  In order to deduce properties of dust in astrophysical environments where
dust growth through aggregation is important, knowledge of the way aggregated
particles interact with radiation, and what information is encoded in the
thermal radiation they emit, is needed. The emission characteristics are
determined by the size and structure of the aggregate and the composition and
shape of the constituents. We thus aim at performing computations of
compositionally inhomogeneous aggregates composed of irregularly shaped
constituents. In addition we aim at developing an empirical recipe to compute
the optical properties of such aggregates in a fast and accurate manner. We
performed CDA computations for aggregates of irregularly shaped particles with
various compositions. The constituents of the aggregate are assumed to be in
the Rayleigh regime, and in addition we assume that the dominant interaction of
the aggregate constituents is through dipole-dipole interactions. We computed
the spectral structure of the emission efficiency in the 10 micron region for
aggregates with 30% amorphous carbon and 70% silicates by volume with various
fractions of crystalline and amorphous components. We find that the spectral
appearance of the various components of the aggregate are very different and
depend on their abundances. Most notably, materials that have a very low
abundance appear spectroscopically as if they were in very small grains, while
more abundant materials appear, spectroscopically to reside in larger grains.
We construct a fast empirical approximate method, based on the idea of an
effective medium approximation, to construct the spectra for these aggregates
which almost perfectly reproduces the more exact computations. This new method
is fast enough to be easily implemented in fitting procedures.
