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.