Friday, 1 February 2019

What is Crystalline Silicates

Crystalline Silicates

What is Crystalline Silicates


One of the big surprises of the Infrared Space Observatory (ISO) has been discovery of crystalline silicates outside our own Solar system. It was generally assumed before that all cosmic silicates in space were of amorphous structure. Thanks to ISO we know now that crystalline silicates are ubiquitous in the Galaxy (except for the diffuse ISM) and sometimes even in very large quantities (>50% of the small dust particles). The evolution of the crystalline silicates is still not completely clarified, but the combination of theoretical modeling and observations have already shed light on their life-cycle. The absence of crystalline silicates in the diffuse ISM provides us with information about the dust amorphization rate in the ISM.


Before the Infrared Space Observatory (ISO) opened the mid and far-infrared range for high resolution spectroscopy, it was generally assumed that cosmic dust silicates were of amorphous structure. The crystalline silicates, the highly ordered counterparts of the amorphous silicates, were only known to be present on earth, in the solar system in comets (Hanneretal.,1994;Hanner,1996),Interplanetary Dust Particles (IDPs) (MacKinnon and Rietmeijer, 1987; Bradley et al., 1992) and in the dust disk of β-Pictoris (Knacke et al., 1993; Fajardo-Acosta and Knacke, 1995), also a crystalline olivine feature was reported in the polarized 10 μm spectrum of AFGL2591 (Aitken et al., 1988). Apart from the crystalline silicates in the IDP’s, that were found with the aid of transmission electron microscopy and only later confirmed by infrared spectroscopy (Bradley et al., 1992), in all other cases the crystalline silicate features were found by infrared spectro (polari) metry around 10 μm.

With the present day knowledge it is relatively easy to understand why crystalline silicates were only discovered to be ubiquitous after ISO was operational. Before ISO was launched the primary MIR/FIR window for observations was around 10μm. And although the crystalline silicates do have strong features in this area, they are in general overwhelmed by emission from the much more abundant and warmer amorphous silicates. Furthermore, most of the crystalline silicates have a (relatively) low temperature (<150K), which suppresses the intrinsically strong crystalline silicate features in the 10μm region. Thanks to the extended wave length range (up to 200μm) of the spectrographs on board ISO, the composition of the cold (<150K) dust, which has the top of its SED at wavelengths above 15μm, could be studied in detail for the first time. With ISO, crystalline silicates have been found around young stars (Waelkens et al., 1996), comets (Crovisieret al., 1997), and evolved stars (Waters et al., 1996) (see Figure 2), but not convincingly so in the interstellar medium. The presence of the crystalline silicates in the different galactic environments will be discussed in Section 2. The properties of the crystalline silicates will be discussed in Section 3. In Section 4 we will discuss the crystalline silicate (trans-)formation and destruction processes based on the ISO (and other astronomical) observations in comparison with laboratory measurements.

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