For fractional crystallization, early formed plagioclase does not react with the liquid. The result is that the liquid will migrate to a lower point on the boundary curve than for the same bulk composition which has undergone equilibrium crystallization.
Examination of plagioclase in igneous rocks indicates that such a process of incomplete reaction is common, as observed in the presence of zoned plagioclase grains which exhibit normal, reverse and oscillatory zoning.
For pure fractional crystallization the last liquid would have a composition represented by the binary eutectic composition on the Di-Ab join and would produce a solid mixture of Di and Ab in the proportions given by this binary eutectic.
