Cordierite

Cordierite is a high temperature metamorphic magnesium-aluminosilicate. It is interesting because it contains no water in its structure, but contains large channels (see picture) into which water will readily bind, with up to about 1.2 H₂O per formula unit (Mg₂Al₄Si₅O₁₈). Unlike most non-structural water, cordierite can hold onto water in this site under very high temperature conditions, such as granulite facies metamorphism.

Because the water is not bound to a specific site, it has high entropy for a non-vapor phase, which of course explains the high temperature stability. It also leads to some funky phase transitions.