1978 K | ||
| faujasite | cubic cF576, Fd3m, No.227 |
1.92 | sodalite cages connected by hexagonal prisms; 12-membered ring pore opening; faujasite structure. | |
| melanophlogite | cubic (cP*, P4232, No.208) or tetragonal (P42/nbc) | 2.04 | Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces-a clathrasil | |
| keatite | tetragonal tP36, P41212, No. 92 |
3.011 | Si5O10, Si4O14, Si8O16 rings; synthesised from glassy silica and alkali at 600–900K and 40–400 MPa | |
| moganite | monoclinic mS46, C2/c, No.15 |
Si4O8 and Si6O12 rings | ||
| coesite | monoclinic mS48, C2/c, No.15 |
2.911 | Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa | |
| stishovite | Tetragonal tP6, P42/mnm, No.136 |
4.287 | One of the densest (together with seifertite) polymorphs of silica; rutile-like with 6-fold coordinated Si; 7.5–8.5 GPa | |
| fibrous | orthorhombic oI12, Ibam, No.72 |
1.97 | like SiS2 consisting of edge sharing chains, melts at ~1700 K | |
| seifertite | orthorhombic oP, Pbcn |
4.294 | One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa. |
Read more about this topic: Silicon Dioxide
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