I have conducted three experiments of melt crystallization with different temperature gradients (from 1,300 ° C to room temperature in 24h, 3.5 days and 7 days). Each time crystallization begins with melilite. Regardless of the temperature gradient, the crystallization process is stopped with a melilite share of about 30%. The composition of the residual glass lies close to the melilite - pseudowollastonite cotectic.
Why does crystallization stop instead of following cotectic?
Always ask yourself what is crystallizing and how this changes the parent liquid or glass phase. If the chemical composition of what is crystallizing is the same as the chemical composition of the parent phase, then all the building blocks for crystal growth are available at the crystal-liquid interface (assuming a chemical homogeneous material).
Now, if the chemical composition of the phase that is crystallizing is different than the liquid phase, then diffusion will play an important role. As the crystal grows the chemical composition of its interface will somehow reflect the difference in composition between the crystal and the liquid.
One example: if the phase that crystallizes is richer in lithium than the non-crystalline phase, then it will deplete lithium ions in the crystal-liquid interface as the crystal grows. This process will change the thermochemical properties of the interface and probably change the rate in which the crystal is growing.
This is what is probably happening in your case. Remember that equilibrium phase diagrams only show you what happens when thermodynamic equilibrium is reached, it does not say anything about the kinetics of the process.
Cheers
Thank you for your response. I will be happy to learn about your work regarding the crystal growth in supercooled liquid. Of course, I considered the possible limitations resulting from reaction kinetics. But still in my opinion this does not explain such a rapid end of the crystallization process in the same point at such different temperature gradients.
It seems that the solution to this problem results from two factors: the non-equilibrium crystallization of "akermanite" which eventually changes the composition of the melt to almost purely "gehlenite" which, however, has a much higher solidus temperature. For this reason, it has a much narrower window of crystallization and can only appear as a small envelope on an earlier Akermanite (confirmed).
At the same time, this amount of melilite is almost enough but not quite for reaching the melilite pseudowollastonite cotectic.
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