Origin of Name
Named after occurrence at Cerro San Cristobal, Mexico.
Hand Specimen Identification
Cristobalite is difficult to identify in hand specimen. X-ray or optical techniques are required. Cristobalite is Sometimes confused with zeolites in hand specimen.
Physical Properties
| Hardness | 6.5 |
| Specific Gravity | 2.33 |
| Cleavage | Absent |
| Fracture | Conchoidal |
| luster | vitreous |
| Transparency | Translucent to Transparent |
| Color | Colorless |
| Streak | white |
Optical Properties
Crystal habit, low birefringence, and moderate negative relief characterize cristobalite in thin section. Low cristobalite: uniaxial (—), Womega = 1.489, Epsilon = 1.482, Delta = 0.007.
3D Model
Crystallography
Low cristobalite is tetragonal, a = 4.97, c = 6.93, Z = 4; point group 422. High cristobalite is cubic, a = 7.13, Z= 8.
Habit
Cubic, octahedral, or coarse aggregates typify cristobalite. Although low cristobalite crystals belong to crystal class 422, they often appear as small octahedra or globby aggregates.
Structure and Composition
Cristobalite, always nearly pure SiO2, may contain minor amounts of Al³+ and alkalis. As with quartz, the structure is based on a three dimensional framework of SiO4 tetrahedra. The difference between cristobalite, quartz, and other SiO2 polymorphs is the way in which the tetrahedra are linked. Mineralogists have described two cristobalite polymorphs: low cristobalite (alfa - cristobalite) is tetragonal, high cristobalite (beta - cristobalite) is cubic.
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Occurrence and Associations
Cristobalite is only found in high temperature silicic extrusive igneous rocks. Rapid cooling may keep it from changing into the more stable, low quartz. Typically it occurs as small spherical grains or aggregates in vugs, as misty inclusions in volcanic glass, or as principal components in finegrained ground mass. It is associated with other high temperature minerals including sanidine and tridymite.
Related Minerals
SiO2 polymorphs include quartz, cristobalite, tridymite, coesite, and stishovite.