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Polymorphism of SiO2

Alpha quartz is the most stable form of solid SiO2 at room temperature. The high-temperature minerals cristobalite and tridymite have lower densities and refraction indices than quartz. The transformation from α-quartz to beta-quartz takes place abruptly at 573 °C. Since a significant volume change accompanies the shift, it can quickly induce the 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.[10] Stishovite has a rutile-like structure where silicon is 6-coordinate. The thickness of stishovite is 4.287 g/cm3, which compares to α-quartz, the densest of the low-pressure forms, which has a thickness of 2.648 g/cm3. The difference in thickness 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. If you are looking for high quality, high purity, and cost-effective Silicon dioxide, or if you require the latest price of Silicon dioxide, please feel free to email contact mis-asia.

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