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We report the detection of interstellar silicate dust in the z_abs=0.685absorber along the sightline toward the gravitationally lensed blazar TXS0218+357. Using Spitzer Space Telescope Infrared Spectrograph data we detectthe 10 micron silicate absorption feature with a detection significance of10.7-sigma. We fit laboratory-derived silicate dust profile templates obtainedfrom literature to the observed 10 micron absorption feature, and find that thebest single-mineral fit is obtained using an amorphous olivine template with ameasured peak optical depth of tau_10=0.49+/-0.02, which rises totau_10~0.67+/-0.04 if the covering factor is taken into account. We alsodetected the 18 micron silicate absorption feature in our data with a >3-sigmasignificance. Due to the proximity of the 18 micron absorption feature to theedge of our covered spectral range, and associated uncertainty about the shapeof the quasar continuum normalization near 18 micron, we do not independentlyfit this feature. We find, however, that the shape and depth of the 18 micronsilicate absorption are well-matched to the amorphous olivine templateprediction, given the optical depth inferred for the 10 micron feature. Themeasured 10 micron peak optical depth in this absorber is significantly higherthan those found in previously studied quasar absorption systems. Thereddening, 21-cm absorption, and velocity spread of Mg II are not outliersrelative to other studied absorption systems, however. This high optical depthmay be evidence for variations in dust grain properties in the ISM between thisand the previously studied high redshift galaxies.

Interstellar Silicate Dust in the z=0.685 Absorber Toward TXS 0218+357