Mid-infrared spectra of a handful of Class II Young Stellar Objects taken using the Infrared 
Spectrograph (IRS) on board the Spitzer Space Telescope show very prominent narrow emission 
features indicating silica (crystalline silicon dioxide). Silica is not believed to be a 
major constituent of the interstellar medium; therefore, any silica present in the 
circumstellar disks of Class II YSOs must be largely the result of processing of primitive 
dust material in the protoplanetary disks surrouding these stars. We model the silica 
features in our spectra using the opacities of various polymorphs of silica and amorphous 
versions thereof computed from earth-based laboratory measurements. This modeling suggests 
that the polymorphs of silica that form at high temperatures and low pressures are the 
dominant forms of silica in some Class II YSO disks. In turn, this constrains the locations 
within the disk and the physical processes involved in the thermal processing of dust 
grains. 


This work is based on observations made with the Spitzer Space Telescope, which is operated 
by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 
1407. Support for this work was provided by NASA through contract number 1257184 issued by 
JPL/Caltech, JPL contract 960803 to Cornell University, and Cornell subcontracts 31419-5714 
to the University of Rochester.