Compositional dependence of infrared absorption spectra of crystalline silicate - II. Natural and synthetic olivines

The mass absorption spectra of crystalline olivine particles with different Mg/(Mg+Fe) ratios, between forsterite (Mg2SiO4, Fo) and fayalite (Fe2SiO4, 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 mum region, these trends are very clear, and the peak positions are a good indicator of the composition of dust grains. In the 20-100 mum 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 mum bands of forsterite shift linearly as the Fo concentration decreases to about Fo(70). Those of the double band in the 50 mum 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 mum region are a good indicator of the composition of dust grains.