  We present low-resolution mid-infrared spectra of nine classical T Tauri
stars associated with the Chamaeleon I dark cloud. The data were obtained with
the PHOT-S instrument on-board the Infrared Space Observatory (ISO) in the two
wavelength ranges 2.5-4.9 and 5.9-11.7 microns. All nine stars show evidence of
silicate emission at 10 microns, which is the only prominent feature in the
spectra. We discuss a model for the origin of these features in a hot
optically-thin surface layer of the circumstellar disks surrounding the central
young stars (i.e. a disk atmosphere). We report excellent agreement of our
observations with predictions based upon this simple model for most stars in
our sample, assuming that a mixture of amorphous silicates of radius ~< 1
microns is the dominant source of opacity. These observations support the
notion that extended disk atmospheres contribute substantially to the mid-IR
flux of young stars.
