Shape and lattice distortion effects on infrared absorption spectra of olivine particles

¹ Department of Earth and Space Science, Osaka University, Toyonaka 560-0043, Japan e-mail: imai@astroboy.ess.sci.osaka-u.ac.jp
² Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Hakozaki, 812-8581, Japan

Received: 27 February 2009
Accepted: 18 May 2009

Abstract

We prepared sub-micron olivine (Fo₉₂) particles of different shapes and different degrees of crystal lattice distortion, and studied both effects on the infrared absorption spectra experimentally. We found that the peak positions of the spectra shifted to short wavelength when the number fraction of spherical grains became large, consistent with spectral calculations using the continuous distributions of ellipsoids (CDE) model. We also found for the first time that the spectral features become broader by crystal lattice distortion. The peak intensities are affected by both effects. The peak intensity decreases with lattice distortion in general, but some peaks are largely affected by particle shape. The spectral difference of olivine measured in different laboratories can be explained by both the shape and lattice distortion effects through sample preparation methods. Broadening features in the infrared spectra of forsterite ejecta from comet Tempel 1 obtained in the Deep Impact mission, and forsterite crystals in circumstellar regions, suggest that forsterite crystals undergo lattice distortion.