  A set of 45 dust envelopes of carbon stars has been modeled. Among them, 34
were selected according to their dust envelope class (as suggested by Sloan,
Little-Marenin & Price, 1998) and 11 are extreme carbon stars. The models were
performed using a code that describes the radiative transfer in dust envelopes
considering core/mantle grains composed by an alpha-SiC core and an amorphous
carbon (A.C.) mantle. In addition, we have also computed models with a code
that considers two kinds of grains - alpha-SiC and A.C. - simultaneously.
Core-mantle grains seem to fit dust envelopes of evolved carbon stars, while
two homogeneous grains are more able to reproduce thinner dust envelopes. Our
results suggest that there exists an evolution of dust grains in the carbon
star sequence. In the beginning of the sequence, grains are mainly composed of
SiC and amorphous carbon; with dust envelope evolution, carbon grains are
coated in SiC. This phenomena could perhaps explain the small quantity of SiC
grains observed in the interstellar medium. However, in this work we consider
only alpha-SiC grains, and the inclusion of beta-SiC grains can perhaps change
some of there results.
