![]() 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. Polymorphism Īlpha quartz is the most 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 silicon dioxides, 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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