You can melt and freeze water repeatedly, and the crystals will re-form. With SiO2 things are different. When this liquid cools down, the SiO2 molecules find it very difficult to form a crystal again. It’s almost as if they can’t quite remember how to do it: which molecule goes where, who should be next to whom, appears to be a difficult problem for the SiO2 molecules. As the liquid gets cooler, the SiO2 molecules have less and less energy, reducing their ability to move around, which compounds the problem: it gets even harder for them to get to the right position in the crystal structure. The
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