Cristobalite

Cristobalite
Cristobalite spheres formed via devitrification (loss of silica) from the obsidian matrix (California, USA) 5.9×3.8×3.8 cm.
General
Category	Silicate mineral
Chemical formula	SiO2
Strunz classification	4.DA.15
Dana classification	75.1.1.1
Crystal symmetry	Tetragonal (α) space group P41212, cubic (β) Fd3m
Unit cell	a = 4.9709(1) A, c = 6.9278(2) A, Z = 4 (α); a = 7.166 A, Z = 8 (β)
Identification
Color	Colorless, white
Crystal habit	Octahedra or spherulites up to several cm large
Twinning	on {111}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	6–7
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Specific gravity	2.32–2.36
Optical properties	Uniaxial (-)
Refractive index	nω = 1.487 nε = 1.484
Birefringence	0.003
Pleochroism	None
Melting point	1713 °C (β)[1]
References	[2][3][4]

The mineral cristobalite is a high-temperature polymorph of silica, meaning that it has the same chemical formula, SiO₂, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group which also include coesite, tridymite and stishovite. Cristobalite occurs as white octahedra or spherullites in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of the US state of California and similar areas. Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures.

The persistence of cristobalite outside of its thermodynamic stability range occurs because the transition from cristobalite to quartz or tridymite is "reconstructive", requiring the breaking up and reforming of the silica framework. These frameworks are composed of SiO₄ tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula of silica is SiO₂. The breaking of these bonds required to convert cristobalite to tridymite and quartz requires considerable activation energy and may not happen on a human time frame. Framework silicates are also known as tectosilicates.

There is more than one form of the cristobalite framework. At high temperatures, the structure is cubic, Fd3m, No.227, Pearson symbol cF104.^[5] A tetragonal form of cristobalite (P4₁2₁2, No. 92, Pearson symbol tP12^[6]) occurs on cooling below ca. 250 °C at ambient pressure, and is related to the cubic form by a static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or α − β transition. It may be termed "displacive"; i.e., it is not generally possible to prevent the cubic β-form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. The exact transition temperature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature.

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 phonons called rigid unit modes. It is the "freezing" of one of these rigid unit modes that is the soft mode for the α–β transition.

In the α–β phase transition only one of the three degenerate cubic crystallographic axes retains a fourfold rotational axis in the tetragonal form. The choice of axis is arbitrary, so that various twins can form within the same grain. These different twin orientations coupled with the discontinuous nature of the transition can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks.

When devitrifying silica, cristobalite is usually the first phase to form, even when well outside of its thermodynamic stability range. The dynamically disordered nature of the β-phase is partly responsible for the low enthalpy of fusion of silica.

The micrometre-scale spheres that make up precious opal are made of cristobalite, crystallized metastably at low temperature.

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  An idealized model of β-cristobalite, showing corner-bonded SiO₄ tetrahedra. In reality the tetrahedra are constantly tumbling.

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  β-cristobalite viewed along another direction.

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  The crumpled framework of α-cristobalite, related to the β-form by static tilting of the tetrahedra.

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  Unit cell of α-cristobalite; red spheres are oxygen atoms.

References

1. ^ Deer, W. A., R. A. Howie and J. Zussman, An Introduction to the Rock Forming Minerals, Logman, 1966, pp. 340–355 ISBN 0-582-44210-9
2. ^ Cristobalite. Handbook of Mineralogy.
3. ^ Cristobalite. Mindat.
4. ^ Cristobalite Mineral Data. Webmineral.
5. ^ Wright A.F., Leadbetter A.J. (1975). "The structures of the b-cristobalite phases of SiO2 and AlPO4". Philosophical Magazine 31: 1391–1401. doi:10.1080/00318087508228690. 
6. ^ Downs R.T., Palmer D.C. (1994). "The pressure behavior of a cristobalite". American Mineralogist 79: 9–14. http://www.geo.arizona.edu/xtal/group/pdf/AM79_9.pdf. 

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)

External links

• International Chemical Safety Card 0809