Signatures of aging silicate dust

A self-consistent explanation for the width, strength, and position of spectral features which may be diagnostic of the structure and composition of circumstellar and interstellar silicate grains is presented based on a long series of laboratory experiments using simple oxide analogs. The ratio of the integrated absorption strength of the grains near 10 to that near 20 is shown to decrease monotonically with increased processing (e.g., annealing/hydration) and this ratio may, therefore, be diagnostic of the relative age of the silicate. The oxidation state of iron in the grains is directly related to both the near-infrared grain opacity and enhanced absorption in the ultraviolet. The oxidation state of both iron and silicon is directly related to the rate of nucleation, growth, and processing in a circumstellar outflow, as well as to the processing history of the grain after it reaches the interstellar medium. Other diagnostic indicators of the average silicon oxidation state, degree of structural polymerization and dispersion in the average degree of structural polymerization are discussed.