4
$\begingroup$

I've been reading a lot of sources about the hydrothermal synthesis of quartz using autoclaves. I want to try doing this myself, but before I can I need to know the minimum pressure required.

If you need a refresher, hydrothermal synthesis uses steam as a solvent, and creates a supersaturated solution that places the quartz molecules onto a seed to grow a pure crystal.

I've gotten many different numbers in terms of how much pressure [from the steam] is required for the process, ranging from 7500 psi to 30000 psi. I want to work with as low of a pressure as possible.

So then my question is: is high pressure necessary for quartz crystal formation? My understanding is that the pressure in the case of hydrothermal synthesis is created from having large amounts of steam in order for more material to be dissolved in said steam, so the crystal will form faster. So is this the case, in which having a lower pressure will work, or is pressure required for the crystal to form properly? As in, does the pressure actually affect the crystal structure in some significant way?

$\endgroup$
  • $\begingroup$ What would happen if you just melted SiO2 sand and cooled it down very very slowly. It would allow crystallization and all quartz is is SiO2So I don’t see any reason it wouldn’t work. Also it shouldn’t take too long because it’s just cooling into a shape but exposed sides would crystallize. $\endgroup$ – Quartz man May 26 at 18:17
  • $\begingroup$ You're offering an alternative way of making quartz crystals. Whether or not it's correct, it doesn't seem to answer the question and it would best serve as a comment. $\endgroup$ – M.A.R. May 26 at 18:32
  • $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$ – airhuff May 26 at 19:10
  • $\begingroup$ Not sure you get quartz. My understanding of silica (assuming the 4-coordinate-silicon, 2-coordinate-oxygen varieties) is cristobalite and then tridymite are the first stable firms upon cooling the liquid, and only after that quartz. What if you form one of the first two phases and then things freeze up, so to speak, kinetically? $\endgroup$ – Oscar Lanzi May 26 at 19:47
7
$\begingroup$

Pressure does not affect the crystal structure (unless it gets insanely great, so that stishovite formation kicks in; that's not going to happen in an autoclave, though). Pressure, however, does affect the temperature (indirectly), and hence solubility, and hence the rate of crystal formation. There is no lower threshold, that's why you keep getting wildly different estimates for the "required" pressure. It is just that the higher you get, the faster your crystals grow. They would grow at normal atmospheric pressure as well, just very slow (so that only your great-grandchildren may possibly be able to see them).

I suggest that you aim at something less extreme. Calcite, maybe?

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.