Milky Way Mid-Infrared Spitzer Spectroscopic Extinction Curves:
  Continuum and Silicate Features
  We measured the mid-infrared (MIR) extinction using Spitzer photometry and
spectroscopy (3.6--37 micron) for a sample of Milky Way sightlines (mostly)
having measured ultraviolet extinction curves. We used the pair method to
determine the MIR extinction that we then fit with a power law for the
continuum and modified Drude profiles for the silicate features. We derived 16
extinction curves having a range of A(V) (1.8-5.5) and R(V) values (2.4-4.3).
Our sample includes two dense sightlines that have 3 micron ice feature
detections and weak 2175 A bumps. The average A(lambda)/A(V) diffuse sightline
extinction curve we calculate is lower than most previous literature
measurements. This agrees better with literature diffuse dust grain models,
though it is somewhat higher. The 10 micron silicate feature does not correlate
with the 2175 A bump, for the first time providing direct observational
confirmation that these two features arise from different grain populations.
The strength of the 10 micron silicate feature varies by $\sim$2.5 and is not
correlated with A(V) or R(V). It is well fit by a modified Drude profile with
strong correlations seen between the central wavelength, width, and asymmetry.
We do not detect other features with limits in A(lambda)/A(V) units of 0.0026
(5--10 micron), 0.004 (10--20 micron), and 0.008 (20-40 micron). We find that
the standard prescription of estimating R(V) from C times E(K_s-V)/E(B-V) has C
= -1.14 and a scatter of $\sim$7%. Using the IRAC 5.6 micron band instead of
K_s gives C = -1.03 and the least scatter of $\sim$3\%.
