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Cristobalite

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Formula:
SiO2
System:
Tetragonal
Colour:
Colorless, white, also ...
Lustre:
Vitreous
Hardness:
6 - 7
Name:
Named after its locality (Cerro San Cristobal, Mexico).
Cristobalite is a silica polymorph that is thermodynamically stable only at temperatures above 1470°C, up to the melting point at 1705°C, at atmospheric pressures. It commonly metastably persists or even forms at much lower temperatures in silica-rich volcanic and sedimentary environments. It can form crystals in cavities, probably vapour-deposited, or spherulites in obsidian or other silicic volcanics.

It is commonly reported as a major constituent of certain types of opal (opal-C and opal-CT) in marine sedimentary rocks derived from biogenic opaline sediments, and as cavity fills fromed from low temperature groundwater.

Some precious opal is opal-CT. However some workers note that this material is hydrous and lacks any long-range ordering, the opal structure just mimicking cristobalite and tridymite, so may not be considered true cristobalite (Smith, 1998).

At ambient temperatures, cristobalite is tetragonal, and specimens from volcanic environments are paramorphs of cubic β-Cristobalite or high-cristobalite (stable above 1470°C), still showing the initially cubic crystal morphology. To distinguish it from the high-temperature cubic type, tetragonal cristobalite is also called α-cristobalite or low-cristobalite.

May be intergrown with Tridymite.

Classification of Cristobalite

Approved, 'Grandfathered' (first described prior to 1959)
4.DA.15

4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
D : Metal: Oxygen = 1:2 and similar
A : With small cations: Silica family
75.1.1.1

75 : TECTOSILICATES Si Tetrahedral Frameworks
1 : Si Tetrahedral Frameworks - SiO2 with [4] coordinated Si
7.8.5

7 : Oxides and Hydroxides
8 : Oxides of Si
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Type Occurrence of Cristobalite

Occurrences of Cristobalite

Geological Setting:
Individual crystals in vesicles and lithophysae. Common constituent of opal (opal-C and opal-CT) in sedimentary rocks and silicious sinter.

Physical Properties of Cristobalite

Vitreous
Diaphaneity (Transparency):
Transparent, Translucent
Colour:
Colorless, white, also blue grey, brown, grey, yellow
Comment:
Often white due to numerous cracks
Streak:
White
Hardness (Mohs):
6 - 7
Tenacity:
Brittle
Density:
2.32 - 2.36 g/cm3 (Measured)    2.33 g/cm3 (Calculated)

Crystallography of Cristobalite

Crystal System:
Tetragonal
Class (H-M):
4 2 2 - Trapezohedral
Space Group:
P41 21 2
Cell Parameters:
a = 4.9709(1) Å, c = 6.9278(2) Å
Ratio:
a:c = 1 : 1.394
Unit Cell Volume:
V 171.18 ų (Calculated from Unit Cell)
Z:
4
Morphology:
Pseudo octahedral crystals, commonly dendritic, spherulites.
Twinning:
On {111} (paramorphs of cubic β-Cristobalite twins)

Crystallographic forms of Cristobalite

Crystal Atlas:
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Cristobalite no.1 - Goldschmidt (1913-1926)
3d models and HTML5 code kindly provided by www.smorf.nl.

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Transparency
Opaque | Translucent | Transparent

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X-Ray Powder Diffraction Data:
d-spacingIntensity
4.05 (100)
3.135 (11)
2.841 (13)
2.485 (20)
2.465 (5)
2.118 (5)
1.870 (7)
Comments:
For synthetic material.

Optical Data of Cristobalite

Type:
Uniaxial (-)
RI values:
nω = 1.487 nε = 1.484
Max Birefringence:
δ = 0.003
Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:
Moderate

Chemical Properties of Cristobalite

Formula:
SiO2
All elements listed in formula:
Common Impurities:
Fe,Ca,Al,K,Na,Ti,Mn,Mg,P

Relationship of Cristobalite to other Species

Common Associates:
4.DA.Carbon Dioxide IceCO2
4.DA.05QuartzSiO2
4.DA.10OpalSiO2 · nH2O
4.DA.10TridymiteSiO2
4.DA.20MogániteSiO2
4.DA.25Melanophlogite46SiO2 · 6(N2,CO2) · 2(CH4,N2)
4.DA.30LechatelieriteSiO2
4.DA.35CoesiteSiO2
4.DA.40StishoviteSiO2
4.DA.45KeatiteSiO2
4.DA.50SeifertiteSiO2
7.8.1QuartzSiO2
7.8.2CoesiteSiO2
7.8.3TridymiteSiO2
7.8.4StishoviteSiO2
7.8.6LechatelieriteSiO2
7.8.7Silhydrite3SiO2 · H2O
7.8.8OpalSiO2 · nH2O
7.8.9MogániteSiO2

Other Names for Cristobalite

Other Information

Thermal Behaviour:
Inverts from high cristobalite at 268C.
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Cristobalite

Reference List:
vom Rath, G. (1887) Ueber Cristobalit vom Cerro S. Cristóbal bei Pachuca (Mexico). Neues Jahrbuch für Mineralogie, Geologie und Paläontologie: 1987: 198-199.

Murdoch, J. (1942) Crystallographic notes, cristobalite, stephanite, natrolite. American Mineralogist: 27: 500-506.

Henderson, J.H., Jackson, M.L., Syers, J.K., Clayton, R.N., Rex, R.W. (1971) Cristobalite Authigenic origin in relation to montmorillonite and quartz origin in bentonite. Clays and Clay Minerals: 19: 229-238.

Murata, K.J., Nakata, J.K. (1974) Cristobalite stage in the diagenesis of diatomaceous shale. Science: 184: 567-568.

Richet, P., Bottinga, Y., Deniélou, L., Petitet, J.P., Téqui, C. (1982) Thermodynamic properties of quartz, cristobalite, and amorphous SiO2: drop calorimetry measurements between 1000 and 1800 K and a review from 0 to 2000 K. Geochimica et Cosmochmica Acta: 46: 2639-2658.

Heaney, P.J. (1994) Structure and chemistry of the low-pressure silica polymorphs. In: Reviews in Mineralogy, Volume 29, Silica - Physical behavior, geochemistry and materials applications. Mineralogical Society of America, Washington, D.C.

Pabst, W., Gregorová, E. (2013) Elastic properties of silica polymorphs - a review. Ceramics - Silikáty: 57: 167-184.

Smith, D.K. (1998) Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography. Powder Diffraction: 13: 2-19.

Internet Links for Cristobalite

Localities for Cristobalite

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
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