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The transition that occurs when a liquid is cooled to an amorphous or glassy solid. This can occur only if the cooling rate is fast enough to prevent crystallization which would otherwise occur if time had been sufficient for the sample to reach true equilibrium at each temperature. Since the crystal is invariably the thermodynamically stable low-temperature phase, the glass transition corresponds to a transition from a high-temperature liquid into a nonequilibrium meta-stable low-temperature solid. See Amorphous solid, Crystal, Viscosity
For many organic and polymeric systems, the difficulty of molecular packing and the steric hindrances are sufficient to prevent crystallization, and glass formation in these systems is relatively easy. In other systems, for example, metallic systems, rapid quench rates on the order of 106 K/s (2 × 106 °F/s) may be necessary to avoid crystallization, suggesting that any system can be quenched from the liquid state to an amorphous glassy state assuming that the system can be cooled rapidly enough.