Interstellar Iron and Silicon Depletions in Translucent Sight Lines

We report interstellar Fe II and Si II column densities toward six translucent sight lines (A sub(v) [unk] 1) observed with the Space Telescope Imaging Spectrograph (STIS). The abundances were determined from the absorption of Si II] at 2335 AAA, and several weak Fe transitions including the first reported detections of the A2234 line. We derive an empirical f-value for the Fe II A2234 transition of log ( integral of A) = -1.54 plus or minus 0.05. The observed sight lines sample a variety of extinction characteristics as indicated by their R sub(v) values, which range from 2.6 to 5.8. The dust-phase abundances of both Si and Fe are positively correlated with the small-grain population (effective radii smaller than a few hundred mu m) toward the targets. The physical conditions along the sight lines suggest that this relationship may be due to differences in the survival of small particles in some interstellar environments. The chemical composition of the small grains could either resemble dust mantles or be silicate-rich.