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We have examined the relationship between the optical depth of the 9.7 micronsilicate absorption feature (tau_9.7) and the near-infrared color excess,E(J-Ks) in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra,spans E(J-Ks)=0.3 to 10 mag (corresponding to visual extinction between about 2and 60 mag.). All lines of sight show the 9.7 micron silicate feature. Unlikein the diffuse ISM where a tight linear correlation between the 9.7 micronsilicate feature optical depth and the extinction (Av) is observed, we findthat the silicate feature in dense clouds does not show a monotonic increasewith extinction. Thus, in dense clouds, tau_9.7 is not a good measure of totaldust column density. With few exceptions, the measured tau_9.7 values fall wellbelow the diffuse ISM correlation line for E(J-Ks) > 2 mag (Av >12 mag). Graingrowth via coagulation is a likely cause of this effect.

The Relationship between the Optical Depth of the 9.7 micron Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds