  Infrared spectra of a number of evolved O-rich stars have been obtained with
the Short- and Long- Wavelength spectrometers on board the Infrared Space
Observatory. The very broad wavelength coverage (2.4-197 micron) obtained by
combining observations made with the two spectrometers includes practically all
of the flux emitted by the sources, and allows us to determine the emission and
absorption features of the dense circumstellar dust shells. Agreement between
the fluxes obtained by the two instruments is generally very good; the largest
discrepancies are probably due to source variability. Our sample of oxygen-rich
AGB stars exhibits a wealth of spectral features due to crystalline silicates
and crystalline water ice in emission and absorption. In this study a
qualitative overview of all features due to crystalline silicates and water ice
in these high mass loss rate objects is presented. It seems that there is a
certain onset value for the mass loss rate above which these features appear in
the spectrum. Moreover, crystalline silicate emission features have been
detected for the first time at wavelengths where the amorphous silicates are
still in absorption, implying different spatial distributions for the two
materials. A spherically symmetric and an axi-symmetric geometry are proposed.
