Pixie Dust : The Silicate Features in the Diffuse Interstellar Medium

We have analyzed the 9.7 and " 18 " µm interstellar silicate absorption features along the line of sight toward four heavily extincted galactic WC-type Wolf-Rayet (WR) stars. We construct two interstellar extinction curves from 1.25 to 25 µm using near-IR extinction measurements from the literature along with the sili-cate profiles of WR 98a (representing the local ISM) and GCS 3 (representing the Galactic Center). We have investigated the mineralogy of the interstellar sil-icates by comparing extinction profiles for amorphous silicates with olivine and pyroxene stochiometry to the 9.7 and " 18 " µm absorption features in the WR 98a spectrum. In this analysis, we have considered solid and porous spheres and a continuous distribution of ellipsoids. While it is not possible to simultaneously provide a perfect match to both profiles, we find the best match requires a mixture of these two types of compounds. We also consider iron oxides, alu-minosilicates and silicate carbide (SiC) as grain components. Iron oxides cannot be accommodated in the observed spectrum, while the amount of Si in SiC is limited to < 4%. Finally, we discuss the cosmic elemental abundance constraints on the silicate mineralogy, grain shape and porosity.