  We characterize the crystalline silicate content and spatial distribution of
small dust grains in a large sample of protoplanetary disks in the
Taurus-Auriga young cluster, using Spitzer Space Telescope mid-infrared
spectra. In turn we use the results to analyze the evolution of structure and
composition of these 1-2 Myr-old disks around Solar- and later-type young
stars, and test the standard models of dust processing which result in the
conversion of originally amorphous dust into minerals. We find strong evidence
of evolution of the dust crystalline mass fraction in parallel with that of the
structure of the disks, in the sense that increasing crystalline mass fraction
is strongly linked to dust settling to the disk midplane. We also confirm that
the crystalline silicates are confined to small radii, r < 10 AU. However, we
see no significant correlation of crystalline mass fraction with stellar mass
or luminosity, stellar accretion rate, disk mass, or disk/star mass ratio, as
would be expected in the standard models of dust processing based upon
photo-evaporation and condensation close to the central star,
accretion-heating-driven annealing at r < 1 AU, or spiral-shock heating at r <
10 AU, with or without effective radial mixing mechanisms. Either another
grain-crystallizing mechanism dominates over these, or another process must be
at work within the disks to erase the correlations they produce. We propose one
of each sort that seem to be worth further investigation, namely X-ray heating
and annealing of dust grains, and modulation of disk structure by
giant-planetary formation and migration.
