![]() For example, it maintains a high degree of long-range molecular order or crystallinity even after boiling in concentrated hydrochloric acid. Faujasite-silica has very high thermal and acid stability. The resulting product contains over 99% silica, and has high crystallinity and specific surface area (over 800 m 2/g). More importantly, any deviations from these standard parameters constitute microstructural differences or variations, which represent an approach to an amorphous, vitreous, or glassy solid.įaujasite silica, another polymorph, is obtained by dealumination of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment. The change in the coordination increases the ionicity of the Si-O bond. 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 density of stishovite is 4.287 g/cm 3, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648 g/cm 3. Stishovite has a rutile-like structure where silicon is 6-coordinate. The high-pressure minerals, seifertite, stishovite, and coesite, though, have higher densities and indices of refraction than quartz. 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 transformation from α-quartz to beta-quartz takes place abruptly at 573 ☌. The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. PolymorphismĪlpha quartz is the stable form of solid SiO 2 at room temperature. 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 bond length is 161 pm, whereas in α-tridymite it is in the range 154–171 pm. SiO 2 has several distinct crystalline forms, but they almost always have the same local structure around Si and O. The starkly different structures of the dioxides of carbon and silicon are a manifestation of the Double bond rule. Thus, SiO 2 forms 3-dimensional network solids in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms. In the majority of silicates, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom ( see 3-D Unit Cell). Relationship between refractive index and density for some SiO 2 forms
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