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Since glass is amorphous quartz and when you slow the cooling of something enough it will crystallize, couldn’t you melt $\ce{SiO2}$ (sand) and then very slowly cool it to cause spontaneous nucleation and form quartz crystals?

If so, then why hasn’t it been used before? It would seem to be a better method than flux because in flux you need many chemicals and a dissolving chemical or mix and it is a complicated and very long process to grow very small crystals throughout the whole mix. If you simply cooled a molten mixture slowly enough to cause complete crystallization, then the entire mix of molten crystal would need to form crystal because it all is cooling down.

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    $\begingroup$ About the second part of your question, both the α- and the β-phase of quartz have what we call an enantiomorphic space group. It forms spirals that can be 'enantiomers' to each other, which I guess also gives it its properties. For many applications you often need the pure enantiomer which will grow in a seed of the correct orientation under for example hydrothermal conditions. $\endgroup$
    – Justanotherchemist
    Commented May 27, 2019 at 8:27
  • $\begingroup$ Melting and cooling will give you a bunch of crystals lumped together. Why would you want that? Then again, I've seen single crystals of quartz as big as a tabletop, and they were not made by melting. $\endgroup$
    – Ivan Neretin
    Commented May 27, 2019 at 8:29
  • $\begingroup$ see eg ndk.com/catalog/AN-SQC_GG_e.pdf $\endgroup$
    – Buck Thorn ♦
    Commented May 27, 2019 at 8:47
  • $\begingroup$ In principle, maybe. But that isn't how big crystals are formed in nature. Most deposit from solutions not from melted pure silica. $\endgroup$
    – matt_black
    Commented May 27, 2019 at 13:03
  • $\begingroup$ @NightWriter Your example post is about hydrothermal growth of a crystal. $\endgroup$
    – Buttonwood
    Commented May 27, 2019 at 21:37

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While the production of quartz crystals by slow cooling of pure silicon dioxide melt is theoretically possible, there are multiple reasons this is not done in practice. Let’s delve into the many reasons quartz crystals are not grown from pure molten silica.

  1. Melting of silica requires very high temperatures.
  2. Silica melt is extremely viscous, therefore crystallisation is slow.
  3. Silica melt tends to vitrify, due to high viscosity, or crystallise into cristobalite due to the high temperatures.

For these reasons, the growth of quartz crystals by slow cooling of silica melt is inefficient and prohibitively expensive.

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