Compositional dependence of infrared absorption spectra of crystalline silicate

II. Natural and synthetic olivines

¹ Kyoto Pharmaceutical University, Yamashina, Kyoto 607-8412, Japan
² Department of Earth and Space Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
³ Subaru Telescope, National Astronomical Observatory of Japan, Hilo, Hawaii, USA
⁴ Gunma Astronomical Observatory, Agatsuma, Gunma 377-0702, Japan

Corresponding author: C. Koike, koike@mb.kyoto-phu.ac.jp

Received: 30 July 2002
Accepted: 10 December 2002

Abstract

The mass absorption spectra of crystalline olivine particles with different ratios, between forsterite (, Fo) and fayalite (, 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 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 . Those of the double band in the 50 μm region for Fe-rich olivine shift linearly as the Fa concentration increases from 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.