  As comets journey into the inner solar system, they deliver particulates and
volatile gases into their comae that reveal the most primitive materials in the
solar system. Cometary dust particles provide crucial information for assessing
the physico-chemical conditions in the outer disk from which they formed.
Compared to the volatiles and soluble organics, the refractory dust particles
are more robust and may be traceable to other small bodies. Using data from the
Spitzer Heritage Archive, we present thermal dust models of 57 observations of
33 comets observed spectroscopically with the NASA Spitzer Space Telescope.
This comet spectral survey offers the opportunity to study comets with data
from the same instrument, reduced by the same methods, and fitted by the same
thermal model using the same optical constants. The submicron dust tends to be
dominated by amorphous carbon, and the submicron silicate mass tends to be
dominated by amorphous silicate materials. We discuss the implications of these
findings as they relate to Mg-rich crystalline silicates, which are
high-temperature condensates, as well as to potential ion irradiation of
amorphous Mg:Fe silicates prior to their incorporation into comets. These
results impact our understandings of the protoplanetary disk conditions of
planetesimal formation. Lastly, we cannot definitively conclude that a distinct
difference exists in the dust composition between Oort cloud and Jupiter-family
comet dynamical population as a whole.
