  Silicon carbide, a refractory material, condenses near the photospheres of
C-rich AGB stars, giving rise to a conspicuous emission feature at 11.3 mu. In
the presence of a stellar wind, the SiC grains are carried outwards to colder
regions, where less refractory carbonaceous material can condense, either by
itself or in mantles upon SiC grains. Enough carbon can condense on the latter
that their specific feature is completely veiled. Thus may be explained a) the
coexistence of the SiC feature protruding above a carbonaceous continuum, with
a range of contrasts, corresponding to various volume ratios of mantle to core;
b) the ultimate disappearance of the 11.3-$\mu$m feature from the interstellar
medium, where the mantle has become completely opaque due to the much higher
cosmic abundance of carbon.
