"what substance makes ion migration easier in the later stages of magma crystallization, which allows abnormally large mineral crystals to form and create pegmatites? "

The answer is water .

Water is the substance that makes ion migration easier in the later stages of magma crystallization, which allows abnormally large mineral crystals to form and create pegmatites.

So the substance that makes ion migration easier in the later stages of magma crystallization, which allows abnormally large mineral crystals to form and create pegmatites is water.