Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico. The name is from the Greek tridymos for triplet as tridymite commonly occurs as twinned crystal trillings.[1]
Structure [edit]
Crystal structure of α-tridymite
Tridymite can occur in about seven crystalline forms. Two of them are known as α and β. The α-tridymite phase is favored at elevated temperatures and it converts to β-cristobalite at 1470 °C.[2][3]
Crystal phases of tridymite[3]
| Name |
Symmetry |
Space group |
T (°C) |
| HP (β) |
Hexagonal |
P63/mmc |
460 |
| LHP |
Hexagonal |
P6322 |
400 |
| OC (α) |
Orthorhombic |
C2221 |
220 |
| OS |
Orthorhombic |
|
100–200 |
| OP |
Orthorhombic |
P212121 |
155 |
| MC |
Monoclinic |
Cc |
22 |
| MX |
Monoclinic |
C1 |
22 |
In the 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.[3] Mineralogy handbooks often arbitrarily assign tridymite to the triclinic crystal system, yet use hexagonal Miller indices because of the hexagonal crystal shape (see infobox image).[1]
See also [edit]
References [edit]
- ^ a b c Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C. (ed.). "Tridymite" (PDF). Handbook of Mineralogy. III (Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0-9622097-2-4. Retrieved December 5, 2011.
- ^ 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 fur Kristallographie 177: 27–38. Bibcode:1986ZK....177...27K. doi:10.1524/zkri.1986.177.1-2.27.
- ^ a b c 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–. ISBN 978-1-86239-144-4. Retrieved 2 January 2012.
External links [edit]
