We report the detection of interstellar silicate dust in the z _ { abs } =0.685 absorber along the sightline toward the gravitationally lensed blazar TXS 0218+357 .
Using Spitzer Space Telescope Infrared Spectrograph data we detect the 10 \micron silicate absorption feature with a detection significance of 10.7 \sigma .
We fit laboratory-derived silicate dust profile templates obtained from literature to the observed 10 \micron absorption feature , and find that the best single-mineral fit is obtained using an amorphous olivine template with a measured peak optical depth of \tau _ { 10 } =0.49 \pm 0.02 , which rises to \tau _ { 10 } \sim 0.67 \pm 0.04 if the covering factor is taken into account .
We also detected the 18 \micron silicate absorption feature in our data with a > 3 \sigma significance .
Due to the proximity of the 18 \micron absorption feature to the edge of our covered spectral range , and associated uncertainty about the shape of the quasar continuum normalization near 18 \micron , we do not independently fit this feature .
We find , however , that the shape and depth of the 18 \micron silicate absorption are well-matched to the amorphous olivine template prediction , given the optical depth inferred for the 10 \micron feature .
The measured 10 \micron peak optical depth in this absorber is significantly higher than those found in previously studied quasar absorption systems .
The reddening , 21-cm absorption , and velocity spread of Mg II are not outliers relative to other studied absorption systems , however .
This high optical depth may be evidence for variations in dust grain properties in the ISM between this and the previously studied high redshift galaxies .