  Spitzer Space Telescope spectra reveal 10 micron silicate emission from
circumstellar dust orbiting six externally-polluted white dwarfs. Micron-size
glasses with an olivine stoichiometry can account for the distinctively broad
wings that extend to 12 microns; these particles likely are produced by
tidal-disruption of asteroids. The absence of infrared PAH features is
consistent with a scenario where extrasolar rocky planets are assembled from
carbon-poor solids.
