Unusual depletions toward the SMC star Sk 155 - Differences in dust composition in the SMC interstellar medium?

We report initial results from an analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph echelle spectra of interstellar absorption t oward Sk 155, located in the "wing" region of the SMC. There are at least 1 2 Milky Way (disk and halo) and at least 25 SMC components discernible in t he profiles of various neutral and singly ionized species. Fits to the line profiles yield column densities and relative elemental abundances for the individual components. In the SMC components, the depletions of Fe and Ni r ange from mild ([Fe, Ni/Zn] similar to -0.3 to -0.8 dex) to severe ([Fe, Ni /Zn] similar to -1.7 dex); Mg and Si, however, appear to be essentially und epleted ([ME, Si/Zn] similar to -0.1 to + 0.2 dex) throughout. The combinat ion of severe depletion for Cr, Mn, Fe, and Ni and negligible depletion for Mg and Si has not been seen in any Galactic sight line. If Si is generally undepleted in the SMC, then current models of the SMC dust-which rely heav ily on silicates-will have to be reconsidered; oxides and/or metallic grain s may dominate. We briefly discuss possible the implications of these SMC d epletions for understanding the elemental abundances in QSO absorption-line systems.