Quantitative Laboratory Measurements Of IR Spectra Of Diverse Glasses Suggest That Dust In Space Is Crystalline

Details Quantitative Laboratory Measurements Of IR Spectra Of Diverse Glasses Suggest That Dust In Space Is Crystalline Quantitative Laboratory Measurements Of IR Spectra Of Diverse Glasses Suggest That Dust In Space Is Crystalline

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0334h&link_type=abstract

American Astronomical Society, AAS Meeting #212, #3.34; Bulletin of the American Astronomical Society, Vol. 40, p.190

Astronomy and Astrophysics

Astrophysics

Scientific paper

There have been a great many observations of dust in diverse astrophysical environments. Observational spectra of this cosmic dust often exhibit a broad, almost structureless band in the Si-O stretching region (ca 10 microns) which has been assigned to amorphous (glassy) silicates. This attribution is based on comparison to laboratory IR transmission measurements of powder dispersions. Some glasses are difficult to produce: consequently some of the existing data were obtained from polycrystalline grains rather than glasses. It is also difficult to produce fine and even dispersions of soft materials like glasses which shear. Because of these problem and because powder dispersions commonly contain spectral artifacts, such as rounding of peaks due to leakage of light around particles and surface reflections, it is imperative to make a comparison with more quantitative measurements. To this end, we acquired new IR data from thin-films and shards of various glass compositions, most of which are analogous to astrominerals identified in a variety of astrophysical environments (e.g., MgSi2O4, near MgSiO3, near CaMgSi2O6, Ca2Al2SiO7, NaAlSi3O3, SiO2, and additional more complex compositions) and compared this to previous IR reflectivity data on similar glass compositions. Our results show that the Si-O stretching peak is moderately, but not extremely, broader in glasses than in chemically analogous crystals. Pyroxene glasses and silica-rich compositions in particular have peaks nearly as sharp as crystalline material. Peaks tend to be sharper for simple compositions, like those expected in space.
We infer that broad peaks in observational data are not due to presence of glass in space, but rather indicate that dust in space is comprised of crystalline silicate that are either relatively large (ca 1 micron), or are agglomerations of randomly oriented nanocrystals.

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