Origin of Name
From Greek for “threefold”, a reference to its habit of forming compound crystals of three individuals of threetriangular wedge-shaped crystals.
Hand Specimen Identification
Tridymite is usually sufficiently fine grained that X-ray or optical measurements are needed for identification. It is sometimes confused with zeolites.
Physical Properties
| Hardness | 6 to 7 |
| Specific Gravity | 2.28 |
| luster/transparency | vitreous/transparent to translucent |
| Color | colourless |
| streak | white |
| Fracture | Conchoidal |
| Cleavage | Absent |
Optical Properties
In thin section, tridymite can be distinguished by its low birefringence, low refractive index (RI), moderate relief, and typical habit often showing wedge-shaped twins. Low tridymite: biaxial (+); alfa = 1.478, beta = 1.479, gama = 1.481, delta = 0.003, 2V = 70°.
3D Crystal Structure
Crystallography
Low tridymite is orthorhombic, a = 9.9, b = 17.1, c= 16.3, Z = 64; space group P222; point group 222.
Habit
Characteristic habit includes wedge-shaped crystals in vesicles or on the walls of cavities of volcanic rocks. Crystals belong to crystal class 222, but often appear as twinned pseudomorphs after high tridymite (6/m2/m2/m).
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Structure and Composition
Tridymite may contain minute amounts of Al³+ and alkalis. Its structure consists of sheets of SiO4 tetrahedra joined together by bridging oxygens. Three tridymite polymorphs are known (low, middle, and high tridymite).
Occurrence and Associations
Tridymite is found in high-temperature silicic igneous rocks, where it commonly associates with other high-temperature minerals, including sanidine and cristobalite. It is also found in some stony meteorites and lunar basalts.
Related Minerals
SiO2 polymorphs include quartz, cristobalite, tridymite, coesite, and stishovite.