  Silicates are an important component of interstellar dust and the structure
of these grains -- amorphous versus crystalline -- is sensitive to the local
physical conditions. We have studied the infrared spectra of a sample of
ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow
absorption features at 11, 16, 19, 23, and 28 microns, characteristic of
crystalline silicates, superimposed on the broad absorption bands at 10 and 18
microns due to amorphous silicates in a subset of this sample. These features
betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of
the olivines. Previously, crystalline silicates have only been observed in
circumstellar environments. The derived fraction of forsterite to amorphous
silicates is typically 0.1 in these ULIRGs. This is much larger than the upper
limit for this ratio in the interstellar medium of the Milky Way, 0.01. These
results suggest that the timescale for injection of crystalline silicates into
the ISM is short in a merger-driven starburst environment (e.g., as compared to
the total time to dissipate the gas), pointing towards massive stars as a
prominent source of crystalline silicates. Furthermore, amorphization due to
cosmic rays, which is thought to be of prime importance for the local ISM, lags
in vigorous starburst environments.
