Compositional dependence of infrared absorption spectra of crystalline silicates - I. Mg-Fe pyroxenes

Crystalline Mg-Fe pyroxenes with different Mg/(Mg + Fe) ratio (MgSiO3 similar to Mg0.5Fe0.5SiO3 and FeSiO3) were synthesized in laboratory and their absorption properties were investigated in the infrared region. The absorption spectrum of ferrosilite (FeSiO3) 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.