LAGUNA DESIGN / SCIENCE PHOTO LIBRARY LAGUNA DESIGN / SCIENCE PHOTO LIBRARY
Cristobalite crystal, molecular model. Cristobalite is a form of quartz (silicon dioxide) that forms at high temperatures, although once formed it persists at lower temperatures in a metastable state. In this model, silicon ions are red and oxygen ions are grey. The unit cell (repeat structure) of the crystal is highlighted by the blue box. This regular cubic arrangement is predominant above around 1400 degrees Celsius, and it reverts to a more disordered arrangement of SiO4 tetrahedra (grey) when cooled below 250 degrees. However the regular cubic form can persist when cooled if the crystal is constrained by its surroundings, as the reordering changes the shape of the crystal.
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