For crystallization in a Solid Solution Phase Diagram, beginning with a 30% melt.
What will be the composition of the last drop of melt? And at what temperature?
Answers: The last drop will be about 3% anorthite, at a temperature of about 1185o.
If the system stays in equilibrium at all times the final composition must end up where we started, 30% anorthite. So, bring the liquid composition down the liquidus line until it is opposite where the 30% line crosses the solidus line. Straight down from there is the composition of the last drop. The temperature is read straight cross from the intersection of the 30%/solidus line intersection to the temperature scale.