Editing Silicon dioxide (section)

===Polymorphism===
[[Quartz inversion|Alpha quartz]] is the most stable form of solid SiO<sub>2</sub> 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&nbsp;°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.<ref>{{cite book|url=https://books.google.com/books?id=cUwwoR-RuJ0C&pg=PA93|title=Ceramic Technology for Potters and Sculptors|vauthors=Cuff YH|publisher=University of Pennsylvania|year=1996|isbn=9780812213775|location=Philadelphia|pages=93–95}}</ref> The high-pressure minerals, [[seifertite]], stishovite, and coesite, though, have higher densities and indices of refraction than quartz.<ref>{{cite book|title=Silica Stories|vauthors=De La Rocha C, Conley DJ|publisher=Springer|year=2017|isbn=9783319540542|location=Cham|pages=50–55|chapter=Mystical Crystals of Silica|doi=10.1007/978-3-319-54054-2_4}}</ref> Stishovite has a [[rutile]]-like structure where silicon is 6-coordinate. The density of stishovite is 4.287&nbsp;g/cm<sup>3</sup>, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648&nbsp;g/cm<sup>3</sup>.<ref name = "Greenwood"/> 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.<ref>{{cite book|title=Structural Inorganic Chemistry|vauthors=Wells AF|publisher=Oxford Science Publications|year=1984|isbn=9780198553700}}</ref>
The change in the coordination increases the ionicity of the Si–O bond.<ref>{{cite journal|display-authors=3|vauthors=Kirfel A, Krane HG, Blaha P, Schwarz K, Lippmann T|year=2001|title=Electron-density distribution in stishovite, SiO<sub>2</sub>: a new high-energy synchrotron-radiation study|journal=[[Acta Crystallographica Section A]]|volume=57|issue=6|pages=663–77|doi=10.1107/S0108767301010698|pmid=11679696|doi-access=free|bibcode=2001AcCrA..57..663K }}</ref>

[[Faujasite]] silica, another polymorph, is obtained by the [[wikt:dealumination|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 m<sup>2</sup>/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]].<ref name="fau">{{cite journal|vauthors=Scherzer J|year=1978|title=Dealuminated faujasite-type structures with SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> ratios over 100|journal=[[Journal of Catalysis|J. Catal.]]|volume=54|issue=2|page=285|doi=10.1016/0021-9517(78)90051-9}}</ref>