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We investigate the composition of interstellar grains along the line of sighttoward Zeta Ophiuchi, a well-studied environment near the diffuse-dense cloudtransition. A spectral decomposition analysis of the solid-state absorbers isperformed using archival spectroscopic observations from the Spitzer SpaceTelescope and Infrared Space Observatory. We find strong evidence for thepresence of sub-micron-sized amorphous silicate grains, principally comprisedof olivine-like composition, with no convincing evidence of H2O ice mantles.However, tentative evidence for thick H2O ice mantles on large (a ~ 2.8microns) grains is presented. Solid-state abundances of elemental Mg, Si, Fe,and O are inferred from our analysis and compared to standard referenceabundances. We find that nearly all of elemental Mg and Si along the line ofsight are present in amorphous silicate grains, while a substantial fraction ofelemental Fe resides in compounds other than silicates. Moreover, we find thatthe total abundance of elemental O is largely inconsistent with the adoptedreference abundances, indicating that as much as ~156 ppm of interstellar O ismissing along the line of sight. After taking into account additional limits onthe abundance of elemental O in other O-bearing solids, we conclude that anymissing reservoir of elemental O must reside on large grains that are nearlyopaque to infrared radiation.

The Composition of Interstellar Grains Toward Zeta Ophiuchi: Constraining the Elemental Budget Near the Diffuse-Dense Cloud Transition