For crystallization in a Solid Solution Phase Diagram, beginning with a 30% melt.
If the system remained in equilibrium at all times, what will be the composition of the crystals just before the last drop of melt crystallizes?
Answers: The last crystal will be about 31% anorthite.
If the system stays in equilibrium at all times the final composition must end up where we started, 30% anorthite. The crystals are continuously reacting with the melt during cooling, always shifting (reacting) to a more Na rich crystal to stay in equilibrium with the prevailing temperature. Therefore, at the very end, before the last drop of melt crystallizes (and reacts with the last crystals) the last crystals will be just slightly more Ca rich than the beginning (and final) compositions of 30%.