For a crystal melting in a Solid Solution Phase Diagram, beginning with a 60% rock.
What will be the composition of the last crystal to melt? And at what temperature?
Answers: The last crystal will melt at about 1480o with a composition of about 88% anorthite.
If the system stays in equilibrium at all times the final composition must end up where we started, 60% anorthite. But the crystals are continuously reacting with the melt during melting, always shifting (reacting) to a more Ca rich crystal (as more and more Na melts away) so as to stay in equilibrium with the prevailing temperature. Thus, crystal composition will move up the solidus line toward the Ca end until it reaches the temperature where the last crystal will melt. This temperature will be where the original composition of 60% crosses the liquidus line, about 1480o
At the same time, of course, the melt, having started out quite Na rich is shifting to a more and more Ca rich compositon. Therefore, at the very end, before the last drop of crystal melts its composition will be approaching the starting composition, about 59% Ca.