Steps toward interstellar silicate mineralogy -: VI.: Dependence of crystalline olivine IR spectra on iron content and particle shape

Crystalline olivines are an important component of silicate dust particles in space. ISO observations revealed the presence of crystalline silicates in comets, protoplanetary accretion disks, and outflows from evolved stars. For the interpretation of astronomical spectra, the relevant material data at a variety of temperatures and over a broad wavelength range, are urgently needed. In contrast to this need, optical properties of the astronomically interesting olivines are scarcely available at present. In order to close this gap, we studied the optical properties of three minerals of the olivine group by reflection spectroscopy on single crystals in the infrared spectral range. We measured the iron endmember (fayalite, Fe(2)SiO(4)), an Mg-rich olivine (Mg(1.9)Fe(0.1)SiO(4)), and the magnesium endmember (forsterite, Mg(2)SiO(4)) of the (Mg, Fe)(2)SiO(4) series. For a direct comparison with astronomical observations, we present calculated mass absorption coefficients in the Rayleigh limit for different shapes and varying iron content of the dust particles. The laboratory data together with a set of ISO data for envelopes around evolved stars (Molster 2000) are used to constrain the properties of circumstellar silicates. We find that essentially all band positions are shifted to larger wavelengths with increasing iron content. The particle shape influences very significantly the strong bands such as the B(1u):nu (3) mode that appears as the "11.4 mum" band of forsterite, whereas e.g. the two FIR modes longward of 40 mum remain practically unaffected by the particle shape but shift due to increasing iron content. The comparison with the band positions in ISO spectra points to the presence of olivine crystals strongly elongated along the crystallographic c-axis. In addition, we apply the calculated mass absorption coefficients to evaluate transmission measurements of particles embedded in a matrix-a technique which is frequently used in laboratory astrophysics. All data shown in this paper will be made available in digital form via the electronic database http://www.astro.uni-jena.de.