Astronomy & Astrophysics

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Issue		A&A Volume 391, Number 1, August III 2002
Page(s)		267 - 273
Section		Diffuse matter in space
DOI		10.1051/0004-6361:20020791

A&A 391, 267-273 (2002)
DOI: 10.1051/0004-6361:20020791

Compositional dependence of infrared absorption spectra of crystalline silicates

I. Mg-Fe pyroxenes

H. Chihara^{1, 2}, C. Koike², A. Tsuchiyama¹, S. Tachibana³ and D. Sakamoto¹

¹ Department of Earth and Space Science, Osaka University, 1-1, Machikaneyama, Toyonaka, Osaka 560-0043, Japan
² Laboratory of Physics, Kyoto Pharmaceutical University, Misasagi, Yamashina, Kyoto 607-8414, Japan
³ Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404, USA

(Received 14 March 2002 / Accepted 21 May 2002 )

Abstract
Crystalline Mg-Fe pyroxenes with different ${\rm Mg/(Mg+Fe)}$ ratio ( ${\rm MgSiO_ \sim {\rm Mg_Fe_SiO_$ and ${\rm FeSiO_$ ) were synthesized in laboratory and their absorption properties were investigated in the infrared region. The absorption spectrum of ferrosilite ( ${\rm FeSiO_$ ) is reported for the first time in this work. Our study confirmed that the Mg end members have sharp and characteristic features in the far-IR region and in particular that it is easy to distinguish the two types of crystalline structures (orthorhombic and monoclinic) in this wavelength range. In addition we find that the absorption spectra depend on the chemistry and crystal structure of the material: (1) The far-IR features which are prominent in the Mg end members, vanish when the iron concentration is increased. However, even in the iron bearing pyroxenes, the variation of the mid-IR features with Fe concentration is less significant in comparison to that of the far-IR features. (2) Peak positions shifted to longer wavelength an increase in iron concentration; the dependence of the shift is approximately linear in wavenumber (cm ^-1). (3) Bandwidths of the far-IR bands for the end members are significantly smaller than those of solid solutions. These results suggest that the far-IR features of pyroxenes are very sensitive to chemical composition and crystal structure. Therefore, far-IR features are a very useful constraint on the chemical composition and crystal structure of circumstellar pyroxenes.

Key words: circumstellar matter -- line: profiles -- infrared: ISM: lines and bands -- methods: laboratory -- ISM: dust, extinction

Offprint request: H. Chihara, chi@ess.sci.osaka-u.ac.jp

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