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Tridymite

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Formula:
SiO
 
2
System:TriclinicColour:Colourless, white, ...
Lustre:VitreousHardness:6½ - 7
Name:From the Greek "Tridymos" , triplet, alluding to its common twinning as trillings.
Polymorph of:Coesite, Cristobalite, Quartz, Stishovite


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

IMA status:Approved
Strunz 8th edition ID:4/D.01-20
Nickel-Strunz 10th (pending) edition ID: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
Dana 8th edition ID:75.1.2.1

75 : TECTOSILICATES Si Tetrahedral Frameworks
1 : Si Tetrahedral Frameworks - SiO2 with [4] coordinated Si
Hey's CIM Ref.:7.8.3

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

Type Locality:Cerro San Cristóbal, Mun. de Pachuca, Hidalgo, Mexico

Occurrences of Tridymite

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

Physical Properties of Tridymite

Lustre: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 (measured):2.25 - 2.28 g/cm3
Density (calculated):2.28 g/cm3

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
Unit Cell Volume:V 13,996.16 ų (Calculated from Unit Cell)
Z:320
Morphology:Pseudohexagonal plates, wedge-shaped, tabular.
Twinning:Trilling {1016} contact twins on {3034}
Comment:Orthorhombic, pseudohexagonal; triclinic below 100°C
Crystal Atlas:
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Tridymite no.26 - Goldschmidt (1913-1926)

About Crystal Atlas

The mindat.org Crystal Atlas allows you to view a selection of crystal drawings of real and idealised crystal forms for this mineral and, in certain cases, 3d rotating crystal objects. The 3d models and HTML5 code are kindly provided by www.smorf.nl.

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X-Ray Powder Diffraction:
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Radiation - Copper Kα
Data Set:
Horizontal Axis: ° to ° Vertical Axis: % Source Data: Filtered Data:
Data courtesy of RRUFF project at University of Arizona, used with permission.
X-Ray Powder Diffraction:
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°
Maximum Birefringence:δ = 0.006

Chart 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:
SiO
 
2
Simplified for copy/paste:SiO2
Essential elements:O, Si
All elements listed in formula:O, Si

Relationship of Tridymite to other Species

Related Minerals - Nickel-Strunz Grouping):

- +
4.DA.05Quartz
SiO
 
2
4.DA.10Opal
SiO
 
2
· nH
 
2
O
4.DA.15Cristobalite
SiO
 
2
4.DA.20Mogánite
SiO
 
2
4.DA.25Melanophlogite
46SiO
 
2
· 6(N
 
2
,CO
 
2
) · 2(CH
 
4
,N
 
2
)
4.DA.30Lechatelierite
SiO
 
2
4.DA.35Coesite
SiO
 
2
4.DA.40Stishovite
SiO
 
2
4.DA.45Keatite
SiO
 
2
4.DA.50Seifertite
SiO
 
2
Related Minerals - Hey's Index Grouping:

- +
7.8.1Quartz
SiO
 
2
7.8.2Coesite
SiO
 
2
7.8.4Stishovite
SiO
 
2
7.8.5Cristobalite
SiO
 
2
7.8.6Lechatelierite
SiO
 
2
7.8.7Silhydrite
3SiO
 
2
· H
 
2
O
7.8.8Opal
SiO
 
2
· nH
 
2
O
7.8.9Mogánite
SiO
 
2

Other Names for Tridymite

Synonyms:
α-TridymiteAlpha-TridymiteAsmaniteTridymite-αTridymite-alpha
Other Languages:
Dutch:Tridymiet
Estonian:Tridümiit
French:Tridymite
German:Tridymit
α-Tridymit
Asmanit
Japanese:鱗珪石
Polish:Trydymit
Portuguese:Tridimita
Russian:Тридимит
Simplified Chinese:鳞英石
Slovak:Tridymit
Spanish:α-Tridymita
Asmanita
Tridymita
Traditional Chinese:鳞英石
Varieties:
Christensenite

Other Information

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

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  • Localities for Tridymite

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