Abstract. The mass absorption spectra of crystalline olivine particles with different Mg/(Mg + Fe) ratios, between forsterite (Mg 2 SiO 4 , Fo) and fayalite (Fe 2 SiO 4 , Fa), were measured for synthetic and natural samples in the mid-and far-infrared regions. The main strong peaks show a systematic shift to longer wavelengths as the Mg/(Mg + Fe) ratio decreases. In the 10-17 µm region, these trends are very clear, and the peak positions are a good indicator of the composition of dust grains. In the 20-100 µm region, the trends are also rather clear, though they are slightly complicated; the intensity and width of the various bands become weak and broad, respectively, or the bands disappear as the concentration of Fo decreases. However, the trends are clear only in a limited composition range near Fo and Fa; the peak positions of 30, 50, and 69 µm bands of forsterite shift linearly as the Fo concentration decreases to about Fo 70 . Those of the double band in the 50 µm region for Fe-rich olivine shift linearly as the Fa concentration increases from Fo 20 to Fa. When the spectral signature of Mg-rich or Fe-rich olivine can be observed, the peak positions in the 20-100 µm region are a good indicator of the composition of dust grains.
The infrared reflectivities of crystalline forsterite (Mg2SiO4) were measured for the temperature range 295–50 K for each crystal axis, between wavenumber 5000 and 100 cm−1. The reflection spectra show clear dependence of temperature; most of the bands become more intense, sharper and their peak positions shift to higher wavenumber with decreasing temperature. Reflection spectra were fitted with dispersion formula of damped oscillator model of the dielectric constants and the oscillator parameters in the model were derived. The absorption spectra of forsterite particle are calculated with the derived dielectric constants to show that the forsterite features are good thermal indicator for cold temperature range below 295 K.
Polarized infrared reflectance of large single crystals of forsterite was measured for each crystal axis at frequencies between 5000 cm −1 and 100 cm −1 with a resolution of 1 cm −1 . The reflectance spectra were analyzed based on classical dispersion theory. A set of oscillator parameters for crystalline axes of forsterite was obtained. Those parameters are used to derive optical constants of forsterite, which are important for analyses of observed spectra of astronomical objects and laboratory spectra of particle samples. Calculations of absorption for small particles using the bulk optical constants are compared with mid-infrared and far-infrared absorption spectra measured in the laboratory.
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