  We have analyzed the 9.7 and ``18'' micron 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 micron using near-IR extinction measurements from the literature
along with the silicate profiles of WR 98a (representing the local ISM) and GCS
3 (representing the Galactic Center). We have investigated the mineralogy of
the interstellar silicates by comparing extinction profiles for amorphous
silicates with olivine and pyroxene stochiometry to the 9.7 and ``18'' micron
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, aluminosilicates 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.
