ABSTRACT
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neutral SiO2 units (i.e., silica) with no need for charge-compensating cations, which we also consider as a (nano)silicate herein. Terrestrial olivine and pyroxene are typically very rich in Mg and thus oen are close to the n = 0 end members, forsterite and enstatite, respectively (see Figure 13.1). Such a situation is not specic to the Earth’s intrinsic geology; Mg-rich silicates are a common constituent of meteorites and are found in a wide range of extraterrestrial environments (e.g., comets, circumstellar disks, supernovae, quasars) [1]. Astronomical detection of such silicates typically takes advantage of the fact that, depending on the environment, silicate dust absorbs or emits light in the infrared (IR) part of the spectrum. In some astronomical environments (e.g., circumstellar shells), and accounting for about 10% of all observed cosmic dust, detailed IR spectral characteristics of crystalline forsterite and enstatite have been identied. Typically, however, cosmic silicate dust spectra consist of two signicantly broadened characteristic peaks with wavelengths at around 10 and 18 μm (corresponding to Si-O stretching and O-Si-O bending modes respectively), which are judged to correspond to noncrystalline silicate dust, usually with pyroxene or olivine chemical composition [1]. Because of their broad spectral signatures, our knowledge of dust particles in space is more limited than that of specic molecules. rough a combination of lab characterization of pristine material [2,3], astronomical observations [4,5], and analyses of spectral features [6], the general properties of dust particles in various environments have been deduced to a certain extent [2]. Fitting the observed spectra with a combination of lab spectra of materials of dierent crystallinity, shape, size, and composition gives insight into the possible identity of dust particles in space [2,7]. Such dust is found to be ubiquitous throughout the interstellar medium (ISM) and is the source of all cosmic and terrestrial silicate solids. In fact, silicates are thought to be the most abundant form of solid atomic matter (as opposed to the much higher fraction of gas phase atoms and molecules) in the known Universe.
