Cristobalite
Cristobalite | ||
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Specific gravity 2.32–2.36 | | |
Optical properties | Uniaxial (−) | |
Refractive index | nω = 1.487 nε = 1.484 | |
Birefringence | 0.003 | |
Pleochroism | None | |
Melting point | 1,713 °C (3,115 °F) (β)[2] | |
References | [3][4][5][6] |
Cristobalite (
It is used in dentistry as a component of
Properties
Metastability
Cristobalite is stable only above 1470 °C, but can crystallize and persist
When devitrifying silica, cristobalite is usually the first phase to form, even when well outside its thermodynamic stability range. This is an example of Ostwald's step rule. The dynamically disordered nature of the β phase is partly responsible for the low enthalpy of fusion of silica.
Structures
There is more than one form of the cristobalite framework. At high temperatures, the structure is called β-cristobalite. It is in the
The cubic β phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency
In β-cristobalite, there are right-handed and left-handed helices of tetrahedra (or of silicon atoms) parallel to all three axes. In the α–β phase transition, however, only the right-handed or the left-handed helix in one direction is preserved (the other becoming a two-fold screw axis), so only one of the three degenerate cubic crystallographic axes retains a fourfold rotational axis (actually a
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An idealized model of β-cristobalite, showing corner-bonded SiO4 tetrahedra. There is a right-handed four-fold screw axis at the centre of half the white squares, and a left-handed one at the centre of the others. In this projection we see glide planes parallel to the axes and mirrors on the diagonals. In reality the tetrahedra are constantly wobbling.
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β-cristobalite viewed along the 101 direction
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The crumpled framework of α-cristobalite, related to the β form by static tilting of the tetrahedra. This view corresponds to the view along the 101 direction of the previous illustration, except that the "b" axis of that picture is now horizontal. The two-fold screw axes appear here as two-fold axes of rotation going through the middle of the white areas and between the pairs of almost superimposed oxygen atoms.
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Unit cell of α-cristobalite; red spheres are oxygen atoms. We see here five silicon atoms in a helix (the first and the last are equivalent atoms in the lattice) going in the "c" direction (into the page). The horizontal and vertical axes are the "a" axes.
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Unit cell of β-cristobalite; red spheres are oxygen atoms.
Occurrence
Cristobalite occurs as white octahedra or spherulites in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of the US state of California and similar areas.
The micrometre-scale spheres that make up precious opal exhibit some X-ray diffraction patterns that are similar to that of cristobalite, but lack any long-range order so they are not considered true cristobalite. In addition, the presence of structural water in opal makes it doubtful that opal consists of cristobalite.[12][13]
Cristobalite is visible as the white inclusions in
References
- S2CID 235729616.
- ISBN 0-582-44210-9.
- ^ Mineralienatlas.
- ^ Cristobalite Archived 2010-07-15 at the Wayback Machine. Handbook of Mineralogy.
- ^ Cristobalite. Mindat.
- ^ "Cristobalite Mineral Data". Webmineral.
- OCLC 934359978.
- .
- ^ Downs R. T., Palmer D. C. (1994). "The pressure behavior of a cristobalite" (PDF). American Mineralogist. 79: 9–14. Archived from the original (PDF) on 2019-05-15. Retrieved 2009-12-15.
- ISBN 978-0-7506-4564-5.
- .
- S2CID 97394861.
- ^ "Silica, Crystalline - Overview | Occupational Safety and Health Administration" (PDF). www.osha.gov. Archived from the original (PDF) on 2016-03-04.
Further reading
- American Geological Institute Dictionary of Geological Terms.
- Durham, D. L., "Monterey Formation: Diagenesis". in: Uranium in the Monterey Formation of California. US Geological Survey Bulletin 1581-A, 1987.
- Reviews in Mineralogy and Geochemistry, vol. 29., Silica: behavior, geochemistry and physical applications. Mineralogical Society of America, 1994.
- R. B. Sosman, The Phases of Silica. (Rutgers University Press, 1965)