Steps toward interstellar silicate mineralogy IV. The crystalline revolution

Midand far-infrared spectra gained by the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) satellite have provided striking evidence for the presence of crystalline silicates in comets, circumstellar envelopes around young stars and, most of all, evolved stars and planetary nebulae. Since optical properties of astrophysically relevant crystalline silicates are lacking in the literature, in this paper mass absorption coefficients (MACs) of olivines and pyroxenes for a wide range of Mg/Fe ratios are presented, which cover the whole ISO wavelength range . The MAC have been derived from transmission spectra of small grains embedded in potassium bromide and polyethylene pellets. Only in the case of natural enstatite (MgSiO3), was a monocrystalline sample available, which allowed the measurement of optical constants for the different crystallographic orientations of this anisotropic silicate. Since not all Mg/Fe ratios are represented among the natural minerals, we supplemented the series by synthetic products prepared in our lab. We also included two inhomogeneous synthetic materials, one of olivine and the other one of pyroxene composition, which are expected to be similar to the primary condensate in cosmic environments. For all samples the chemical composition, the purity, and the homogeneity have been determined by energy-dispersive X-ray analysis and by scanning electron microscopy. Especially for the minerals, it is important to exclude the spectral influence of differently composed inclusions. The peak positions of the samples are influenced by different factors which are discussed: chemical composition (FeO content), size and shape distribution of the grains, and the matrix in which the grains are embedded for spectroscopy. The continuum-subtracted ISO SWS spectrum of the source AFGL 4106 has been compared with simple optically thin model spectra calculated for our olivine and pyroxene samples. The main result was that a combination of the pure magnesium silicates (forsterite and enstatite) gives a good agreement between observations and laboratory measurements.