Heating amorphous silica causes conversion to the crystalline form under appropriate conditions.   Maybe you can find some information in the floowing link:

http://www.iapa.ca/pdf/Silica-in-the-workplace-FEB03.pdf

Annealing during enough time (the higher the tremperature, the shorter the treatment) will convert the (metastable) amourphous silica into a more stable (crystallised) one. Traces of alkali ions (such as those left by touching fused silica by hands) will significantly catalyse the process.

Alain

Thanks Dr Alain, Do we need to ball mill the sample into uniform size before annealing to get uniform crystals?

Did this work? I need an AFM probe apex made out quartz or cristobalite. It'd be pretty great if I could just thermally oxidize the probe in air and then turn the resulting silica crystalline with a simple anneal in vacuum or N2.

I'm still lit searching but haven't found any useful protocols as of yet. The PDF link above is dead.

You can convert amorphous silica to crystalline silica by heating to about 1000°C in air. What you may find is that you form a type of cristobalite that can be referenced in some X-ray Diffraction references, but the literature on this topic is limited. You can check the crystalline pattern using XRD, as distinct peaks will develop from your single x-ray hump that is common for glassy materials.

Thank you. I eventually found some references that confirm what you say, although I think you need to go a little higher than 1000C to get reasonably fast kinetics if the silica is very pure as it would be from oxidizing a Si single crystal. It seems that impurities or standard glass compositions seem to transform more quickly at lower temperatures. I'm using quartz for now but will keep this idea in mind if I ever want to try cristobalite.