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Tridymite

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Formula:
SiO2
System:
Triclinic
Colour:
Colourless, white, ...
Lustre:
Vitreous
Hardness:
6½ - 7
Name:
From the Greek "Tridymos" , triplet, alluding to its common twinning as trillings.
Frequently occurs in cavities in volcanic rocks. Common constituent of certain types of opal (opal-CT) in marine sedimentary rocks derived from biogenic opaline sediments.
Both commensurately and incommensurately modulated structure variants are known. Stabilised by very small amounts of impurity cations (mainly alkalis).

May be intergrown with cristobalite.

Classification of Tridymite

Approved
4.DA.10

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
75.1.2.1

75 : TECTOSILICATES Si Tetrahedral Frameworks
1 : Si Tetrahedral Frameworks - SiO2 with [4] coordinated Si
7.8.3

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

Occurrences of Tridymite

Geological Setting:
Vapor phase deposition in vesicles and lithophysae, phenocrysts in volcanic rocks, contact metamorphosed sandstones.

Physical Properties of Tridymite

Vitreous
Diaphaneity (Transparency):
Transparent, Translucent
Comment:
May be pearly on {0001}
Colour:
Colourless, white, yellowish white, or grey
Streak:
White
Hardness (Mohs):
6½ - 7
Tenacity:
Brittle
Cleavage:
Poor/Indistinct
[0001] Indistinct, [1010] Imperfect
Fracture:
Conchoidal
Density:
2.25 - 2.28 g/cm3 (Measured)    2.28 g/cm3 (Calculated)

Crystallography of Tridymite

Crystal System:
Triclinic
Class (H-M):
1 - Pedial
Cell Parameters:
a = 9.932(5) Å, b = 17.216(6) Å, c = 81.854(9) Å
α = 90°, β = 90°, γ = 90°
Ratio:
a:b:c = 0.577 : 1 : 4.755
Z:
320
Morphology:
Pseudohexagonal plates, wedge-shaped, tabular.
Twinning:
Trilling {1016} contact twins on {3034}
Comment:
Orthorhombic, pseudohexagonal; triclinic below 100°C

Crystallographic forms of Tridymite

Crystal Atlas:
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Tridymite no.26 - 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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X-Ray Powder Diffraction:
Image Loading

Radiation - Copper Kα
Data Set:
Data courtesy of RRUFF project at University of Arizona, used with permission.
X-Ray Powder Diffraction Data:
d-spacingIntensity
4.30 (100)
4.09 (90)
3.80 (60)
3.249 (30)
2.964 (16)
2.483 (16)
2.305 (8)

Optical Data of Tridymite

Type:
Biaxial (+)
RI values:
nα = 1.468 - 1.482 nβ = 1.470 - 1.484 nγ = 1.474 - 1.486
2V:
Measured: 40° to 86°, Calculated: 50° to 72°
Max Birefringence:
δ = 0.006
Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:
Moderate
Dispersion:
none

Chemical Properties of Tridymite

Formula:
SiO2
Essential elements:
All elements listed in formula:

Relationship of Tridymite to other Species

4.DA.05QuartzSiO2
4.DA.10OpalSiO2 · nH2O
4.DA.15CristobaliteSiO2
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
7.8.4StishoviteSiO2
7.8.5CristobaliteSiO2
7.8.6LechatelieriteSiO2
7.8.7Silhydrite3SiO2 · H2O
7.8.8OpalSiO2 · nH2O
7.8.9MogániteSiO2

Other Names for Tridymite

Name in Other Languages:
Dutch:Tridymiet
Estonian:Tridümiit
French:Tridymite
Japanese:鱗珪石
Polish:Trydymit
Portuguese:Tridimita
Simplified Chinese:鳞英石
Slovak:Tridymit
Traditional Chinese:鳞英石

Other Information

Thermal Behaviour:
High tridymite between 870 and 1470°C.
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 Tridymite

Reference List:
vom Rath, G., (1868): Vorläufige Mitteilung über eine neue Kristallform der Kieselsäure (Tridymit), Annalen der Physik und Chemie, Vol. 209, 507ff.

vom Rath, G., (1868): Über den Tridymit eine neue krystallisierte Modification der Kieselsäure, Annalen der Physik und Chemie, Vol. 211, 437ff.

Ann. Physical Chemistry; Poggend. (1868), 133, 508.

vom Rath, G., (1872): Tridymit von Quito. Annalen der Physik und Chemie, Vol. 223, 279ff.

Lukesh, Joseph & Martin J. Buerger (1942), The tridymite problem (abstract): American Mineralogist: 27: 143.

Mitchell, R.S. (1967), Tridymite pseudomorphs after wood: In: Virginian Lower Cretaceous sediments: Science: 158: 905-906.

Konnert, J.H. and Karle, J. (1972) Tridymite-like structure in silica glass. Nature Phys. Sci.: 236: 92-94.

Wennemer, M., Thompson, A.B. (1984): Tridymite polymorphs and polytypes. Schweizerische mineralogische und petrographische Mitteilungen, 64: 335.

Cellai, D., Carpenter, M.A., Wruck, B., Salje, E.K.H. (1994): Characterization of high-temperature phase transitions in single crystals of Steinbach tridymite. American Mineralogist: 79: 606.

Graetsch, H. A. (2009): Modulated crystal structure of incommensurate low tridymite. Acta Crystallogr. B65, 543-550. [ doi:10.1107/S0108768109029127 ]

Internet Links for Tridymite

Localities for Tridymite

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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