Silicate minerals

Silicate minerals of the olivine and pyroxene classes have been shown to be present in outflows of evolved stars as well as in comets and protoplanetary disks. The positions of the infrared emission bands produced by these minerals are diagnostic for the crystal structure as well as for the chemical composition, especially the iron content. Comparison of the laboratory data with observed features can constrain the conditions in these environments which have led to the formation or processing of the dust grains. We have used the infrared optical constants of forsterite contained in the database for calculating the absorption cross sections of spherical and non-spherical particles in the Rayleigh limit (see Fig. ). The spectra are obtained by averaging the cross sections calculated for the three different crystallographic directions. The spectra show resonances due to surface modes which shift very strongly in dependence on the aspect ratio of the particles. This effect probably is very important for the identification of emission features in astronomical spectra [7,8]. Interstellar polarization measurements and laboratory experiments on the growth of silicate particles [9] support the presence of elongated grains in astrophysical environments. Information about the grain shape may provide constraints for the formation mechanism of crystalline silicate grains, i.e. the role of direct condensation vs. processing of previously amorphous material.

Figure: Left panel: Imaginary part of the refractive index for crystalline forsterite (Mg$_2$SiO$_4$) in the three different crystallographic directions. Right panel: Mass-normalized absorption cross section (MAC) of prolate spheroidal forsterite particles (rotationally averaged) with different axis ratios. The dots and asterisks below the spectra indicate positions of astronomically observed emission bands (after [6]).