  We review the composition of Jupiter-family comet dust as inferred from
infrared spectroscopy. We find that Jupiter-family comets have 10 micron
silicate emission features with fluxes roughly 20-25% over the dust continuum
(emission strength 1.20-1.25), similar to the weakest silicate features in Oort
Cloud comets. We discuss the grain properties that change the silicate emission
feature strength (composition, size, and structure/shape), and emphasize that
thermal emission from the comet nucleus can have significant influence on the
derived silicate emission strength. Recent evidence suggests that porosity is
the dominant parameter, although more observations and models of silicates in
Jupiter-family comets are needed to determine if a consistent set of grain
parameters can explain their weak silicate emission features. Models of 8 m
telescope and Spitzer Space Telescope observations have shown that
Jupiter-family comets have crystalline silicates with abundances similar to or
less than those found in Oort Cloud comets, although the crystalline silicate
mineralogy of comets 9P/Tempel and C/1995 O1 (Hale-Bopp) differ from each other
in Mg and Fe content. The heterogeneity of comet nuclei can also be assessed
with mid-infrared spectroscopy, and we review the evidence for heterogeneous
dust properties in the nucleus of comet 9P/Tempel. Models of dust formation,
mixing in the solar nebula, and comet formation must be able to explain the
observed range of Mg and Fe content and the heterogeneity of comet 9P/Tempel,
although more work is needed in order to understand to what extent do comets
9P/Tempel and Hale-Bopp represent comets as a whole.
