  We present a SOFIA FORCAST grism spectroscopic survey to examine the
mineralogy of the circumstellar dust in a sample of post-asymptotic giant
branch yellow supergiants that are believed to be the precursors of planetary
nebulae. Our mineralogical model of each star indicates the presence of both
carbon rich and oxygen rich dust species-contrary to simple dredge-up
models-with a majority of the dust in the form of amorphous carbon and
graphite. The oxygen rich dust is primarily in the form of amorphous silicates.
The spectra do not exhibit any prominent crystalline silicate emission
features. For most of the systems, our analysis suggests that the grains are
relatively large and have undergone significant processing, supporting the
hypothesis that the dust is confined to a Keplerian disk and that we are
viewing the heavily processed, central regions of the disk from a nearly
face-on orientation. These results help to determine the physical properties of
the post-AGB circumstellar environment and to constrain models of post-AGB mass
loss and planetary nebula formation.
