Origin of Asteroids

SiO₂ has many different crystal structures, which are stable or metastable at various ranges of temperatures and pressures.Per An Introduction to the Rock Forming Minerals by Deer, Howie, & Zussman, the ultimate melting point of SiO₂ is 1730^o Celsius, which (if I did the conversions correctly) equals 3115^o Fahrenheit.Now, I believe the above is for pure SiO₂. The presence of water effects the crystallization temperatures. Offhand, I think this is getting into a hybrid area that also involves solution effects.1300^o Fahrenheit equals 690^o Celsius. Offhand, this seems to be in the ballpark of the temperature that the last Quartz will crystallize out, when in a cooling high water environment. Once again, this is more a matter of crystallizing out of an water solution, rather that crystallizing out of a melt.Or something like that.MooseEdited to repair coding, which I lost when I did a non-raw text copy and paste.This message has been edited by minnemooseus, 12-06-2005 03:32 PM

The lowest temperature Quartz crystallization scenario is how pegmatites are formed. These are the VERY coarse grained rocks that might be found as the final phase of a granite bodies formation. I don't know if there are any examples of the reverse process having happened.Obviously, the final pegmatite crystallization is happening at something greater than standard atmospheric pressures, as the boiling point of water is at or above that c. 700^o Celcius temperature.In all, Walt Brown seems to be, at best, working in the realm of dubious quality science fiction.Moose

That 1730 Celsius number was for a high pressure crystal form of SiO₂ (Beta-Cristobalite). My guess is that temperature was such at much higher than atmospheric pressure. The previous mention of 1600 Celsius may well be correct for SiO₂ at atmospheric pressure.I tried and failed to find a good online silica temperature/presure phase diagram. If one wishes to venture into some highly technical material, you can look at http://arxiv.org/PS_cache/cond-mat/pdf/0406/0406192.pdf.One also may find some amusement in consulting this Scirus.com search, for "silica phase diagram". That is where I came up with the previous link.Moose