What is the difference between congruent crystallization and incongruent crystallizationÉ?

CONGRUENT  MELTING
        Material changes directly from a solid to a melt of the same composition at the temperature of melting, ie melting occurs all at the one time

     INCONGRUENT  MELTING
        Material starts to melt and the first melt formed has a different composition to the starting material. The melt only has the same composition as the starting material when it becomes completely molten
        Example: Orthoclase starts to melt at around 1150°C where it forms a mixture of leucite crystals and a melt of composition intermediate between KAlSi3O8 and SiO2. As the temperature increases leucite starts to dissolve in the melt until a temperature of 1500°C when all the leucite dissolves and the melt has an orthoclase composition.
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