When rocks (poly-crystalline materials) are heat treated, its microstructure may changes, what will happen to its crystal or grains? will there be any grain growth or recrystallization?
Because of the anisotropy of their expansion, rocks will form cracks. Of course, it depends on the type of phases, possible phase transformations, the size of the grains (and the sample). If grains contain stored energy, e.g from plastic deformation, crystals can also grow. If they don't contain any other driving force (energy-rich grain boundaries) there is no reason why crystal should "invest" energy. Insofar, if you observe recrystallization, you have some indication for stored energy within phases. Some phases may also melt at certain temperature. Then you have to look, which change the phase diagram predict. As you see....it is a wide field... It would be better, if you would say a bit more about your "rock".
When you are using the very generalized term rock it becomes very difficult to say precisely.
even in a single rock, as you have mentioned, like granite there will be about ten minerals....which all have different physico-chemical changes with increasing temperature.
A simple mineral like quartz will just recrystalize.....according to changing temperatures...alpha quartz, beta quartz, trydimite, cristobalite.....are various forms of same composition, but different structures.
Hello
In the theoretical processing of ores, there are changes in the structure of the individual elements in it. Depending on the heating temperature, partial changes or full changes may occur at higher temperatures. It also depends on retention time. The longer the time we hold at a certain temperature, a crystal recrystallization is obtained therein. This leads to a decrease in the hardness of the material and its strength.
Wishing for success in scientific work.
Emil Yankov
In addition to an illuminating answer by Dr Nolze, I’d like to mention that predicting any possible changes in a mixture of crystalline materials (including rocks) one should be based on thermodynamics and kinetics considerations. Grain growth is determined by kinetics (diffusion), and ionic mobility is agitated by heating, thus leading to enlarging crystallites and their coalescence. On the other hand, if thermodynamics-driven phase transitions happen, with their characteristic volume changes, particles/crystallites crack and even experience subdivision; the tin pest can serve as a notorious example.
Furthermore, thermodynamics rules the game in the case of possible chemical reactions of the components of a mixture with atmospheric gases and between each other. Melting of some components or reaction products should also be taken into account. If we add energetic considerations regarding the role of stored energy mentioned by Dr Nolze, you will see that the phenomenon is extremely complicated and no simple answer could be given. It requires careful, material-related preliminary analysis.
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