Stoichiometric hydrates

Peritectic
Binary phase diagram with the formation of a eutectic (E) and a peritectic (P).

In general, stoichiometric hydrates contain the water molecules as integral part of the crystal lattice, in which they interact strongly with the host molecule and each other. That implies that the removal of these water molecules will cause instability of the crystal network, which subsequently collapses and either stays as an amorphous phase or recrystallises as a new crystal form with reduced water content.

As water is an integral part of the crystal structure of stoichiometric hydrates, these crystals can show a distinct melting point. In a two component phase diagram this behaviour resembles the eutectic temperature. In contrast to a clear melting point, the peritectic describes a temperature above which the hydrate as a composite crystal form is not stable anymore. This is normally detectable as incongruent melting, a concomitant melting and recrystallisation event, during which the water of crystallisation is released.