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We characterize the crystalline silicate content and spatial distribution ofsmall dust grains in a large sample of protoplanetary disks in theTaurus-Auriga young cluster, using Spitzer Space Telescope mid-infraredspectra. In turn we use the results to analyze the evolution of structure andcomposition of these 1-2 Myr-old disks around Solar- and later-type youngstars, and test the standard models of dust processing which result in theconversion of originally amorphous dust into minerals. We find strong evidenceof evolution of the dust crystalline mass fraction in parallel with that of thestructure of the disks, in the sense that increasing crystalline mass fractionis strongly linked to dust settling to the disk midplane. We also confirm thatthe crystalline silicates are confined to small radii, r < 10 AU. However, wesee no significant correlation of crystalline mass fraction with stellar massor luminosity, stellar accretion rate, disk mass, or disk/star mass ratio, aswould be expected in the standard models of dust processing based uponphoto-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 anothergrain-crystallizing mechanism dominates over these, or another process must beat work within the disks to erase the correlations they produce. We propose oneof each sort that seem to be worth further investigation, namely X-ray heatingand annealing of dust grains, and modulation of disk structure bygiant-planetary formation and migration.

Crystalline silicates and dust processing in the protoplanetary disks of the Taurus young cluster