  In this paper we demonstrate that distinguishing between the polytypes of
silicon carbide by means of infrared features in small-grain spectra is
impossible. Therefore, the infrared spectra of carbon stars, unfortunately, do
not provide a means for drawing comparisons between the crystal structures of
grains condensed in these environments and found in meteorites.
  This is proven first by comparing theoretical band profiles calculated for
ellipsoidal particles, which show clearly a strong dependence on the axis ratio
of the ellipsoids but negligible differences for the two most common polytypes.
Second, spectra measured on submicron particle samples in the laboratory do not
show any obvious correlation of band position or shape to the polytype.
However, we demonstrate by measurements on SiC whiskers that grain shape is
able to determine the spectrum completely. A further strong systematic
influence on the band profile can be exerted by plasmon-phonon coupling due to
conductivity of the SiC material. The latter fact probably is responsible for
the confusion in the astronomical literature about spectral properties of SiC
grains. We show that, although the conductivity seems to be a common property
of many SiC laboratory samples, it is, however, independent of the polytype.
