Infrared spectroscopy silicate dust

Both the analysis of Comet Wild 2 dust particles (Brownlee et al. 2006) and infrared spectroscopy of dust particles from comets Halley, Hale-Bopp, and Tempel 1 (Lisse et al. 2006) have shown that crystalline silicates are common constituents of comets. The presence of crystalline silicates in comets indicates that the crystallization of amorphous interstellar silicates occurred in the comet-forming region... 

With improved possibilities for infrared spectroscopy, broad extinction bands around 9.7 pm and 18 pm have been detected, which were ascribed to the stretching (Woolf Ney 1969) and bending (Treffers Cohen 1974) modes in the SiC>4 tetrahedron forming the building block of silicates, because they correspond to known absorption bands seen in all terrestrial silicates. These bands are also seen in the emission from dust shells around O-rich stars. This gave the first observational hints on the mineralogy of the silicate dust. The smooth, structureless nature of the bands indicated that the silicates in the ISM and in circumstellar dust shells are amorphous. 

From infrared spectroscopy it is very difficult to obtain the composition of the amorphous silicates. This is because the spectral signature observed is a combination of grain composition, shape, size, and structure, making it difficult to isolate the pure amorphous silicate signal. This, in combination with the relatively small spectral changes caused by the composition of the silicates, makes it hard to get a definitive answer in most cases. In the case of interstellar dust we have a unique opportunity the grains are very small and (almost) all silicates are amorphous. 

Infrared absorption spectroscopy of interstellar clouds shows that the interstellar dust population varies with the line of sight, yet it maintains a similar character. In particular, submicron-sized amorphous silicate grains are the dominant component in every direction. The absence of crystalline grains is likely the result of rapid amorphization by the interstellar radiation field. 

Spectroscopy allows one to see not only the elements in the gas phase, but also those in the dust grains. Using bright X-ray binaries as background sources, Ueda et al. (2005) were able to determine the total abundances of the elements in the diffuse ISM and found that most of them are approximately solar, with the exception of oxygen. From the details of the X-ray absorption spectra it could be determined to some extent what fraction of the elements were in the solid phase and also to a lesser extent the lattice structure could be determined. From this, they found the silicates to be predominantly rich in magnesium and poor in iron, in agreement with the infrared absorption spectroscopic study of the diffuse ISM by Min et al. (2007). 

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