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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.
Tridymite is a low pressure, mostly high-temperature-stable polymorph of silica that can also form or persist metastably at low temperatures. The high temperature form occurs most notably as vapour-deposited, platey crystals in vesicles in some volcanic rocks, also rarely as phenocrysts in some felsic volcanics, or as a contact metamorphic material in some hornfels.

Tridymite can occur in seven polytypes and the most common at standard atmospheric pressure are known as α and β; both commensurately and incommensurately modulated structure variants are known. Below 100°C the triclinic form, α-tridymite is stable; there are also orthorhombic, monoclinic and hexagonal polytypes stable at higher temperatures. The orthorhombic β-tridymite polytype is most stable at elevated temperatures (>870°C) and it converts to β-cristobalite above 1470°C. However, tridymite does not usually form directly from pure β-quartz, and it is usually stabilised by alkali metals. Otherwise β-quartz transitions directly to cristobalite at 1050°C without occurrence of the tridymite phase.

Some polytypes of tridymite ( from Deer et al. 2004)
NameSymmetrySpace groupT (°C)
HP (β)HexagonalP63/mmm460
LHPHexagonalP6322400
OC (α) Orthorhombic C2221. 220
OSOrthorhombic100–200
OPOrthorhombicP212121155
MCMonoclinicCc22
MXMonoclinicC122

In the above table, M, O, H, C, P, L and S stand for monoclinic, orthorhombic, hexagonal, centered, primitive, low (temperature) and superlattice. T indicates the temperature, at which the corresponding phase is relatively stable, though the interconversions between phases are complex and sample dependent, and all these forms can coexist at ambient conditions. Mineralogy handbooks often arbitrarily assign tridymite to the triclinic crystal system, yet use hexagonal Miller indices because of the hexagonal crystal shape.

A low temperature form is commonly reported as a constituent of certain types of opal, intergrown with cristobalite (opal-CT), found in many environments including marine sedimentary rocks derived from biogenic opaline sediments, and low temperature cavity infillings, replacements, etc., including some precious opal (Sanders, 1975). Some wood opal is mostly tridymite (Mitchell & Tufts, 1973). However some workers note that opal is hydrous and lacks any long-range ordering, the opal structure just mimicking cristobalite and tridymite, so may not contain true tridymite (Smith, 1998).

Large tridymite deposits have been detected on Mars, but the nature and origin are uncertain (Lakdawalla, 2015).


Classification of Tridymite

Approved, 'Grandfathered' (first described prior to 1959)
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

Occurrences of Tridymite

Type Occurrence of Tridymite

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
Unit Cell Volume:
V 13,996.16 ų (Calculated from Unit Cell)
Z:
320
Morphology:
Pseudohexagonal plates, wedge-shaped, tabular.
Twinning:
Trilling, multiple contact twins or simple twins on {1016} and contact or penetration twins on {3034}
Comment:
Orthorhombic, pseudohexagonal; triclinic below 100°C

Crystallographic forms of Tridymite

Crystal Atlas:
Image Loading
Click on an icon to view
Tridymite no.26 - Goldschmidt (1913-1926)
3d models and HTML5 code kindly provided by www.smorf.nl.

Toggle
Edge Lines | Miller Indicies | Axes

Transparency
Opaque | Translucent | Transparent

View
Along a-axis | Along b-axis | Along c-axis | Start rotation | Stop rotation
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
All elements listed in formula:

Relationship of Tridymite to other Species

4.DA.Carbon Dioxide IceCO2
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 or β-tridymite forms 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: 209: 507.

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

Poggend. (1868) Ann. Physical Chemistry: 133: 508.

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

Lukesh, J., Buerger, M.J. (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., Karle, J. (1972) Tridymite-like structure in silica glass. Nature Physical Science: 236: 92-94.

Mitchell, R. S. & Tufts, S. 1973. Wood opal - a tridymite-like mineral. Amer. Min., 58, 717-20.

Sanders, J.V. (1975) Microstructure and crystallinity of gem opals. American Mineralogist, 60, 749-757.

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

Kuniaki Kihara; Matsumoto T.; Imamura M. (1986). "Structural change of orthorhombic-I tridymite with temperature: A study based on second-order thermal-vibrational parameters". Zeitschrift für Kristallographie. 177: 27–38.

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.

William Alexander Deer; R. A. Howie; W. S. Wise (2004). Rock-Forming Minerals: Framework Silicates: Slica Minerals, Feldspathoids and the Zeolites. Geological Society. pp. 22

Graetsch, H.A. (2009) Modulated crystal structure of incommensurate low tridymite. Acta Crystallographica B: 65: 543-550. [ doi:10.1107/S0108768109029127 ]

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

Lakdawalla, Emily (December 18, 2015). "Curiosity stories from AGU: The fortuitous find of a puzzling mineral on Mars, and a gap in Gale's history". The Planetary Society. Retrieved December 21, 2015

Internet Links for Tridymite

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http://www.mindat.org/min-4015.html
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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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