Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic . Its chemical formula is siliconoxygen2. 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 Crystal twinning crystal [ (compound crystals comprising three twinned crystal components).
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Structure
Tridymite can occur in seven crystalline forms. Two of the most common at standard pressure are known as α and β. The α-tridymite phase is favored at elevated temperatures (above 870 °C) and it converts to β-cristobalite at 1,470 °C.[ However, tridymite does usually not form from pure β-quartz, one needs to add trace amounts of certain compounds to achieve this.][ Otherwise the β-quartz-tridymite transition is skipped and β-quartz transitions directly to cristobalite at 1,050 °C without occurrence of the tridymite phase.
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>+Crystal phases of tridymite[
!Name!!Symmetry!!Space group!! (°C)
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| 460 |
| 400 |
| 220 |
| 100–200 |
| 155 |
| 22 |
| 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.[ Mineralogy handbooks often arbitrarily assign tridymite to the triclinic crystal system, yet use hexagonal Miller index because of the hexagonal crystal shape (see image).][
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Mars
In December 2015, the team behind NASA's Mars Science Laboratory announced the discovery of large amounts of tridymite in Marias Pass on the slope of Mount Sharp, popularly known as Mount Sharp, on the planet Mars.
DAN's high neutron readings would normally have been interpreted as meaning the region was hydrogen-rich, and ChemCam's high-silica readings were not surprising given the ubiquity of silica-rich deposits on Mars, but taken together it was clear that further study of the region was needed. Following conjunction, NASA directed the Curiosity rover back to the area where the readings had been taken and discovered that large amounts of tridymite were present. How they were formed was unknown, .
See also
External links
