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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)
Post-volcanic, magmatic, and secondary. Common constituent of certain types of opal (opal-C and opal-CT) in marine sedimentary rocks derived from biogenic opaline sediments.

May be intergrown with Tridymite.

Classification of Cristobalite

Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
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

Year of Discovery:
1887

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}

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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Edge Lines | Miller Indicies | Axes

Transparency
Opaque | Translucent | Transparent

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Along a-axis | Along b-axis | Along c-axis | Start rotation | Stop rotation
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
Essential elements:
All elements listed in formula:
Common Impurities:
Fe,Ca,Al,K,Na,Ti,Mn,Mg,P

Relationship of Cristobalite to other Species

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:
Murdoch, Joseph (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.

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.
Mineral and/or Locality  
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