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Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the InfraredSpectrograph (IRS) on board the Spitzer Space Telescope are modeled using dustat two temperatures to probe the radial variation in dust composition in theuppermost layers of protoplanetary disks. Most spectra indicating crystallinesilicates require Mg-rich minerals and silica, but a few suggest otherwise.Spectra indicating abundant enstatite at higher temperatures also requirecrystalline silicates at temperatures lower than those required for spectrashowing high abundance of other crystalline silicates. A few spectra show 10micron complexes of very small equivalent width. They are fit well usingabundant crystalline silicates but very few large grains, inconsistent with theexpectation that low peak-to-continuum ratio of the 10 micron complex alwaysindicates grain growth. Most spectra in our sample are fit well without usingthe opacities of large crystalline silicate grains. If large grains grow byagglomeration of submicron grains of all dust types, the amorphous silicatecomponents of these aggregates must typically be more abundant than thecrystalline silicate components. Crystalline silicate abundances correlatepositively with other such abundances, suggesting that crystalline silicatesare processed directly from amorphous silicates and that neither forsterite,enstatite, nor silica are intermediate steps when producing either of the othertwo. Disks with more dust settling typically have greater crystallineabundances. Large-grain abundance is somewhat correlated with greater settlingof disks. The lack of strong correlation is interpreted to mean that settlingof large grains is sensitive to individual disk properties. Lower-mass starshave higher abundances of large grains in their inner regions.

Dust Processing and Grain Growth in Protoplanetary Disks in the Taurus-Auriga Star-Forming Region