  We present evidence of a >10-sigma detection of the 10 micron silicate dust
absorption feature in the spectrum of the gravitationally lensed quasar PKS
1830-211, produced by a foreground absorption system at redshift 0.886. We have
examined more than 100 optical depth templates, derived from both observations
of Galactic and extragalactic sources and laboratory measurements, in order to
constrain the chemical structure of the silicate dust. We find that the best
fit to the observed absorption profile is produced by laboratory crystalline
olivine, with a corresponding peak optical depth of tau_10=0.27+/-0.05. The fit
is slightly improved upon by including small contributions from additional
materials such as silica, enstatite, or serpentine, which suggests that the
dust composition may consist of a blend of crystalline silicates. Combining
templates for amorphous and crystalline silicates, we find that the fraction of
crystalline silicates needs to be at least 95%. Given the rarity of
extragalactic sources with such a high degree of silicate crystallinity, we
also explore the possibility that the observed spectral features are produced
by amorphous silicates in combination with other molecular or atomic
transitions, or by foreground source contamination. While we cannot rule out
these latter possibilities, they lead to much poorer profile fits than for the
crystalline olivine templates. If the presence of crystalline interstellar
silicates in this distant galaxy is real, it would be highly unusual, given
that the Milky Way interstellar matter contains essentially only amorphous
silicates. It is possible that the z=0.886 absorber towards PKS 1830-211, well
known for its high molecular content, has a unique star-forming environment
that enables crystalline silicates to form and prevail.
