Observations of the Cas A supernova remnant in the 16 - 49 micron
 range with the KAO have revealed a stong continuum with a broad
 emission feature at 22 microns and a strong line at 26 microns
 (Moseley et al. in preparation). However, the KAO data lack the
 sensitivity and spectral resolution to clearly identify the nature of
 these features. The 26 micron line could potentially be either [O IV]
 or [Fe II]. The broad feature in the continuum spectrum is
 displaced to slightly longer wavelengths than the 20 micron feature
 found in most common silicate minerals. The position and shape of this
 emission feature are clearly unlike those previously observed in any
 astronomical source.
   The SWS observations we propose will have enough sensitivity and
 spectral resolution to allow more precise fitting of the continuum,
 and determination of the emissivity and nature of dust in Cas A. This
 unusual dust feature could either reflect the change in the optical
 properties of regular astrophysical silicates as they are exposed to
 the harsh post shock environment, or it could reflect the optical
 properties of pristine dust that has formed in the supernova ejecta.
   The wavelengths of the oxygen and iron lines are close enough and
 the range of velocities in the Cas A ejecta is high enough that
 spectral resolution alone will not allow unambiguous identification of
 the 26 micron line. The full spectral coverage of the SWS data should
 reveal the presence of another iron line at 35.4 microns, or other
 lines that were only hinted at in the KAO data ([S III] 18.7 microns,
 [Si II] 34.8 microns, and [Ne III] 36.0 microns). If the 26
 micron line is iron it samples some of the heaviest ejecta
 (previously unseen) of the Cas A supernova. If the line is oxygen,
 then we have a new tracer of oxygen sampling hotter material than that
 observed at visual wavelengths and unobservable in the UV (where
 other O IV lines are found) due to the high extinction towards Cas A.