  Analysis of all archival 5--14 micron spectra of field ultracool dwarfs from
the Infrared Spectrograph on the Spitzer Space Telescope has shown that
absorption by silicates in the 8--11 micron region is seen in most L-type (1300
K to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the
atmospheres of L dwarfs and is strongest at L4--L6 spectral types. Herein we
compare averages of the mid-infrared silicate absorption signatures of L3--L7
dwarfs that have low ($\lesssim$10$^{4.5}$ cm s$^{-2}$) vs.\ high
($\gtrsim$10$^5$ cm s$^{-2}$) surface gravity. We find that the silicate
absorption feature is sensitive to surface gravity and indicates a difference
in grain size and composition between dust condensates in young and old mid-L
dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs
matches expectations for $\sim$1 micron-sized amorphous iron- and
magnesium-bearing pyroxene (Mg$_x$Fe$_{1-x}$SiO$_3$) grains. High-gravity mid-L
dwarfs have silicate absorption better represented by smaller ($\lesssim$0.1
$\mu$m) and more volatile amorphous enstatite (MgSiO$_3$) or SiO grains. This
is the first direct spectroscopic evidence for gravity-dependent sedimentation
of dust condensates in ultracool atmospheres. It confirms theoretical
expectations for lower sedimentation efficiencies in low-gravity atmospheres
and independently confirms their increased dustiness.
