  An accurate knowledge of the mineralogy (chemical composition and crystal
structure) of the silicate dust in the interstellar medium (ISM) is crucial for
understanding its origin in evolved stars, the physical and chemical processing
in the ISM, and its subsequent incorporation into protostellar nebulae,
protoplanetary disks and cometary nuclei where it is subjected to further
processing. While an appreciable fraction of silicate dust in evolved stars, in
protoplanetary disks around pre-main sequence stars, in debris disks around
main sequence stars, and in cometary nuclei is found to be in crystalline form,
very recent infrared spectroscopic studies of the dust along the sightline
toward the Galactic center source Sgr A* placed an upper limit of ~1.1% on the
silicate crystalline fraction, well below the previous estimates of ~5% or ~60%
derived from the observed 10 micron absorption profile for the local ISM toward
Cyg OB2 No.12. Since the sightline toward SgrA contains molecular cloud
materials as revealed by the detection of the 3.1 and 6.0 micron water ice
absorption features, we argue that by taking into account the presence of ice
mantles on silicate cores, the upper limit on the degree of silicate
crystallinity in the ISM is increased to ~3--5%.
