We propose to make high quality observations of the 2.75 micron feature of
interstellar hydrated silicates which has been recently discovered by ISO
towards Cyg.OB2 #12. A number of sources suffering from extinction by the
diffuse interstellar medium are selected. Together with guaranteed time data
of dense clouds sources and of mass-loosing giant stars our program could
reveal the importance of hydration for grains in various interstellar
environments. This information could yield important clues on the origin and
evolution of interstellar silicates, i.e., whether cold formation in the
interstellar medium or hot formation in the outflow of stars dominate. Showing
considerable variation in different materials, comparison of high quality
spectra of the hydration band with laboratory analog materials could
furthermore clarify the composition of interstellar minerals.