I have more than 20 XRD spectra of quartz extracted from different granitoids. I want to refine these spectra in GSAS-II, but I am confusing that how to choose the most suitable or right cif? There are many cif cards of quartz under different conditions as shown in picture.
Should I choose one cif card that have the same conditions to my sample,and choose another one for next sample? Or just use one standard cif for all of my samples?
Many thanks!
I suggest that you work through some of the cards (and draw the structures) and compare these to find out by yourself what the differences are and what the differences imply physically. Then you can make the decision by yourself. The only thing what can be mentioned here, is that many entries my refer to non-ambient conditions (temperature, pressure). If you yourself have ambient conditions data, you may look for data pertaining to ambient conditions.
It is even more important at which temperature your measurements have been performed. Certain aspects of the high-temperature and high-pressure conditions can be quenched upon returning to ambient conditions, but not thermal expansion and the pure compression. So in particular for the situation of quartz the ambient-temperature ambient pressure data are relevant (if you perform the measurement AT ambienet temperature and pressure)
Andreas Leineweber I finally understand why you mentioned "which temperature your measurements have been performed". The quartz formed at 600 °C is β-quartz, and it will finally transform to α-quartz at earth surface temperature and pressure. I compared differences in the phase transition of quartz and rutile. β-quartz spontaneously transforms to α-quartz, but rutile does not spontaneously transform to anatase. I think the reason is the energy potential of transition from rutile to anatase is little higher than β to α-quartz. I don't know if I'm thinking correctly？ Thanks for your help!
The phase transition from beta- to alpha-quartz involves only tilting of the tetrahedra against each other, which is ordered in alpha and (I think to remember) dynamically distordered in the beta quartz. I think the high- and low-temperature versions of tridymite and crystobalite are related similarly. Hence, also the corresponding transitions cannot be quenched. Anatase vs. rutile share different arrangements of TiO6 octahedra, so that bonds have to be broken to reshuffle the atomic arrangments. Likewise the above SiO2 polymorphs (quartz, tridymite, cristobalite in whatever alpha or beta form) cannot be easly intercovertet, because again bonds have to be broken.
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