Tridymite

Summit Rock, Klamath Co., Oregon, USA
© Saul Krotki 2004

Show Tridymite Photos (66)

Formula:

SiO

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.
Both commensurately and incommensurately modulated structure variants are known.

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

mindat.org URL:

http://www.mindat.org/min-4015.html
Please feel free to link to this page.

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 Å³ (Calculated from Unit Cell)

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)

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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. You need Java to see these. You can download Java for free - click here to download Java

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X-Ray Powder Diffraction:

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Radiation - Copper Kα
Data courtesy of RRUFF project at University of Arizona, used with permission.

Optical Data of Tridymite

Type:

Biaxial (+)

RI values:

n_α = 1.468 - 1.482 n_β = 1.470 - 1.484 n_γ = 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

Essential elements:

O, Si

All elements listed in formula:

O, Si

Relationship of Tridymite to other Species

4.DA.05

Quartz

SiO

4.DA.10

Opal

SiO

·nH

4.DA.15

Cristobalite

SiO

4.DA.20

Mogánite

SiO

4.DA.25

Melanophlogite

46SiO

·6(N

,CO

)·2(CH

)

4.DA.30

Lechatelierite

SiO

4.DA.35

Coesite

SiO

4.DA.40

Stishovite

SiO

4.DA.45

Keatite

4.DA.50

Seifertite

SiO

7.8.1

Quartz

SiO

7.8.2

Coesite

SiO

7.8.4

Stishovite

SiO

7.8.5

Cristobalite

SiO

7.8.6

Lechatelierite

SiO

7.8.7

Silhydrite

3SiO

·H

7.8.8

Opal

SiO

·nH

7.8.9

Mogánite

SiO

Other Names for Tridymite

Synonyms:

Other Languages:

Dutch:	Tridymiet
Estonian:	Tridümiit
French:	Tridymite
German:	Tridymit Asmanit α-Tridymit
Japanese:	鱗珪石
Polish:	Trydymit
Portuguese:	Tridimita

Russian:	Тридимит
Slovak:	Tridymit
Spanish:	Asmanita α-Tridymita Tridymita

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.

Schweizerische mineralogische und petrographische Mitteilungen (1984): 64: 335.

American Mineralogist (1994): 79: 606.

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