Tridymite is a high-temperaturepolymorph ofsilica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities infelsicvolcanic rocks. Itschemical formula isSiO₂. Tridymite was first described in 1868 and thetype location is in Hidalgo,Mexico. The name is from theGreektridymos fortriplet as tridymite commonly occurs astwinned crystaltrillings^[2] (compound crystals comprising three twinned crystal components).

Tridymite
Tabular tridymite crystals from Ochtendung, Eifel, Germany
General
Category	Tectosilicates,quartzgroup
Formula	SiO2
IMA symbol	Trd[1]
Strunz classification	4.DA.10
Crystal system	Orthorhombic (α-tridymite)
Crystal class	Disphenoidal (222) H–M symbol: (222)
Space group	C2221
Identification
Formula mass	60.08 g/mol
Color	Colorless, white
Crystal habit	Platy – sheet forms
Cleavage	{0001} indistinct, {1010} imperfect
Fracture	Brittle – conchoidal
Mohs scale hardness	7
Luster	Vitreous
Streak	white
Specific gravity	2.25–2.28
Optical properties	Biaxial (+), 2V = 40–86°
Refractive index	nα=1.468–1.482 nβ=1.470–1.484 nγ=1.474–1.486
Birefringence	δ < 0.004
Pleochroism	Colorless
Other characteristics	non-radioactive, non-magnetic; fluorescent, short UV=dark red
References	[2][3]

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.^[4][5] However, tridymite does usually not form from pure β-quartz, one needs to add trace amounts of certain compounds to achieve this.^[6] 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.

Crystal phases of tridymite^[5]

Name	Symmetry	Space group	Temp. (°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 formonoclinic,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.^[5] Mineralogy handbooks often arbitrarily assign tridymite to thetriclinic crystal system, yet use hexagonalMiller indices because of the hexagonal crystal shape (see image).^[2]

In December 2015, the team behindNASA'sMars Science Laboratory announced the discovery of large amounts of tridymite in Marias Pass on the slope ofAeolis Mons, popularly known as Mount Sharp, on the planetMars.^[7] This discovery was unexpected given the rarity of the mineral on Earth and the apparent lack of volcanic activity where it was discovered, and at the time of discovery no explanation for how it was formed was forthcoming. Its discovery was serendipitous: two teams, responsible for two different instruments on theCuriosity rover, both happened to report what in isolation were relatively uninteresting findings related to their instruments: theChemCam team reported a region of high silica while theDAN team reported high neutron readings in what turned out to be the same area. Neither team would have been aware of the other's findings had it not been for a fortuitous Marsconjunction in July 2015, during which the various international teams took advantage of the downtime to meet inParis and discuss their scientific findings.

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 theCuriosity 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, as of December 2015^[update].^[8]

• Cristobalite
• Coesite
• Stishovite

• Mineral galleries
• Standard pressure polymorphs of SiO2
• Curiouser And Curiouser: NASA's Curiosity Rover Finds Piles Of Silica On Mars
• Study: Explosive volcanic eruption produced rare mineral on Mars